University of KentuckyCollege of Agriculture

ID-125: A Comprehensive Guide to Wheat Management in Kentucky

Acknowledgments

Authors

Table of Contents (Click here for Table of Contents only)

Writing and printing of this publication was funded in part by: KySGA

Section 1. Introduction

Chad Lee, James Herbek, and Richard L. Trimble
Photo1.1 Soft red winter wheat(Triticum aestivum) grown in kentucky is a valuable commodity and an important component to crop rotations.

The soft red winter wheat (Triticum aestivum L.) grown in Kentucky provides flour for cookies, cakes, pastries, and crackers and is the fourth most valuable cash crop in the state (Figure 1-1). Winter wheat has been an integral part of crop rotation for Kentucky farmers. Wheat is normally harvested in June in Kentucky and provides an important source of cash flow during the summer months. Several trends should be examined when considering the economic potential of wheat production in the state (see Section 9 Economics of the Intensively Managed Wheat Enterprise).

Improvements in varieties and adoption of intensive wheat management practices have resulted in dramatically increased wheat yields. Prior to 1987, the highest average yield achieved in Kentucky was 42 bushels per acre; since 1987, averages have been at least 49 bushels per acre in all but two years (Figure 1-2). State average yields have been 59 bushels per acre for the past decade and 62 bushels per acre for the past five years. State averages were above 70 bushels per acre in 2006 and 2008. Continued increases in yield help to keep wheat in the crop rotation.

The average yield of wheat trend has been upward, but the number of acres of wheat planted in the state has declined since 1981. Harvested acres were 680,000 in 1981 and were 460,000 in 2008 (Figure 1-3). Fluctuation in wheat acres harvested is a function of government programs, crop condition and economics.

Fig1.1 Kentucky crop values according to the Kentucky Agricultural Statistics Service
Kentucky crop values according to the kentucky Agricultural statistics service
Fig1.2 Kentucky average wheat yields according to the Kentucky Agricultural Statistics Service
Kentucky average wheat yields according to the Kentucky Agricultural Statistics Service
Fig1.3 Kentucky planted and harvested wheat acres according to the Kentucky Agricultural Statistics
Kentucky planted and harvested wheat acres according to the Kentucky Agricultural Statistics Service

This publication will help you use wheat management practices to improve the competitiveness of wheat in your crop rotation. There is no single best wheat management prescription for all circumstances, but this comprehensive publication explains the principles of wheat growth and management so you can make decisions appropriate to your situation. This publication also will help troubleshoot problems encountered during the growing season. If you use and adopt the following principles and practices, you should see increased yields, higher profits, and improved environmental protection from your wheat fields.

The important steps for intensive wheat management can be summarized in 18 steps. The application of these steps at the proper stage of growth and time of year is the basis for obtaining maximum and efficient wheat yields. (See Winter Wheat Calendar [ID-125A].)

18 Steps for Maximum Winter Wheat Yields

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Section 2. Growth and Development

James Herbek and Chad Lee

Germination and Seedling Growth | Tillering | Stem Elongation/Jointing | Boot | Heading | Grain Fill/Ripening | Yield Components

Wheat responds best to inputs at certain stages of plant development. Therefore, it is important to understand wheat development and recognize wheat growth stages in order to properly time applications of pesticides, nitrogen, and other inputs.

Photo2.1 Wheat at about Feekes 2 (Zadoks 21) in corn residue.
Wheat at about Feekes 2 (Zadoks 21) in corn residue.

Wheat plants progress through several growth stages, which are described in terms of developmental events. Wheat plant growth and development can be broadly divided into the following progressive stages: germination/seedling emergence, tillering, stem elongation, boot, heading/anthesis, and grain-fill/ripening. Several different systems have been developed to identify wheat growth stages. These systems use a numerical designation for the development or formation of specific plant parts. The two most widely used methods for identification of wheat growth stages are the Feekes scale and the Zadoks scale. The Feekes scale is the traditional, most common scale and has been widely used by Kentucky growers. Developmental stages are designated on a scale of 1 (seedling growth) through 11 (ripening). The Zadoks scale is much more descriptive of various stages of development. It uses a two-digit system for wheat plant development, divided into 10 primary stages, each of which is divided into 10 secondary stages, for a total of 100 stages. The Zadoks scale goes from primary stage 00 (dry seed) to 90 (ripening). Both the Zadoks and Feekes scales are shown for comparison (Figure 2-1 and Table 2-1).

Germination and Seedling Growth

Fig 2.1 Wheat at about Feekes 2 (Zadoks 21) in corn residue.

Adequate temperature and moisture are needed for wheat seeds to germinate. Wheat seeds germinate at temperatures of 39°F or higher; temperatures between 54° and 77°F are considered optimum for rapid germination and growth. Germination begins when the seed imbibes water from the soil and reaches 35 to 45 percent moisture on a dry weight basis. During germination, the seedling (seminal) roots, including the primary root (radicle), emerge from the seed along with the coleoptile (leaflike structure), which encloses the primary leaves and protects the first true leaf during emergence from the soil. The coleoptile extends to the soil surface, ceases growth when it emerges, and the first true leaf emerges from its tip. Under favorable conditions, seedling emergence occurs within seven days. Until the first leaf becomes functional, the seedling depends on energy and nutrients stored in the seed.

Seedling growth begins with the emergence of the first leaf above the soil surface and continues until the next stage, tillering. Normally three or more leaves develop in the seedling stage before tillering is initiated. Each new leaf can be counted when it is over one-half the length of the older leaf below it. During this phase the fibrous root system develops more completely, helping plant establishment.

The crown (a region of lower nodes whose internodes do not elongate) is located between the seed and the soil surface. It tends to develop at the same level, about one-half to one inch below the soil surface, regardless of planting depth. Leaves, tillers and roots (including the main root system) develop from the crown nodes. The growing point is located at the crown until it is elevated above the soil surface at the stem elongation stage.

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Tillering

The tillering stage begins with the emergence of lateral shoots (tillers) from the axils of the true leaves at the base of the main stem of the plant. The tillers are formed from the auxiliary buds located at each crown node. Primary tillers form in the axils of the first four or more true leaves of the main stem. Secondary tillers may develop from the base of primary tillers if conditions favor tiller development. A tiller may also develop from the coleoptile node (coleoptilar tiller), but this occurs sporadically and its appearance is dependent on genotype, planting practices, and environmental conditions. At the base of each tiller is a sheath (small leaf like structure) called the prophyll, from which the tiller leaves emerge. The prophyll acts like the coleoptile and protects the auxiliary bud before it elongates its first leaf to become a tiller. Identifying the prophyll, which encloses the base of the tiller, will help differentiate tiller leaves from the leaves on the main stem and from other tillers. Tillering usually begins when the seedling plant has three or more fully developed leaves. Tillers depend on the main stem for nutrition during their development. Once a tiller has developed three or more leaves, it becomes nutritionally independent of the main stem and forms its own root system.

Photo2.2 Wheat field at about Feekes 4 or 5 (Zadoks 30).

Tillers are an important component of wheat yield because they have the potential to develop grain-bearing heads. In Kentucky, each plant normally develops two or more tillers in the fall when planted at optimum dates. The total number of tillers eventually developed will not all produce grain-bearing heads. Under recommended plant populations, usually two or three tillers, in addition to the main shoot, will produce grain. Tiller development occurs in the fall until low temperatures stop plant growth. In Kentucky, during the tillering stage, winter wheat goes through the winter months in a dormant condition in which plant growth (including tiller production) essentially ceases due to cold temperature. Tiller production and development resumes in late winter/early spring with an increase in temperature as the plants break dormancy and resume growth. Due to cooler temperatures, late planted winter wheat may have little or no fall tillering because of limited seedling growth or because no wheat has emerged; late planted wheat will rely heavily on spring tiller development. Spring tillers generally contribute less to yield potential than do fall tillers. Consequently, fall tillering is important for winter wheat to achieve maximum yield potential.

Tillers develop sequentially on a plant, resulting in a prioritization for development. The main stem and older (first-formed) tillers have priority to complete development and form a grain-bearing head. This same priority also exists regarding the size of the grain-bearing head on the main stem and subsequent tillers.

Table 2.1 Wheat Growth Stages
Stage
General Description
Feekes Scale
Zadoks Scale
Additional Comments
Germination Dry seed   00  
Start of imbibition   01  
Imbibition complete   03 Seed typically at 35 to 40% moisture.
Radicle emerged from seed (caryopsis)   05  
Coleoptile emerged from seed (caryopsis)   07  
Leaf just at coleoptile tip   09  
Seedling Growth First leaf through coleoptile 1 10  
First leaf unfolded   11  
2 leaves unfolded   12  
3 leaves unfolded   13  
4 leaves unfolded   14  
5 leaves unfolded   15  
6 leaves unfolded   16  
7 leaves unfolded   17  
8 leaves unfolded   18  
9 or more leaves unfolded   19  
Tillering Main shoot only   20  
Main shoot and 1 tiller 2 21  
Main shoot and 2 tillers   22  
Main shoot and 3 tillers   23 Many plants will only have 2 or 3 tillers per plant at recommended populations.
Main shoot and 4 tillers   24  
Main shoot and 5 tillers   25  
Main shoot and 6 tillers 3 26 Leaves often twisting spirally.
Main shoot and 7 tillers   27  
Main shoot and 8 tillers   28  
Main shoot and 9 tillers   29  
Stem Elongation Pseudostem erection 4-5 30  
1st detectable node 6 31 Jointing stage
2nd detectable node 7 32  
3rd detectable node   33  
4th detectable node   34 Only 4 nodes may develop in modern varieties.
5th detectable node   35  
6th detectable node   36  
Flag leaf visible 8 37  
Flag leaf ligule and collar visible 9 39  
Booting Flag leaf sheath extending   41 Early boot stage.
Boot swollen 10 45  
Flag leaf sheath opening   47  
First visible awns   49 In awned varieties only.
Head (Inflorescence) Emergence First spikelet of head visible 10.1 50  
1/2 of head visible 10.2 52  
1/2 of head visible 10.3 54  
3/4 of head visible 10.4 56  
Head completely emerged 10.5 58  
Pollination (Anthesis) Beginning of flowering 10.51 60 Flowering usually begins in middle of head.
10.52   Flowering completed at top of head.
10.53   Flowering completed at bottom of head.
1/2 of flowering complete   64  
Flowering completed   68  
Milk Development Kernel (caryopsis) watery ripe 10.54 71  
Early milk   73  
Medium Milk 11.1 75 Milky ripe.
Late Milk   77 Noticeable increase in solids of liquid endosperm when crushing the kernel between fingers
Dough Development Early dough   83  
Soft dough 11.2 85 Mealy ripe: kernels soft but dry.
Hard dough   87  
Ripening Kernel hard (hard to split by thumbnail) 11.3 91 Physiological maturity. No more dry matter accumulation.
Kernel hard (cannot split by thumbnail) 11.4 92 Ripe for harvest. Straw dead.
Kernel loosening in daytime   93  
Overripe   94  
Seed dormant   95  
Viable seed has 50% germination   96  
Seed not dormant   97  
Secondary dormancy   98  
Secondary dormancy lost   99  
Sources: Conley, et al. 2003. Management of Soft Red Winter Wheat. IPM1022. Univ. of Missouri. Alley, et al. 1993. Intensive Soft Red Winter Wheat Production: A Management Guide. Pub. 424-803. Virginia Coop. Extension. Johnson, Jr., et al. Arkansas Wheat Production and Management. MP404. Univ. of Arkansas. Coop. Ext. Serv.

The number of tillers a plant develops is not a constant and will vary because of two factors: genetic potential and environmental conditions. Some varieties have a greater potential to develop more tillers than others. Tillering is also a means for the plant to adapt to changing environmental conditions. Plants are likely to produce more tillers when environmental conditions such as temperature, moisture, and light are favorable, when plant populations are low, or when soil fertility levels are high. Under weather stress conditions such as high temperature, drought, high plant populations, low soil fertility, or pests, plants respond by producing fewer tillers or even aborting initiated tillers. Rarely do more than five auxiliary tillers form and complete development on a plant. Although the total number of tillers formed per plant can vary considerably and be quite high, not all of the tillers remain productive. The later developing tillers usually contribute little to yield. Tillers that emerge after the fifth leaf on the main stem are likely to senesce (or die), abort, or not produce a grain head. Very few of the secondary tillers that form usually develop a head unless conditions dictate a need.

As temperatures decrease below the minimum for plant growth in late fall/winter, winter wheat will become dormant. Cooler temperatures induce cold hardiness in wheat plants to protect against cold injury and to help them survive the winter. During this period, the low temperatures initiate in the plant a physiological response called vernalization. During vernalization, the plant converts from vegetative to reproductive growth and the reproductive structures are developed. Because of this vernalization requirement, winter wheat produces only leaves for both the main stem and tillers aboveground in the fall in preparation for winter. The growing point and buds of both the main stem and tillers remain belowground, insulated against the cold winter temperatures. Once vernalization requirements are met, the growing point differentiates and develops an embryonic head. At this time, wheat head size or total number of spikelets per head is determined. Neither seedling growth nor tillering is required for vernalization to occur. This process can begin in seeds as soon as they absorb water and swell. Hence, late planted wheat that has not emerged prior to winter should be adequately vernalized. Following vernalization, exposure to progressively longer photoperiods (longer day length periods) is necessary to initiate and hasten reproductive development.

The vernalization requirement involves exposure to cooler temperatures for a required length of time. Temperatures below 50°F are needed to induce cold hardening and satisfy vernalization requirements; temperatures of 37° to 46°F are considered sufficient and most effective. The required length of low temperature exposure decreases with colder temperatures and advanced plant development. At sufficiently low temperatures, most varieties in Kentucky require three to six weeks of vernalization. Varieties also differ in their response to vernalizing temperature requirements. Generally, early-maturing varieties require less time to vernalize than later-maturing varieties.

In some varieties, vernalization is affected by photoperiod, in which exposure of the wheat plant to short days replaces the requirement for low temperatures. Exposure of wheat to temperatures above 86°F shortly following low temperatures can sometimes interrupt vernalization. Spring wheat varieties do not possess an absolute vernalization requirement. Reproductive development in most spring varieties is induced by light and accumulated heat units (growing degree days).

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Stem Elongation / Jointing

Stem elongation is the next phase of growth (Feekes 4-9; Zadoks 30-39). The leaves of overwintering (dormant) wheat are generally short and lie rather flat. As temperatures increase in the spring, the wheat plants break dormancy and resume growth. The leaf sheaths grow quickly and give a strongly erect appearance known as a pseudostem (not a real stem) (Feekes 4-5; Zadoks 30). At this time, and prior to actual stem elongation, each main stem and tiller of the young plant is a succession of leaves wrapped around each other (i.e., a pseudostem). The actual stem has not elongated at this stage and the immature head (growing point) is still below ground level but has started to advance above the crown region. The growing point is only about one-eighth of an inch in length and has the appearance and shape of a very small pinecone.

As growth continues, stem elongation (jointing) occurs as a result of internode elongation. The embryonic head (growing point) in the main stem and each tiller that has formed at the base of the plant begin to move up the stem. The maximum possible number of kernels per head is determined at this time. The plant allocates nutrients to the main stem and tillers with at least three leaves. Once the plant has jointed, typically no more potential head-bearing tillers will form. However, if the growing point has been killed during stem elongation as a result of damage (physical, freeze, pests) to the immature head and/or supporting stem, that main stem or tiller will die. As a result, the wheat plant will tend to compensate for this loss by development of new shoots from the base of the plant.

During stem elongation, the stem nodes and internodes emerge above the soil surface and become visible. Nodes are areas of active plant cell division from which leaves, tillers and adventitious (crown) roots originate. Leaves originate from the stem nodes above the soil surface and emerge as the stem elongates. As jointing (stem elongation) occurs, the nodes swell, and they look and feel like bumps on the stem. This makes them easier to see or feel and easier to count. An internode is the region between two successive nodes. During stem elongation, the internodes above the soil surface elongate to form the stem. The elongated internode is hollow between the nodes. Wheat stems contain several internodes which can be described as telescopic. Prior to stem elongation, the nodes and internodes are all formed but are sandwiched together at the growing point as alternating layers of cells destined to become the nodes and the internodes of a mature stem. When jointing is initiated, these telescoped internodes begin to elongate, nodes appear one by one, and elongation continues until head emergence. When an internode has elongated to about half its final length, the internode above it begins elongating. This sequence continues until stem elongation is complete, usually at head emergence. Each succeeding stem internode (from the base to the top of the plant) becomes progressively longer. The last elongated internode is the peduncle, which supports the head. It accounts for a good proportion of the overall stem length. Plant height continues to increase during stem elongation until the heads emerge. Plant height is influenced by both genotype (variety) and growing conditions. Generally, variation in height is due more to differences in internode length than internode number.

When stem elongation begins, the first node of the stem is swollen, becomes visible as it appears above the soil surface, and is commonly called jointing (Feekes 6; Zadoks 31). Above this node is the immature head, which is being pushed upward as internodes elongate to eventually emerge (heading stage). Usually a plant has about five to six leaves on the main shoot when jointing begins. The immature head continues to develop and enlarge during stem elongation until it becomes complete at the boot stage. As previously noted, the jointing stage will not occur prior to the onset of cold weather, as vernalization is required in winter wheat to initiate reproductive development. When the growing point moves above the soil surface and is no longer protected by the soil, the head becomes more susceptible to damage (mechanical, freeze, pests).

During stem elongation, the lower four nodes remain in the crown. The fifth node may remain in the crown or be elevated slightly. Nodes six, seven, and possible additional nodes are elevated above the soil. When stem elongation is complete, most wheat varieties usually have three nodes visible above the soil surface, but occasionally a fourth node can be found. The stem elongation stage is complete when the last leaf, commonly called the flag leaf, emerges from the whorl (Feekes 8-9, Zadoks 37-39). On most varieties, the flag leaf begins to emerge just after the third aboveground node is observed (or can be felt). To confirm that the leaf emerging is the flag leaf, split the leaf sheath above the highest node. If the head and no additional leaves are found inside, the emerging leaf is the flag leaf. The flag leaf stage is significant because the flag leaf produces a large proportion (estimates of at least 75%) of the photosynthate (carbohydrates) for filling grain. It must be protected from diseases, insects, and defoliation in order for the plant to develop its full yield potential. Flag leaf emergence is a visual indicator that the plant will soon be in the boot stage.

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Boot

The boot stage (Feekes 10, Zadoks 45) occurs shortly after flag leaf emergence and indicates that the head is about to emerge. The flag leaf sheath (the tubular portion of the leaf that extends below the leaf blade and encloses the stem) and the peduncle (the internode which supports the head) elongate and the developing head is pushed up through the flag leaf sheath. As the developing head beings to swell inside the leaf sheath, the leaf sheath visually obtains a swollen appearance to form a boot. The boot stage is rather short and ends when the awns (or the heads in awnless varieties) are first visible at the flag leaf collar (junction of the leaf blade and leaf sheath) and the leaf sheath is forced open by the head.

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Heading/Flowering (Anthesis)

Photo2.3 Flowering usually begins at the middle of the head and then progresses upward and downward simultaneously.
Flowering usually begins at the middle of the head and then progresses upward and downward simultaneously. (source: William Bruening)

By the time heading occurs, the development of all shoots (main stem and tillers) on the same plant is in synchronization even though there were large differences as to when the initiation of the various shoots occurred (i.e. tiller initiation occurs later than the main stem). However, throughout the pre-heading period, differences also occur in the duration of the various developmental phases among the shoots (i.e. developmental phases for tillers are shortened), which serves to synchronize tiller development with the main stem so that tiller head emergence and flowering occurs soon after the main stem has headed and flowered.

The heading stage begins when the tip of the spike (head) can be seen emerging from the flag leaf sheath (Feekes 10.1; Zadoks 50), and emergence continues until the head is completely emerged (Feekes 10.5; Zadoks 58). The heading date in most wheat varieties is determined by temperature (accumulation of heat units). In some varieties, a combination of heat accumulation and day length determines heading date.

Shortly after the wheat head has fully emerged, flowering (anthesis) occurs. However, flowering and pollination in cereals may occur either before or after head emergence, depending on plant species and variety. Thus, cereals are classified as either open-flowering or closed-flowering types. Flowering occurs in open-flowering types shortly after head emergence. Most varieties of wheat are of the open-flowering type. Generally, flowering in wheat begins within three or four days after head emergence. Open flowering is characterized by extrusion of the anther (reproductive portion of the flower which produces pollen) from each floret on the head. In contrast, closed-flowering types of varieties or cereals (i.e. barley) flower prior to head emergence and the anthers remain inside each floret.

Photo2.4 Many wheat varieties have awns and are called "bearded" wheat, while other varieties are awnless.
Many wheat varieties have awns and are called "bearded" wheat, while other varieties are awnless. (source: William Bruening)

Flowering and pollination of wheat normally begins in the center of the head and progresses to the top and bottom of the head. Pollination is normally very quick, lasting only about three to five days. Pollination occurs slightly later on tillers than on the main stem, but all heads on a plant pollinate within a few days of each other. Wheat is largely self-pollinated, and pollination and fertilization has already occurred before the pollen-bearing anthers are extruded from the florets. Kernels per head are determined by the number of flowers that are pollinated. Pollen formation and pollination are very sensitive to environmental conditions. High temperatures and drought stress during heading and flowering can reduce pollen viability and thus reduce kernel numbers.

Flowering is the transition between two broadly categorized growth stages in wheat. In the first stage, vegetative growth, reproductive initiation, and reproductive development occur and determine the final yield potential of the crop and also provide the photosynthetic factory necessary for maximum yield. The second stage is the grain-filling period in which the potential yield created in the first stage is realized. The extent to which the potential yield is realized will depend on the environment and on management inputs prior to and after anthesis.

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Grain Filling/Ripening

Grain filling follows anthesis and refers to the period during which the kernel matures or ripens. Within a few hours of pollination, the embryo (rudimentary, undeveloped plant in a seed) and endosperm (area of starch and protein storage in the seed) begin to form and photosynthates (products of photosynthesis) are transported to the developing grain from leaves (primarily the flag leaf). In addition, starches, proteins, and other compounds previously produced and stored in leaves, stems, and roots are also transferred to the developing grain. The grain filling period is critical for producing high yields because kernel size and weight are determined during this stage. Yields will be reduced by any stress (high temperatures, low soil moisture, nutrient deficiencies, and diseases) occurring during grain fill. Environmental factors affect the rate and duration of the grain filling period. The longer this filling period lasts, the greater is the probability for higher yields. If this period is shortened, yields will usually be lower. In Kentucky, the average length of the grain filling period is one month. The grain fill period can be as few as 25 days or less in high stress environments (hot and dry weather, heavy disease, and nutrient deficiencies) and may exceed 35 days in high yield, low stress environments (disease-free, high soil moisture, and moderate/cooler temperatures).

The grain development stages are listed in Table 2-1 (Feekes 10.54 to 11.4; Zadoks 70 to 92). A brief description and comments of the grain filling and ripening stages follows below.

It is important for grain quality that the harvest begins as soon as possible. Test weight (and hence grain yield) may be reduced during the ripening process. Decreased test weight results from the alternate wetting (rains or heavy dews) and drying of the grain after the wheat has physiologically matured.

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Wheat Yield Components

Critical yield components include tiller and head number, head size, kernel number per head and kernel size. Table 2-2 highlights the key growth stages that affect yield determination.

Table 2-2 Key growth stages in wheat for yield determination
Critical Yield Component Determined by:
Tiller and head number Jointing (Feekes 6, Zadoks 31)
Head size Mid to late tillering (Feekes 3; Zadoks 23 to 29)
Kernel numer per head Jointing (Feekes 6, Zadoks 31)
Kernel size Begining at flag leaf (Feekes 8, Zadoks 37) and continuing through grain fill

For maximum wheat yields, proper management and favorable weather are necessary during these key growth stages. The final yield of a wheat crop is a function of the yield components in the following formula:

(number of heads / acre)* (number of seeds / head) * (weight / seed) * ( grain yield / acre)

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Section 3. Cultural Practices

Chad Lee, James Herbek, David Van Sanford, and William Bruening

Crop Rotation | Variety Selection | Planting | Winterkill | Stand Counts | Lodging

Wheat grows best on well-drained soils. Since wheat does not tolerate waterlogged conditions well, yields and stands are reduced in fields prone to standing water, flooding, or poor drainage. Wheat can be grown successfully on moderately and somewhat poorly drained soils, but the long-term yields are usually reduced by five to ten bushels per acre due to stress placed on the wheat during wet springs, increased winterkill, higher nitrogen losses, and inability to access fields with application equipment. During springs with normal or below normal rainfall, yields on poorly drained soils approach those on well-drained soils.

Crop Rotation

Photo3.1 Wheat variety trials are conducted across the state to compare relative performances of varieties. Each variety is planted mulitiple times at each location to minimize field variability and to better predict performance potential.
Wheat variety trials are conducted across the state to compare relative performances of varieties. Each variety is planted mulitiple times at each location to minimize field variability and to better predict performance potential.

Most of the wheat in Kentucky harvested for grain is grown in a cropping system of three crops in two years (corn / wheat / double-crop soybeans). Wheat following soybean generally yields more than wheat following corn (Figure 3-1). However, when wheat yields are high, the previous crop has less influence on wheat yield. Wheat is suited to the corn / wheat / double-crop soybean rotation system and offers both economic and agronomic advantages. Yields of all three crops in the rotation are increased over growing any crop without rotation.

Wheat is planted in the fall after summer annual crops are harvested and can be harvested early enough in the summer for a second crop to be planted (double-cropped). Double-cropping is an important economic component of the wheat enterprise in Kentucky. More than 85 percent of the harvested wheat acreage is double-cropped, primarily with soybeans.

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Variety Selection

Fig 3.1 As overall wheat yield potential increases the previous crop has less efect on the yield

Choosing a wheat variety is one of the most important management decisions that Kentucky wheat producers make. Yield potential is clearly important, but the decision is complicated by such factors as the need for disease resistance; the double-cropping system, which requires early maturity; the extreme year to year climatic variation in Kentucky, and the need to spread out the harvest maturity date so every variety is not ready to harvest at once. It makes sense to minimize risks by planting several varieties with good yield and test weight potential that complement one another in terms of disease resistance, maturity, and resistance to spring freeze damage. Proper use of variety test performance data is the first step in making this important decision. The University of Kentucky Small Grain Variety Performance Tests provide the most comprehensive source of information on varieties tested under a broad range of environments. Results of the variety tests are published annually and are available at Cooperative Extension Service offices and online at www.uky.edu/ag/WheatVarietyTest. The best use of University of Kentucky variety performance data for variety selection can be achieved by applying the following basic principles.

Conventional vs. No-till Testing

Based on 10 years of conventional vs. no-till data from the University of Kentucky variety testing program, variety performance can be assessed independently of the tillage system used. This fact enables growers to identify superior varieties based on performance regardless of the tillage system used.

Multi-year/Multi-location Data

While many growers ask about the variety that looked best in this year's test, it is more useful to know which varieties have performed well over a range of conditions. When interpreting the results in the variety performance report, it is important to note that variety yield is relative. This means that comparisons among varieties should only be made among those varieties in the same test or within the same analysis averaged across locations. The state summary table provides performance data averaged across test locations and years. It provides the best estimate of varietal performance, particularly the 2 and 3 year averages. When selecting varieties, growers should first utilize data from the state summary table. Once several candidate varieties have been selected, the grower should examine their performance in the closest regional test. After identifying a group of varieties with high grain yield potential, varietal selection can be based on secondary characteristics such as test weight, disease resistance, lodging, height, maturity and straw yield potential.

Wheat varieties that have performed well under diverse conditions are likely to perform well again. For growers who want to try a new variety, it is best to choose one that has been evaluated for at least one year. If a variety has been tested for one year only, it is best to use the state summary table, rather than using single year data from a single (regional) test. Depending on a grower's location, additional variety performance data may be useful from other (bordering) state variety testing programs. The University of Kentucky Small Grain Variety Testing Program website has links to these programs.

Economic Analysis of Varieties

Photo3.2 While most wheat in Kentucky is grown for grain, some is grown for forages.
While most wheat in Kentucky is grown for grain, some is grown for forages. The University of Kentucky tests wheat varieties for performance both in forage yields and grain yields. (source: William Bruening)

Farmers are always interested in high yields, but the highest yielding variety may not always be the most profitable. One needs to consider other economic factors such as disease susceptibility (may require fungicides), lodging (costs more to harvest), late maturity (delays soybean planting), potential straw yield as a secondary commodity, low test weight (discounts at the elevator) and seed cost. All of these factors require study and evaluation to determine the most profitable varieties for a particular operation. Maximum productivity and profitability begin with careful variety selection. Once varieties have been selected, the best guarantee of obtaining the quality seed necessary for the highest yields is to use certified seed or seed of proven high quality from an established, reputable dealer.

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Planting Practices

The target population for planting wheat is a uniform stand of 25 plants per square foot (225 plants per square yard) (Table 3-1). Usually planting 30 to 35 seeds per square foot (1,524,600 seeds/A) will result in the desired plant population. Planting methods include seedbed preparation or no-tillage planting (see Section 4 Planting Methods), planting date, seed placement, seeding rate, row width, and use of tramlines.

Planting Date: The recommended planting date for most of Kentucky is October 10 through October 30. This window is a compromise between early planting to ensure adequate fall growth and winter survival and later planting to decrease disease and insect infestations. Typically, these dates will fall within a period of one week before to one week after the expected date of the first fall frost. Soil temperatures are usually high enough during this window for the crop to emerge in seven to ten days or less. Also, the length of time between the first frost and winter dormancy for growth is critical for the development of an adequate number of tillers. Tillers developed in the fall are essential to producing high yields. A longer period of growth in the spring and more extensive root systems mean that fall tillers account for most of the grain produced in an intensively managed crop.

Table 3-1. Recommended number of wheat seeds to plant per square foot or per drill-row foot.
Row width (inches)
Length of row needed for 1 sq ft (inches)
Seeds/row foot needed*
30 seeds per sq ft 35 seeds per sq ft
4 36 10 12
6 24 15 18
7 20.6 17 20
7.5 19.2 19 22
8 18 20 23
10 14.4 25 29
*If planting time is delayed, increase seeding rates by two to three seeds/sq ft (one to two seeds/row foot) for every two-week delay beyond the optimum planting date.

Late-planted wheat misses much of the critical fall growing period, generally suffers more winter damage, is more prone to heaving (uplifting of the plant and root system due to alternate freezing and thawing of soil), tillers less, has reduced yields, and matures later than wheat planted at the recommended time. It is difficult, if not impossible, to make up for late planting by management practices employed at later growth stages.

Planting too early, on the other hand, can result in excessive fall growth and create the potential for more winter injury (growth stages too advanced), greater risk of spring freeze injury, fall disease infection, and increased problems with aphids (which vector barley yellow dwarf) and Hessian fly infestations. Delaying planting until October 10 in northern Kentucky and October 15 in southern Kentucky generally averts Hessian fly damage. These dates are known as the fly-free planting dates. The Hessian fly-free date is based partly on the first fall freeze date, so if air temperatures are warmer in the fall, the effective fly-free date would actually be delayed that season.

Seed Placement: Plant seeds 1 to 1.5 inches deep when soil moisture levels are adequate, slightly deeper if moisture is deficient. Do not plant wheat seed more than 2 inches deep. Rapid emergence and good root development start with good seed-soil contact.

Table 3-2. Number of pounds of wheat seed needed, depending on seed size and seeding rate.
Seeds/pound
Seeds/square foot*
30 lbs/acre 35 lbs/acre
10,000 131 152
12,000 109 127
14,000 93 109
16,000 82 95
18,000 73 85
20,000 65 76
*Based on 90 percent or greater germination.

Many wheat varieties have small seed, and when seed is planted deeper than 2 inches, emergence is delayed. Some semi-dwarf varieties with short coleoptiles might open the first leaf below ground and die. Deep seed placement delays emergence and reduces stand, resulting in plants with less vigor, less initial vegetative growth, and reduced tillering.

The other problem is not planting seed deep enough. Planting seed less than 0.5 inch deep can result in uneven germination and emergence because of dry soil. Shallow seed placement also can result in more winter injury and greater susceptibility to heaving. If seed is planted shallow and timely rains accompany planting, then adequate stands can be achieved.

Seed placement is especially critical for no-till planting. Seed must be placed in the soil at the proper depth and below all the plant residue or mulch. The mulch should be distributed evenly on the soil surface to help drills successfully slice through the mulch and place the seed in the soil. Poor seed placement is a major problem in no-tillage planting. Fast, uniform seedling emergence provides quick ground cover and erosion protection.

Photo3.3 Seeding wheat rows at a diagonal to the old corn rows is a good practice in no-till fields.
Seeding wheat rows at a diagonal to the old corn rows is a good practice in no-till fields.

Seeding Rate: Wheat seed size varies dramatically among varieties and can be influenced by production environment and degree of conditioning. Using seeding rates expressed in terms of volume or weight (bushels or pounds) per acre without consideration of seed size can result in stands that are too low or too high. Proper stand establishment requires that the seeding rate be determined in terms of number of seeds per unit area (per square foot or linear row foot). Seeding rates below optimum may reduce yield potential, while excessive seeding rates increase lodging, create a greater potential for disease, and increase seed costs. The optimum planting rate is 35 seeds per square foot (1,524,600 seeds /A) with an objective of obtaining at least 25 plants per square foot. The seed rate and seed size should be determined to calculate how many pounds of seed per acre are needed. Seed sizes and the pounds needed can vary widely (Table 3-2).

For precise seeding, calibrate your planting equipment. Seeding rate charts on drills may not be precise and size and shape of seed can affect seed delivery. (See Section 4 Planting Methods for a five-step procedure for proper grain-drill calibration.)

Photo3.4 Corn residue that piles in the field can prevent the drill from placing the wheat seed in the soil surface. seeds either fail to germinate or seedlings are killed during winter, leaving blank spaces in the field
Corn residue that piles in the field can prevent the drill from placing the wheat seed in the soil surface. seeds either fail to germinate or seedlings are killed during winter, leaving blank spaces in the field

Row Width: The most practical wheat row widths are normally 7 to 8 inches, combining the higher yield potential of narrow rows with the effective movement of planting equipment through the field. Research throughout the growing region of soft red winter wheat has shown 5 percent to 10 percent higher yields when wheat is planted in 4-inch rows versus 8-inch rows. Likewise, research has shown significant yield decreases for wheat grown in row spacings greater than 10 inches. Wheat must be planted at a uniform rate and depth, and conservation requirements must be met.

Drills with units 4 inches apart are likely to clog due to excessive surface residue or clods. Typically, drills with units about 7 to 8 inches apart have minimal clogging, but relatively high yield potential. Some farmers are choosing to use modified planters with units spaced 15 inches apart. These planters are normally used in soybean and corn. The cost of modifying the equipment is less than purchasing a drill, but the yield loss associated with the wider row spacings may not justify 15-inch rows.

Based on limited research in Kentucky, wheat in 15-inch rows will yield about 15 percent to 20 percent less than wheat planted in 7.5-inch rows. For wheat normally yielding 70 bushels per acre, that is a yield loss of 10.5 bushels per acre, or $63 per acre for wheat being sold at $6.00 per bushel. Based on these numbers, not very many acres are needed before a drill becomes more economical than a planter.
Photo3.5 Wheat heads that are bleached white are a clear indication that heads were killed by a freeze event.
Wheat heads that are bleached white are a clear indication that heads were killed by a freeze event. (Source: Cam Kenimer)

Tramlines and/or GPS: Tramlines are roadways placed in the wheat field at planting and used by equipment for applying pesticides and fertilizers. Tramlines should match the width of the applicator tires and be spaced to match the width of the applicator boom. Tramlines allow timely application of input and more uniform applications of nutrients and pesticides with no skips or overlaps.

Tramlines can be formed by blocking drill spouts and not planting wheat seed in specific rows. Tramlines can also be formed by planting wheat in all rows and then running over the same tracks each time an application is made. Tramlines formed by blocking seeding spouts will allow wheat plants in rows beside the tramlines to compensate some for the unplanted area. There is no compensation for plants that have been run over past jointing (Feekes 6, Zadoks 31).

When blocking drill spouts, using tractors with narrow tires so only one drill row needs to be blocked is a recommended practice. Devices that automatically close the selected drill spouts on the appropriate planting pass through the field are available for most grain drills. Fertilizer and spray booms should be at least 40 feet wide to be economical. The distance from the first tramline to the edge of the field should be one-half the width of the sprayer.

When running over wheat to form tramlines, use the same track for each application and do the first track (application) prior to jointing (Feekes 6, Zadoks 31) to allow plants in adjacent rows to compensate for the tramlines. Lightbars enabled with GPS (global positioning system) receivers can be very useful in helping to establish tramlines. Lightbars limit the amount of overlap and skips for nutrient and pesticide applications.

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Winterkill and Freeze Injury

Wheat is subjected to adverse weather conditions during much of its growth period. Autumn frosts and cool temperatures actually help by hardening plants for the months of cold winter weather ahead.

Expect winterkill on poorly drained soils, with extreme temperature fluctuations, where poor fall root development occurred, and with sustained low temperatures (particularly with no snow cover). Extremely cold winters tend to cause more winterkill in varieties developed in more southerly locations because they have less winter hardiness. Heaving is a major cause of late winter or early spring damage to small plants due to extreme temperature fluctuations, especially on poorly drained soils.

Photo3.6 Wheat is re-growing after a freeze event. Normally, development of this re-growth will be delayed, pushing harvest to later in the season.
Wheat is re-growing after a freeze event. Normally, development of this re-growth will be delayed, pushing harvest to later in the season.

Wheat seeded close to the recommended dates typically will receive little damage from a spring freeze. Spring freeze injury can occur when low temperatures coincide with sensitive plant growth stages (Table 3-3). The risk of spring freeze injury is greater when conditions cause wheat to break winter dormancy (greenup) and begin growing and those conditions are followed by freezing temperatures. These scenarios occur with unusually warm temperatures in February or March or from unusually late freeze events in April or May. Injury can occur across large areas of the field but usually is most severe in low areas or depressions in the field where cold air settles. A late spring freeze can reduce yield because of damage to the head and stem. Usually, a week to ten days of good warm temperatures and adequate sunlight are required before head and stem damage from a freeze event becomes visible. If cool, cloudy days persist, then more time may be needed to assess the damage. If the plants are damaged from the freeze, then the wheat stems will likely be damaged close to the ground. Heavy rainfall will knock over the damaged wheat and severely reduce yields.

To check for damage to an unemerged wheat head, cut into the stem to find the growing point (developing head). An undamaged head normally appears light green, glossy, and turgid. A killed head is pale white or tan, limp, shrunken, and not developing in size. Spikelets within a single head can be damaged as well. Growing tissue of plants that have been frozen is dry, bleached, and shrunken. See the Supplement section for more pictures of freeze damage.

Table 3-3. Freeze injury in wheat.*
Growth stage Feekes Zadoks Approximate injurious temp. (2 hrs)
Primary symptoms
Yield effect
Tillering** 1-5 20-29 12°F Leaf chlorosis; burning of leaf tips; silage odor; blue cast to fields Slight to moderate
Jointing (6-7) 31-32 24°F Death of growing point; leaf yellowing or burning; lesions, splitting, or bending of lower stem; odor Moderate to severe
Boot 10 41-49 28°F Floret sterility; spike trapped in boot; damage to lower stem; leaf discoloration; odor Moderate to severe
Heading 10.1-.5 50-58 30°F Floret sterility; white awns or white spikes; damage to lower stem; leaf discoloration Severe
Flowering 10.51-.54 60-71 30°F Floret sterility; white awns or white spikes; damage to lower stem; leaf discoloration Severe
Milk 11.1 75 28°F White awns or white spikes; damage to lower stems; leaf discoloration; shrunken roughened or discolored kernels Moderate to severe
Dough (11.2) 11.2 85 28°F Shriveled discolored kernels; poor germination Slight to moderate
*Information in this table assumes timely rainfall events occurring after the freeze event.
**See Section 2 for more information about growth stages.

The temperatures and growth stages listed in Table 3-3 work well in most situations as a general guideline; however adequate yields may still be produced.

Photo3.7 Using a hand lens or microscope to examine the growing point of wheat can help determine if the crop survived a freeze event.
Using a hand lens or microscope to examine the growing point of wheat can help determine if the crop survived a freeze event. (Source: Tom Miller)

There is some evidence that timing of nitrogen fertilizer application in relation to the freeze event may help reduce the damage from a freeze event. The theory is that for a short period of time, as wheat takes up nitrogen the concentration of nitrogen in the plant cell will be high enough to act as a kind of anti-freeze agent. The problem is that there is no sound recommendation for applying nitrogen to help with this.

In addition, some wheat varieties may be a little more tolerant to spring freezes based on the mechanisms that determine flowering in wheat. Flowering in some wheat varieties seems to be controlled more by day length while flowering in others may be controlled more by temperature. Unusually warm temperatures could accelerate crop development in varieties more responsive to temperature more so than in varieties more responsive to day length. In these cases, varieties more sensitive to temperature would be at a greater risk for spring freeze. Assessing wheat damage from a freeze event can be difficult. In addition to evaluating the stems and heads for freeze damage, one also must look at extended forecasts. If rain is not in the forecast, farmers may be less likely to destroy a damaged wheat crop.

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Determining Plant Populations, Tiller, and Head Counts

Plant Populations:

Photog3.8 A dowel rod with specific lengths marked on it can be used to count plants and tillers on a square-foot-basis.
A dowel rod with specific lengths marked on it can be used to count plants and tillers on a square-foot-basis.

After the wheat has emerged, make a stand count to determine if your target population was achieved and if the final stand is acceptable for maximum yield potential. Make fall stand counts one to two weeks after emergence. Make spring stand counts before greenup of the plants occurs to determine if winter damage has reduced the initial plant population obtained in the fall. Count only whole plants, not tillers. Fields with stand counts below 15 plants per square foot have less than 75 percent yield potential (Table 3-4) and probably should not be kept but used instead for planting corn or soybeans. If stand counts are adequate to keep but somewhat reduced from optimum, consider an early nitrogen application.

To determine the number of plants per square foot:
Table 3-4. Wheat yield potential based on plants per square foot.
Final Stand (%) Plants/sq ft Plants/sq yard Potential yield* (%).
100 30-35 270-315 100
80 24-28 216-252 100
60 18-21 162-189 90-95
50 15-18 135-162 75-80
40 12-14 108-126 60-70
20 6-7 54-63 40-50
*This provides an estimate of the relationship of wheat stand to yield potential and is only a guide. Many factors (plant vigor, weather, disease, fertility management, planting date, and variety) influence how a wheat stand ultimately responds to achieve its final yield potential.

A second method to counting stands is to determine the length of row needed to equal one square foot (Table 3-5). Mark the needed length on a dowel rod or stick and then count the plants in a row.

Table 3-5. Length of row needed for 1 square foot.
Row Width (in) Row Length for 1 sq ft
(ft) (in)
6 2.0 24.0
7 1.7 20.6
7.5 1.6 19.2
8 1.5 18.0
10 1.2 14.4

A third method is to count the plants, or tillers in 1, 2 or 3 feet of row and use Table 3-6 to determine stands.

Table 3-6 Wheat stand count table.
Row width(inches) Row length(ft) Area(sqft) Plants (or tillers) per counted area
10 15 20 25 30 40 60 80 100 120 140 160
Plants (or tillers) per sqft
7 1 0.58 17 26 34 43 51 69 103 137 . . . .
2 1.17 9 13 17 21 26 34 51 69 86 103 120 137
3 1.75 6 9 11 14 17 23 34 46 57 69 80 91
7.5 1 0.63 16 24 32 40 48 64 96 128 . . . .
2 1.25 8 12 16 20 24 32 48 64 80 96 112 128
3 1.88 5 8 11 13 16 21 32 43 53 64 75 85
8 1 0.67 15 23 30 38 45 60 90 120 . . . .
2 1.33 8 11 15 19 23 30 45 60 75 90 105 120
3 2.00 5 8 10 13 15 20 30 40 50 60 70 80
10 1 0.83 12 18 24 30 36 48 72 96 120 . . .
2 1.67 6 9 12 15 18 24 36 48 60 72 84 96
3 2.50 4 6 8 10 12 16 24 32 40 48 56 64
15 1 1.25 8 12 16 20 24 32 48 64 80 96 112 128
2 2.50 4 6 8 10 12 16 24 32 40 48 56 64
3 3.75 3 4 5 7 8 11 16 21 27 32 37 43

Tiller and Head Counts:

Taking a tiller count which includes main shoot and tillers at Feekes 3 (roughly Zadoks 22 through 26) is the first step in all fields for determining nitrogen needs in late winter or early spring. To determine tiller numbers, count all stems with three or more leaves. Tiller counts below 70 per square foot indicate the need for nitrogen at Feekes 3. At recommended populations, many plants will have only three to four stems (main shoot plus two to three tillers, Zadoks 22 or 23). Thus, 70 to 100-plus tillers (stems) per square foot at Feekes 3 are considered adequate.

Head counts can be taken late in the season after heads have fully emerged (Feekes 10.5, Zadoks 58 or later) to help estimate yield potential. An ideal count for maximum yields is 60 to 70 heads per square foot (540 to 630 per square yard) with 35 kernels per head and 16 to 18 spikelets per head. For adequate yields, 55 heads per square foot (500 per square yard) are needed. If the number of heads per square foot is too high (90 to 100), severe lodging can occur and seeding rates were probably too high. Use the same procedure to count tillers or heads as outlined above for plant populations.

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Lodging Control and Plant Growth Regulators

Lodging can be a problem when too much fertilizer nitrogen is used, too thick of a stand is established and/or growing conditions favor excessive growth. Lodged wheat can result in decreased combine speed because of the amount of straw that must be processed through the combine, decreased grain recovery, delayed harvesting after rainfall and heavy dew, and more difficult planting conditions for double-crop soybeans that follow wheat.

Risk of wheat lodging can be reduced by choosing good varieties, establishing the correct stand and using the recommended amount of fertilizer nitrogen. Situations do occur, however, in which there is a large carryover of residual soil nitrogen or weather conditions produce very lush crops and the potential for lodging is high.

When the potential for lodging is high, consider using the growth regulator such as Cerone. Cerone prevents lodging by shortening the wheat plant and strengthening the straw. It does not increase yields if no lodging occurs. Correct application is critical and should be made between Feekes 8 and 10 (Zadoks 37 and 45). Never apply Cerone to crops with exposed heads. Research at the University of Kentucky showed best results when Cerone was applied at Feekes 8 or 9 (Zadoks 37 or 39). Carefully read the label, and follow all directions.

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Section 4. Planting and Drill Calibration

James Herbek and Lloyd Murdock

Drilling | Broadcasting | Aerial Seeding | Grain Drill Calibration

The objective when planting wheat is to establish a uniform stand of at least 25 plants per square foot with adequate fall growth for tiller development and an established root system for winter survival. Planting methods include drilling, broadcast seeding, and aerial seeding. Each has advantages and disadvantages. A planting method should be based on planting equipment, time and labor availability, seeding costs, planting date opportunity, weather, crop usage, yield goals, and stand establishment risks associated with each method. In addition, calibration of planting equipment is critical to getting the correct number of seeds in the soil. Methods for drill calibration are included at the end of this section.
Photo4.1 Proper seeding techniques are critical for an excellent stand of wheat.
Proper seeding techniques are critical for an excellent stand of wheat.

As machinery moves across a field, soil compaction is a concern. Compaction causes the soil to waterlog easily, reduces air movement through the soil, puts the wheat crop under stress, and can reduce the yield. Fields should be tested for compaction by using a penetrometer or similar device when there is ample water in the soil. If soil compaction exists in the field, it should be alleviated before wheat is seeded, when the field is relatively dry. Subsoiling equipment can alleviate deep compaction while a field cultivator can alleviate shallow compaction. These tillage operations should only be conducted when the field is dry. If the field is wet, then these operations could actually worsen compaction. Some types of subsoilers leave most of the residue on the surface and other types cause considerable soil disturbance which would require additional tillage. Once the compaction is remedied and is the field is managed in a complete no-tillage system, the field usually will remain free of compaction.

Drilling

The best results in wheat stand establishment and yield are obtained by seeding with a grain drill. A drill ensures good seed-to-soil contact, promotes rapid germination, results in more uniform and optimum stands, reduces winter injury, and increases yields over broadcast seeding and aerial seeding. (For calibration of a drill, see the end of this section.)

Drills can be used for conventional tillage, reduced tillage, and no-tillage field conditions. Conventional/full tillage provides a level, smooth seedbed for drilling and results in a more uniform planting depth. Drills with additional coulters and more down pressure on the planter units can establish a good stand of wheat in reduced tillage and no-tillage fields. Leaving crop residue on the soil surface protects the soil from erosion until the wheat crop becomes established. About half of the wheat crop in Kentucky is currently planted into no-till conditions with a drill. For fields that still receive tillage, disking is probably the most common method.

Photo4.2 Wheat can be seeded into heavy corn residue with modern no-tillage drills.
Wheat can be seeded into heavy corn residue with modern no-tillage drills. (source: John Grove)

No-tillage conditions provide several advantages over tillage conditions, including reduced soil erosion, reduced equipment requirements, reduced labor costs and reduced fuel costs. No-tillage conditions also allow more timely management, such as spring applications of nitrogen (N) fertilizer. On the other hand, no-till wheat can result in variable planting depths and uneven stands, especially if equipment is not properly adjusted for no-tillage fields. In the early stages of no-tillage adoption by a producer, yields can be a reduced in a high-yield environment. However, management experience seems to eliminate most of these disadvantages. Yield comparisons from many research and on-farm trials over the last 25 years show little or no difference in yield between no-tillage and tillage. The small increase in yields of soybean and corn in a true no-tillage system for wheat, double-cropped soybean and corn is attractive to producers, also.

Residue management varies with the previous crop. Planting into no-tillage conditions after soybeans is ideal but may not be the most economical crop rotation. Planting into corn residue requires proper management of that residue in order to get uniform seed depth and uniform emergence. Combines should have residue choppers and spreaders to distribute the corn residue evenly. In many fields, wheat seeding occurs very soon after corn harvest. Normally, stalk shredding or mowing prior to seeding is not necessary if cornstalks are moist and firmly in the soil. However, if two or three weeks will elapse between stalk shredding and wheat seeding, then shredding the corn residue can improve drill coulter penetration. A rotary mower may have a tendency to windrow the residue. A flail mower is a better tool and distributes the residue more evenly for a more uniform seeding depth. Drilling wheat at an angle to the corn stalk row is also helpful because a drill unit is not continually in a row of corn stalks.

Winterkill is a problem about every four or five years in Kentucky. It can be more pronounced in no-till plantings if the planting depth is 0.5 inch or less. To remove this increased risk, use the proper planting methods and adjustments to plant 1 to 1.5 inches deep. Also, be sure to plant a winter hardy variety.

Drills should be adjusted to target 30 to 35 live seeds per square foot for conventional tillage systems and 35 to 40 live seeds per square foot for no-tillage systems.

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Broadcasting

Wheat seed can be broadcast as either a planned or emergency seeding method. The wheat seed is broadcast on the soil surface with a fertilizer spreader and incorporated into the soil with light tillage (usually disk or field cultivator). Broadcasting is a fast method of seeding wheat and is an acceptable option if corn or soybean harvest is delayed or weather delays push planting dates to the end of or beyond the optimum planting period.

When broadcast seeding into corn stubble, tillage is often conducted before broadcasting. Once broadcasting occurs, then a light tillage operation incorporates the seed into the soil. When broadcasting wheat into a field of soybean stubble, generally a light tillage operation after broadcasting is necessary.

Broadcast seeding often results in uneven seed placement in the soil, which results in uneven emergence and stands. Seeds may be placed as deep as 3 to 4 inches, where many seeds will germinate but will not emerge through the soil surface. Other seeds may be placed very shallow or on the soil surface. These seeds often do not survive due to dry soil or winter damage. The uneven stands from broadcasting often result in lower yields comparing with drilling.

One method of improving stand uniformity is to broadcast seed in two passes across the field, with a half seeding rate for each pass. The second pass is made perpendicular to the first pass. While this method should improve stand uniformity, it also increases time required to seed the field.

Because plant establishment potential is reduced and seed placement is not uniform, seeding rates should be increased for broadcast seeding. Increase broadcast seeding rates by 30 percent to 35 percent over drilled seeding rates. This equates to seeding rates of 45 to 47 seeds per square foot (or approximately 2.5 bushels per acre at average seed size). Soil moisture, crop residue and accuracy of seed incorporation into the soil are crucial to stand establishment.

Broadcasting wheat with fertilizer is a fast way to seed after harvest. Take precautions to ensure that the seed is uniformly blended with the fertilizer and that the fertilizer-seed mixture is uniformly applied. Seed should be mixed with fertilizer as close to the time of application as possible and applied immediately after blending. Allowing the fertilizer-seed mixture to sit after blending (longer than eight hours), particularly with triple super phosphate (0-46-0) or diammonium phosphate (18-46-0), results in seed damage (reduced germination) and, subsequently, a poor stand.

In summary, broadcast seeding is a faster method of seeding and can save time during corn or soybean harvest. The time saved may offset some of the greater costs and potential yield loss associated with broadcast wheat. Disadvantages include inconsistent seed depth and emergence, nonuniform stands, potential for reduced stands, usually lower yields, increased chances of winter injury and higher seed costs.

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Aerial Seeding

Aerial seeding is a risky method for establishing wheat and is not very common. It may be considered as an option when harvest of the summer crop is delayed well into the optimum time for planting wheat. An airplane or helicopter drops a high rate of wheat seed onto the soil surface through the canopy of an established summer crop such as corn or soybean. The wheat seed is not incorporated into the soil, making successful germinate and stand establishment heavily dependent on adequate and timely rainfall. Depending on the weather during stand establishments, yields from aerial seeding can be very high or the crop can be a complete failure.

Aerial seeding normally works best when the summer crop of corn or soybean is turning yellow and leaves are dropping to the ground. This leaf drop can provide a mulch cover and improve the environment for germination. Even in the best conditions, aerial seeding will result in wheat plants with crowns at or above the soil surface, making the wheat crop extremely vulnerable to winterkill.

Historically, aerial seeding was conducting in September prior to the Hessian fly free date. This practice is not recommended, because rainfall is usually low during this period, and there is a greater risk of damage from Hessian fly, aphids, take-all and wheat spindle streak mosaic virus. Aerial seeding is not recommended for late October or November plantings, either. Normally, wheat growth from late aerial seedings will be inadequate for winter survival.

Seeding rates should be 50 to 55 seeds per square foot for aerial seeding, nearly 40 to 50 percent greater than those used for drill seeding. Expected stand establishment will be about 50 to 75 percent of the seeding rate.

In summary, aerial seeding is a high-risk venture and should only be considered for the early window of wheat seeding dates when harvest of the summer crop is delayed. Even in these cases, seeding wheat late with a drill may have better odds of surviving than aerial-seeded wheat.

Table 4-1. Adjusted seeding rate needed based on standard germination and desired live seed-ing rate.
Live Seeding Rate Standard Germination Rate
95% 90% 85% 80% 75%
Adjusted Seeding Rate, seeds/ft
25 26 28 29 31 33
30 32 33 35 38 40
35 37 39 41 44 47

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Grain Drill Calibration

Several methods for calibrating drills are presented below. For any of these methods, ensure that all units are properly delivering seed before conducting any calibration. Look for any loose hoses or chains, gears, etc. that might affect seed delivery.

Photo4.3 Drill calibration takes time, but the final results are worth the effort. (Phil Needham)
Drill calibration takes time, but the final results are worth the effort. (Phil Needham)

For all target recommendations, we are expecting a germination rate of 90 percent. For example, when 30 to 35 seeds per sq ft is recommended, we are expecting 27 to 32 plants to emerge. Seeding rates for no-tillage are slightly higher than conventional tillage, because we anticipate slightly lower emergence rates.

When calibrating a drill, make note of the standard germination of seed as marked on the seed tag. That number can be used with the desired live seeding rate to calculate how many total seeds to drop. For example, if the targeted live seeding rate is 35 live seeds per sq ft and the standard germination is 80 percent, then the total seeds needed are 38 seeds per sq ft. Table 4-1 can help with calculations of standard germination and adjusted seeding rate.

Once desired seeding rate has been determined, based on field conditions and standard germination of the seed, then the following methods can be used.

Method 1: (Most accurate) A five-step process for proper grain-drill calibration follows:

Option: The above procedure also can be used under actual field conditions by catching seed while the drill is traveling a distance of 50 feet. Use Table 4-4 to determine how much seed should be collected from each row unit.

Method 2: (less accurate) Put the wheat seed in the hopper of the drill to cover two or three drill spouts. Keep the seed tag for reference.

Table 4-4. Weight of seed needed for one row unit and 50 feet of row, depending on seed size, target seeding rate and spacing between row units (assuming 90% seed germination).
Seed Size (Seeds/lb) Row Width (in)
7 7.5 8 7 7.5 8
Seed collected from one unit in 50 ft of row
Ounces grams
30 seeds/sqft (target seeding rate)
10000 1.55 1.67 1.78 44.1 47.2 50.3
12000 1.3 1.39 1.48 36.7 39.4 42
14000 1.11 1.19 1.27 31.5 33.7 36
16000 0.97 1.04 1.11 27.5 29.5 31.5
18000 0.86 0.93 0.99 24.5 26.2 28
20000 0.78 0.83 0.89 22 23.6 25.2
35 seeds/sqft (target seeding rate)
10000 1.81 1.94 2.07 51.4 55.1 58.7
12000 1.51 1.62 1.73 42.8 45.9 49
14000 1.3 1.39 1.48 36.7 39.4 42
16000 1.13 1.22 1.3 32.1 34.5 36.7
18000 1.01 1.08 1.15 28.6 30.6 32.6
20000 0.91 0.97 1.04 25.7 27.6 29.4
40 seeds/sqft (target seeding rate)
10000 2.07 2.22 2.37 58.8 63 67.1
12000 1.73 1.85 1.97 49 52.5 55.9
14000 1.48 1.59 1.69 42 45 48
16000 1.3 1.39 1.48 36.7 39.4 42
18000 1.15 1.23 1.32 32.6 35 37.3
20000 1.04 1.11 1.18 29.4 31.5 33.6
Calculation to determine seeds needed:
Ounces of seed needed =[ seeds/sqft*(50 ft*row width in ft) seeds per pound)*16 ounces per pound]/0.9
Where seeding rate is seeds/sqft, Row width is in feet, and 0.9 is 90% germination.

Method 3: (least accurate) Calculate out how many pounds of seed should be planted for each acre. For example, a target of 35 seeds per square foot is 1,524,600 seeds per acre. If the seed size is 10,000 seeds per pound, the total pounds per acre needed is 152 pounds per acre (Table 4-3).

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Section 5. Fertilizer Management

Lloyd Murdock, John Grove, and Greg Schwab

Nitrogen | Phosphorus and Potassium | Other Nutrients | Burning of Wheat Straw

The most important first step in your fertilizer management program is to take a soil sample. Except for nitrogen (N), your fertilizer and lime decisions will be based on the soil test results. It is advantageous to take the sample as soon as possible after harvest of the previous crop to supply the necessary phosphorus (P) and potassium (K) for the seedling wheat plant. However, in drought years, soil testing at this time can result in soil pH and K test values that are artificially low due to extremely dry soil conditions during August and September. Extension publication AGR-189 gives recommendations on taking soil samples under such conditions. Refer to Extension publication AGR-1, Lime and Fertilizer Recommendations, for specific recommendations based on soil tests.

Nitrogen

Nitrogen is the nutrient requiring the most management. Proper N rate and timing are important for high tiller numbers and yield (Figure 5-1). Nitrogen deficiency symptoms consist of pale green (chlorotic) plants that are poorly tillered (Photo 5-2 and Photo 5-3). Excessive N can cause lodging, increased disease incidence and severity, and lower yield. Additionally, excessive N may result in increased levels of N in ground and surface waters, with negative environmental (and economic) consequences.

Rates and Timing.
Photo5.1 The wheat is at about Feekes 2 (Zadoks 21). Stand counts at this stage can help determine how much N to apply for the first application.
The wheat is at about Feekes 2 (Zadoks 21). Stand counts at this stage can help determine how much N to apply for the first application.

Wheat requires a small, but important, amount of N in the fall. This requirement can almost always be met by soil N remaining after the preceding corn or soybean crop. Producers needing additional P may select P fertilizer sources containing N (for example, diammonium phosphate, DAP, 18-46-0). In the unusual event where and when corn yields greatly exceed (by at least 30 bu/acre) the expectations built in to the corn crop's nutrient management plan, residual N for the succeeding wheat crop will likely be low. If the corn yield exceeds expectations, then 20 to 40 lb N/acre should be added at or near planting. Fall N fertilization becomes more important with late planting (after the first week of November) in a wet fall season and poor initial emergence (less than 25 plants per square foot). Sufficient fall N stimulates early tillering, which is important for high yields. The fall N rate should not exceed 40 lb N/acre.

Nitrogen applied in late winter-early spring is most effective for yield. There are two approaches that can be used for spring N applications: a single application or a split spring application. Research indicates that a split spring application of N can increase yield by 3 bu/acre (although this varies from year to year), and split N applications reduce lodging potential. Split spring N applications are recommended when possible, but equipment and logistic problems cause some growers to make a single application.

Fig5.1 Nitrogen uptake during the growth of winter wheat
Nitrogen uptake during the growth of winter wheat

When using the split N strategy, the first application should be made in late winter (mid-February to early March, Feekes 2-3, Zadoks 20-29) at a rate between 30 and 50 lb N/acre. Nitrogen applied at this time encourages further tillering and maintains current tillers. Fields with thin stands or little fall tillering should receive higher early N rates, while those with high tiller counts (above 70 tillers per square foot) should receive lower early N rates (Figure 5-2). Excessive N applied in late winter can increase the potential for lodging, disease, and late spring freeze damage. The second N application should be made to no-till fields in mid to late March (Feekes 5.6, Zadoks 30-31) at a rate sufficient to bring the total amount of spring N to 80 to 110 lb N/acre. In no-till fields with yield potential greater than 70 bu/acre, spring N should total 100 to 120 lb N/acre. If there is some freeze damage or excessively high rainfall in February and March, then the higher N rates should be targeted. For tilled plantings the total should be decreased by 20 lb N/acre because N fertilizers are more efficient with tillage and N loss potential is slightly lower. Higher rates of N application than those recommended here increase lodging potential and do not increase yield potential, unless specific conditions that require more N nutrition are identified.
Fig5.2 Tillering influences on relative yield of wheat
Tillering influences on relative yield of wheat

When making a single N fertilizer application, the best time is when the crop growth stage is Feekes 4-5 (Zadoks 30, usually mid-March), just before the first joint appears on the main stem and, when wheat starts growing rapidly. Rapid growth causes a large demand for N. The rate of N fertilizer for a single application should be between 60 and 90 lb N/acre for fields with a yield potential less than 70 bu/acre and 90 to 100 lb N/acre for fields with greater yield potential. An early (late February) single N application is recommended only when the field's stand or tiller density is low. Earlier N applications are at increased risk of denitrification loss (N loss during extended wet periods). Early single applications increase the risk of spring freeze damage because they encourage earlier heading. Single applications made too early generally result in lower yields and encourage the growth of succulent plants with lush canopies susceptible to diseases like powdery mildew.

Single applications made too late are equally problematic. Nitrogen must be applied in a timely manner to maximize yield potential. Delaying N application after Feekes 6 (Zadoks 31, appearance of the first joint on the main stem) to an N-deficient crop will result in decreased yield potential most years. As plant development advances, yield response to added N progressively declines. After Feekes 9 (Zadoks 39, flag leaf fully developed), there is usually little yield return to added N. However, N applied after Feekes 9 will increase the grain protein concentration.

Fertilizer N Sources.
Photo5.2 Streaks in this field were caused from anyhdrous ammonia applications made before corn where some knife slits were closed and others were not. Where the knife slits were closed, more N was in the soil (for the corn and then for the wheat), resulting in greener wheat.
Streaks in this field were caused from anyhdrous ammonia applications made before corn where some knife slits were closed and others were not. Where the knife slits were closed, more N was in the soil (for the corn and then for the wheat), resulting in greener wheat.

Fertilizer N sources for wheat include ammonium nitrate (33-34% N), urea (45-46% N), and urea-ammonium nitrate solutions (28-32% N). All are equally good sources of wheat N nutrition, when properly managed, in all tillage systems and regardless of previous crop residue. Slow release N is now available for use on wheat. It is a polyurethane (plastic polymer) coated urea prill. The trade name is ESN® . This product should be used at the same rate of N as recommended for other N sources. Since ESN releases N slowly, there is no advantage to split N applications with this source. Research results show that ESN applications between January 15 and February 15 produce wheat yields equal to those observed with uncoated urea applied at Feekes 5 (Zadoks 30). Applying ESN after March 1 increases the risk that too little N will be made available for plant uptake during the critical early growth period.
Photo5.3 Streaks in a field are common at field entrances where sprayers overlaps and skips in N application are more likely to occur.
Streaks in a field are common at field entrances where sprayers overlaps and skips in N application are more likely to occur.

Distribution of Fertilizer N and Leaf Burn. Since the difference between enough and too much N is small, distribution in the field is important. The best distribution will be achieved using liquid N sources or, for solid N sources, an airflow delivery truck. Spinner systems delivering solid materials are less accurate. Distribution of a solid N material that contains a lot of fine material can be improved by double spreading (reducing the distance between passes by half and spreading half the desired rate on each pass). If evenly distributed, N from liquid and solid sources perform equally well in February and March.

Leaf burn can be a concern with liquid N sources, but you can eliminate this concern by using streamjet or flood nozzle application, mixing the liquid N with additional water, applying less than 60 lb N/acre per application, and avoiding applications on cold, windy days. Although wheat fertilizer burn is visually disturbing, research indicates no yield reduction occurs when N is applied late winter-early spring (February and March). Leaf burn after flag leaf emergence (Feekes 9) can cause yield reductions.

Methods to Fine-Tune Wheat Fertilizer N Application Rates.

When the amount of fertilizer N to apply in March is in question, a plant sample collected at Feekes 5 (Zadoks 30) might be helpful. Cut a handful of wheat about 0.5 inch above the ground at 20 to 30 places in the field, and place a subsample of the total plant material collected in a paper bag. Send the sample to a laboratory with a quick turnaround time so fertilizer N application will not be delayed. Table 5-1 shows guidelines for fertilizer N rates recommended at various tissue N concentrations.

Photo5.4 Wheat varieties at Feekes 5 (Zadoks 30) ready for a second application of fertilizer N.
Photo5.5 Variations in green color within a field can be due to application methods as well as weather and soil conditions.
Variations in green color within a field can be do to application methods as well as weather and soil conditions. (source: John Grove)

A chlorophyll meter is a hand-held, non-destructive, field diagnostic tool that actually measures plant leaf greenness. Chlorophyll measurements can provide additional information to help predict the amount of N fertilizer that needs to be added at Feekes 5 or 6 (Zadoks 30 or 31, usually in March). To help calibrate the chlorophyll meter, large amounts of fertilizer N (150 pounds per acre) are added to two or three small areas or strips in the field, in early to mid February. At Feekes 5, chlorophyll readings are taken on 10 to 20 plants in the high N areas and then on 20 to 30 plants in the rest of the field. The measurements are made on the first fully expanded leaf (leaf with a leaf collar) from the top of the plant. Measurements are made about halfway between the tip and base of the leaf. The following formula is used to make the March fertilizer N rate recommendation:

N = 6 + (7 x D)

N = N (lb N/acre) needed for optimum growth at Feekes 5 (March).

D = difference between chlorophyll reading in the bulk of the field and that found in the small areas/strips with high N rates added in February.

Example: Small areas or strips with high N (150 lbs/ac) added at Feekes 3 (Zadoks 26) read an average of 52 at Feekes 5 (Zadoks 30).

Bulk of field reads an average of 45.

So, D = 52 - 45 = 7.

N = 6 + (7 x 7) = 55 lb N/acre recommended fertilizer N rate.

A soil nitrate (NO3) test usually is not helpful unless there are unusual conditions that might cause high N levels in the soil, such as high N carryover due to a very poor corn crop or heavy manure applications. Take soil samples to a depth of 3 feet in February, and place them on brown paper for drying. The NO3-N measured in the samples will be reported in parts per million (ppm), which should be multiplied by 12 to get pounds per acre. If 120 pounds per acre of NO3-N are found, no fertilizer N needs to be added. If the soil test indicates less than 120 pounds per acre of NO3-N, then fertilizer N should be applied.

Table 5-1. Guidelines for fertilizer N ap-plication using wheat tissue N concen-trations at Feekes 5.
Plant N Concentration (%) Recommended fertilizer N rate (lb N/acre)
2.3 100
2.7 80
3.2 60
3.6 40
4.0 20
Murdock (unpublished data)

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Phosphorus and Potassium

Phosphorus is essential for root development, tillering, early heading, grain fill, timely maturity, and winterkill resistance. Wheat takes up about 0.67 pounds of P2O5 for each bushel produced, and 80 percent of this ends up in the grain. A soil test is necessary in order to determine the proper rate of P fertilizer. Apply P fertilizer in the fall, prior to seeding, for best results. See Table 5-2 for the P2O5 concentrations of wheat in grain and straw.

Potassium helps to lower the incidence of some diseases and increases straw strength, which helps reduce lodging. Wheat takes up about 2 pounds of K2O for each bushel produced, but only about 20 percent of this is removed with the grain. A soil test is required in order to determine the proper rate of K fertilizer. Potassium fertilizer should be applied in the fall, but can be applied in the spring if necessary. See Table 5-2 for the K2O concentrations of wheat in grain and straw.

Table 5-2. Nutrients in Wheat Grain and Straw.
Crop Part Yield Unit (%) Nutrient Concen-tration (lb).
N P2O5 (%) K2O
Grain Bu 1.2 0.5 0.3
Straw Ton 12 4 20

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Other Nutrients

Calcium (Ca), magnesium (Mg), and sulfur (S) deficiencies have not been observed on wheat in Kentucky. Calcium and Mg will generally be adequate if the proper soil pH is maintained using agricultural lime. Additional Mg should be added only if soil test Mg is below 60 pounds per acre. Sulfur deficiencies are best determined by analysis of plant tissue sampled at Feekes 5 (Zadoks 30). If the N:S ratio is greater than 15:1, S should be added at a rate of 20 to 40 lb S/acre. Only water-soluble sources that contain sulfate-S (-SO4), such as spray grade ammonium sulfate, should be used at this stage of growth.

Photo5.6 Scientists are constantly working with equipment to improve N application, N timing and N use by the wheat crop.
Scientists are constantly working with equipment to improve N application, N timing and N use by the wheat crop (source: Greg Schwab).
Photo5.7 Most nitrogen fertilizer is applied as a liquid urea ammonium nitrate (UAN) and 28 or 32% N. By using stream bars or stream jet style nozzles, leaf burn from the UAN is minimized.
Most nitrogen fertilizer is applied as a liquid urea ammonium nitrate (UAN) and 28 or 32% N. By using stream bars or stream jet style nozzles, leaf burn from the UAN is minimized.

Micronutrient deficiencies have not been found on wheat grown in Kentucky. The best way to determine micronutrient needs is through plant tissue analysis. See AGR-92 (Sampling Plant Tissue for Nutrient Analysis) for additional sampling instructions and interpretation. Micronutrient deficiencies generally occur when the soil pH is too high or too low. A soil pH between 6.0 and 7.0, with a target pH of 6.4, should provide excellent conditions for micronutrient availability and wheat growth. Lime should be applied prior to planting, in the fall.

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Burning of Wheat Straw

Burning wheat straw in the field will cause loss of some of the nutrients with the vapor and smoke. Research indicates that losses of carbon (C), N, P and K are as follows:

Photo5.8 Nitrogen test strips in a field with tram lines.
Nitrogen test strips in a field with tram lines.

C - 90 to 100%

N - 90 to 100%

P - 20 to 40%

K - 20 to 40%

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Section 6. Weed Management

James R. Martin and J.D. Green

Why Control Weeds in Small Grains? | Weed Scouting | Weed Identification | Weed Control An Ongoing Process

Photo6.1 Italian or ryegrass (lolium rigidum) is a problematic weed in wheat and must be aggresively managed

The crop rotation system typically used in Kentucky can contribute to the control of certain weed species. Practices used in the establishment of no-till corn often break the life cycle of cool-season weeds such as common chickweed, purple deadnettle, or henbit before plants mature and produce seeds. A competitive wheat stand can help weed control in double-cropped soybeans by preventing or delaying emergence of warm-season weeds including crabgrass, cocklebur, and morningglories.

One drawback with this rotation system is that it may perpetuate certain problems. For example, Italian ryegrass often begins in wheat where its seed are easily spread during wheat harvest with combines. Ryegrass seedlings that develop from the scattered seeds during the fall after wheat harvest are able to overwinter and compete the following spring during the establishment of no-till corn. Heavy ryegrass infestations limit no-till corn stands by direct competition as well as harbor voles that feed on corn seed. Studies have shown that if ryegrass is not completely controlled in corn, escaped plants will produce seed and perpetuate the problem in wheat after corn harvest.

Another unique feature about growing wheat in a rotation with corn and double-crop soybeans is associated with the risk of crop injury caused by carryover of herbicide residues. Growers must use caution in selecting herbicides that do not persist in soil for long periods and cause injury to rotational crops.

The spectrum of weeds in conventional and no-tillage plantings of wheat is similar; however, there are some species that tend to be more troublesome where no-tillage practices are used. Wild garlic populations tend to be greater in no-tillage programs compared with programs that use plowing and disking for seedbed preparation. The infestation level of common chickweed, purple deadnettle, and henbit tend to be greater in no-till plantings than in conventional till plantings.

Why Control Weeds in Small Grains?

Photo6.2 Common Chickweed, Stellaria media.

The ability of weeds to compete and limit wheat yield will vary depending on the weed species. Italian ryegrass is the most competitive weed in wheat in Kentucky. One ryegrass plant per square foot can reduce wheat yield by approximately 4 percent. As much as 90 bu/A of yield loss of wheat has been measured in research trials on ryegrass. Common chickweed has a prostrate growth habit that forms dense mats and tends to be more competitive than purple deadnettle or henbit. In no-till plantings infestations of common chickweed can reduce potential wheat yield by 14 percent. However, the impact of these weeds is less where preplant tillage is used for preparing the seedbed.

Weeds can also affect the quality of harvested grain and harvesting efficiency. The aerial bulblets of wild garlic contaminate the grain during the harvesting process. Dockage due to bulblet contamination can vary due to a number of factors determined at the grain elevator. In some cases aerial bulblet contamination may be severe enough to render the grain unfit for sale at the elevator. Giant ragweed, common ragweed, johnsongrass, and marestail are examples of warm-season weeds that produce sufficient amounts of green vegetation in the spring that can reduce harvesting efficiency. The green vegetation may also lead to dockage due to increased moisture and foreign matter. Once wheat has been harvested, the clipped stubble of these weeds may survive and be difficult to control with burndown applications in double-cropped soybeans.

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Weed Scouting

Periodically monitoring fields helps detect problems before weedy plants become too large to control effectively. Critical periods for monitoring weeds are:

Photo6.3 Shepherd's-purse, Caosella bursapastoris
Photo6.4 Field Pennycress, Thlaspi arvense
Photo6.5 Marestail-also referred to as horseweed, Conyza candensis
Photo6.6 Purple deadnettle (lamium purpureum),left and Henbit (lamium amplexicaule), right.

Scouting Procedures for Weeds in Wheat

Pertinent weed information can be recorded on paper or digitally on hand-held recording devices such as a PDA (Personal Data Accessory) or a PC tablet. An advantage for using the computer devices is that they can be equipped with GPS technology or connected to a separate GPS unit to help develop field maps and facilitate keeping permanent records of problem weeds for each field.

Economic Thresholds

Economic thresholds for weeds in wheat are not well defined; consequently, growers need to rely on their personal experience to determine if a herbicide treatment is warranted. General treatment guidelines are in Table 6-2 and vary depending on several factors including weed species, cost of treatment, and price of wheat.

Table 6-2. Treatment guidelines for wheat.
  Infestation level* Treatment Guideline**
Weed cover Wild garlic counts/600 sqft
Light < 5% 1 Probably no economic benefit to treat
Moderate 5 to 30% 2 to 5 plants Treatment may or may not be justified
Severe > 30% > 5 plants Treatment may be justified if implemented in a timely manner.
*The infestation level is the total weed cover (in the fall) or wild garlic counts (in the spring) averaged across survey sites. In some instances the average infestation level may suggest no need for treating, yet a few sites may be heavily infested and warrant control. It may be feasible to spot-treat portions of a field where severe infestations occur based on a weed map.
**Light infestations of problem weeds such as Italian ryegrass may still warrant treatment in order to limit spread of weed seed.

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Weed Identification

Correctly identifying weeds during their early stages of development is important to help select and initiate successful control strategies. Many weed species look similar during early stages of development. Vegetative characteristics such as shape, color, arrangement of leaves, and location of pubescence (hairs) can aid in identification; providing these characteristics remain consistent under a wide variety of conditions. However, it is not unusual for these vegetative characteristics to vary for some weed species, so they are not always reliable for identification. See the illustrations in this section for descriptions and visual aids to be used in identifying weed species.

Photo6.8 Corn Speedwell, Veronica arvensis.
Photo6.9 Wild Garlic, Allium vineale.

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Weed Control An Ongoing Process

The timeline for emergence of various weed species in wheat (Figure 6-4) illustrates why weed management can be an ongoing process beginning prior to planting up through wheat maturity.

An effective overall weed management program for Kentucky wheat involves a combination of cultural and chemical practices.

Fig6.3 Drawings of grass species that occur as weeds in wheat in Kentucky.
Fig6.4 Approximate Time of Significant Emergence of Weedsin Wheat in Kentucky.

Cultural Practices

Establishing and maintaining a competitive wheat stand contributes to weed control. A seeding rate that results in a minimum of 25 wheat seedlings per square foot is ideal for achieving optimum wheat yields and often limits the amount of weedy vegetation. Planting wheat in narrow rows increases the likelihood for achieving early-season shading and competition to weeds compared with wheat planted in wide rows. Applying nitrogen at recommended rates and times can promote tillering of wheat and limit the presence of warm-season weeds that affect harvest.

Crop rotation often reduces weed populations. For example, infestation levels of wild garlic, common chickweed, and henbit tend to be lower following corn than soybeans. A rotation of corn / wheat / double-crop soybeans is common in Kentucky and is often more favorable for managing weeds in wheat than a soybean / wheat / soybean rotation.

Preplant tillage was once the only option for managing such weeds as wild garlic and certain cool-season weedy grasses in wheat. There are a number of drawbacks with tillage including added fuel, time, and erosion. Unless wheat is organically grown, herbicides have often replaced the need for using tillage for weed control.

Growing wheat in rotation with corn and soybeans can be beneficial in controlling broadleaf weeds, such as common chickweed or henbit. The timely use of burndown herbicides or preplant tillage in corn or soybeans limits production of weed seed by destroying cool-season weeds before they mature.

Sanitation is an effective preventative option for limiting the spread of Italian ryegrass. Clipping infested field borders and waterways ahead of wheat harvest can limit the spread of ryegrass seed; however, it is critical to clean equipment after mowing infested areas. Harvest infested fields last. Cleaning combines after harvesting infested areas is especially important. In instances where a portion of wheat seed is being saved for next season's crop, care should be taken to avoid using crop seed harvested from ryegrass infested areas. Also, cleaning the harvested wheat seed is important in limiting the spread of seed in future crops.

Chemical Control

Photo6.10 Hairy Chess (Bromus commutatus), left, and Italian Ryegrass/Annual Ryegrass (Lolium multiflorum)

Herbicides play a major role in managing weeds in wheat. Herbicide recommendations for wheat production are discussed in the Cooperative Extension bulletin Chemical Control of Weeds in Kentucky Farm Crops (AGR-6). Always read and follow the restrictions and precautions stated on the label of herbicide products.

Examples of issues that need to be considered when using herbicides for weed control in wheat are: 1) application timing, 2) compatibility with other chemicals, 3) varietal sensitivity, 4) herbicide-resistant weeds, 5) herbicide carryover, 6) harvesting restrictions for grain or forage, and 7) cleaning spray equipment.

Application timing. The four periods of time when wheat herbicides are applied are: 1) before wheat emergence, 2) postemergence in the fall, 3) postemergence in late winter or early spring, and 4) preharvest. Important issues associated with each timing and some of the factors that determine when treatments should be applied are discussed below.

Before wheat emergence. Fields planted to no-till wheat often require a foliar-applied burndown herbicide such as glyphosate or paraquat. These herbicides will control grasses and broadleaf weeds and can be applied before or after wheat planting but before wheat emerges. Paraquat is a contact herbicide that is labeled to control annual weeds up to six inches in height. It is usually applied in 20 to 40 gallons of clean water or clear liquid fertilizers per acre. Glyphosate is a translocated herbicide used to control annual and perennial weeds. It is often applied in 10 to 20 gallons of water per acre

The use of soil-residual herbicides ahead of wheat emergence is not widely adopted in Kentucky, yet there are occasions where they can help prolong early-season control of some weeds. For example the premix of chlorsulfuron plus metsulfuron (Finesse) can be applied prior to wheat to help suppress emergence of Italian ryegrass. When applied at the high labeled rate, diclofop (Hoelon) offers both premergence and postemergence ryegrass control prior to wheat emergence. Diclofop can also be applied after wheat emergence because of its foliar activity to grassy weeds and safety to wheat. A strategy that growers use to limit expenses with diclofop is to apply preemergence treatments around field borders or areas of heavy infestations where Italian ryegrass problems often begin.

Fall postemergence applications. The likelihood of achieving optimum wheat yield tends to be greater when cool-season broadleaf weeds such as common chickweed, henbit, or purple deadnettle are controlled in the fall rather than in the spring. This is particularly true for no-till plantings. The level of control of these broadleaf weeds is essentially the same regardless of whether treatments are applied in the fall or spring; however, fall tends to be a more favorable timing for optimum control of such weeds as cornflower, annual bluegrass, Italian ryegrass, and certain Brome species.

Fall postemergence sprays can be made soon after wheat emergence and continue though late fall, providing weather conditions are favorable for plant growth. Most postemergence herbicides used in wheat rely on foliar absorption to control weeds; consequently, plants should be actively growing in order to achieve optimum weed control and crop safety. Dry conditions can delay weed emergence, particularly Italian ryegrass. Cold and dry conditions may also delay herbicide activity and in some cases limit weed control. Heavy rainfall, prolonged cold temperatures, or widely fluctuating day / night temperatures before, during, and shortly after application may lead to crop injury, particularly with Acetolactate Synthase (ALS) inhibitor herbicides such as thifensulfuron (Harmony) and mesosulfuron (Osprey).

Photo6.11 2,4-D or Banvel(dicamba)injury.
Photo6.12 Atrazine or Princep(simazine).
Photo6.13 Command(clomazone) carryover.

There are a few soil-residual herbicides that can be applied after wheat emergence. It is unlikely these will provide season-long weed control, yet they can be helpful if conditions are conducive for activity. Extremely dry conditions, or a seedbed that is cloddy or has a lot of surface residue from the previous crop, may limit control from certain soil-residual herbicides. The premix of flufenacet + metribuzin (Axiom) is an example of a soil-residual herbicide that also offers limited foliar activity for seedling weeds present at the time of application. However, pendimethalin (Prowl H2O) is an example of a soil-residual herbicide that has no foliar activity and may need a foliar-applied herbicide as a tank-mix partner for managing weeds that are emerged at the time of application.

Late winter early spring postemergence applications. Wild garlic emerges during the fall and early spring months. Achieving optimum control of this weed is important; therefore, growers tend to delay herbicide applications until late winter or early spring to ensure that most of the population of wild garlic plants has emerged. It is not unusual for growers to apply postemergence herbicides during this time for managing cool-season broadleaf weeds and grasses, especially if conditions in the fall were not favorable for weed emergence and growth.

Several postemergence herbicides can be applied when wheat is coming out of dormancy and in Feekes growth stage 5 (Zadoks 30). This timing usually occurs in March and will vary depending on environmental conditions. Some postemergence herbicides may also be applied up to boot stage, yet growers seldom wait this late to make applications.

Crop injury from 2,4-D is associated with such factors as rate, formulation, and wheat growth stage. Injury may be a risk with the high labeled rate, particularly with ester formulations. The risk of injury from 2,4-D is least when crop plants are fully tillered (Feekes 5, Zadoks 30) but before jointing (Feekes 6, Zadoks 31). Although some 2,4-D labels do not prohibit applications after initiation of the first joint, they do prohibit applying to plants that are in the boot (Feekes 8, Zadoks 37) to dough stage (Feekes 11.2, Zadoks 85). Research has shown that applications of 2,4-D in the fall before wheat is fully tillered can injure wheat and reduce yield by as much as one-third.

Dicamba (Banvel or Clarity) is a growth regulator herbicide that is similar to 2,4-D. While dicamba may be applied in the fall or early spring; it is important that treatments be made prior to jointing (i.e. Feekes 6, Zadoks 31) in order to avoid crop injury. See Photo 6-11 for injury symptoms when 2,4-D or dicamba is applied during the boot stage.

Preharvest treatments. Preharvest treatments are not a part of a planned weed control program but are often used as salvage treatments to help prevent such weeds as Pennsylvania smartweed, ragweeds (common and giant), and johnsongrass from impeding wheat harvest and competing for soil moisture in double-crop soybeans. However, research has shown preharvest treatments are not effective in preventing production of viable seed of such weeds as Italian ryegrass.

Glyphosate and certain formulations of 2,4-D are examples of herbicides registered for preharvest weed control in wheat. The response of weeds to these herbicides is slow and does not occur as rapidly as with certain harvest-aid applications used in other crops. Drift to nearby sensitive crops is a concern when using these treatments. Preharvest treatments can injure wheat or reduce seed germination or seedling vigor and are not recommended for wheat grown for seed production.

Herbicide compatibility with other chemicals. Herbicides can interact with other chemicals when tank mixed with one another or applied near the same time. These interactions can occur between herbicides or other pesticides (especially organophosphate insecticides) as well as fertilizers or additives. Consult the label(s) for potential problems with physical compatibility of the mixtures as well as the potential for crop injury or poor weed control. Also, be certain the application timing is within the recommended period for all chemicals involved.

Photo6.14 Wheat with no injury symptoms(left). Wheat injured by Opsrey herbicide(mesosulfuron-methyl)(right). Leaf burn is more likely to occur when fertilizer N is applied within 14 days of the Opsrey application.

The following are examples of problems associated with compatibility issues:

Osprey and nitrogen fertilizer. Liquid nitrogen fertilizer is often used at low rates as a spray adjuvant with foliar-applied herbicides. However, applying herbicides near the time of topdressing nitrogen fertilizer can lead to crop injury from certain ALS inhibitor herbicides. For example the label for Osprey indicates topdress applications of liquid nitrogen fertilizer may occasionally cause transient leaf burn and stunting when applied within 14 days of an Osprey application (see Photo 6-14). Research has shown that applying Osprey and topdressing nitrogen fertilizer within a few hours of one another on the same day can limit wheat grain yield by 12.6 bu/A. It is important to consult the herbicide label for any precautions regarding timing for topdressing nitrogen fertilizer.

Harmony Extra + liquid fertilizer + nonionic surfactant. Stunting and yellowing of wheat can occur when liquid nitrogen fertilizer is used as the carrier in place of water. Injury associated with this mixture sometimes can be reduced by using the lowest recommended rate of nonionic surfactant and applying the mixture during favorable weather conditions.

Harmony Extra + diclofop (Hoelon). This mixture can reduce ryegrass control with Hoelon. Applying these products separately, approximately seven days apart helps prevent antagonism associated with this mixture.

Harmony Extra + 2,4-D. Theses two herbicides are frequently applied together as a tank mix combination, yet the application timing of 2,4-D is not always compatible with Harmony Extra. This mixture should be applied in the spring after wheat has fully tillered and before jointing. Fall sprays of this mixture can limit tillering and cause other growth regulator symptoms to appear during later stages of wheat development.

Varietal Sensitivity. Wheat varieties may vary in their susceptibility to certain herbicides. Metribuzin is an example of a wheat herbicide that can vary in its ability to cause crop injury based on variety. The labels of products containing metribuzin list wheat varieties sensitive to metribuzin. Testing of varietal response to herbicides is not an ongoing process, which limits the ability to know sensitivity of newly released varieties. When information on varietal sensitivity is not known, treat only a small area until sensitivity is established before treating large acreages.

Herbicide-resistant weeds. Herbicide resistance is the ability of certain biotypes within a weed species to survive a herbicide that would normally control it. A biotype is a naturally occurring individual of a species that often looks the same but has a different genetic makeup than other individuals of the species. The difference in genetics among biotypes within a species accounts for the presence of herbicide-resistant weeds.

There are isolated populations of Italian ryegrass in Kentucky that are resistant to the ACCase inhibitor herbicide diclofop (Hoelon). Scientists have shown that the resistance of Italian ryegrass to ACCase inhibitor herbicides is not well defined. For example, pinoxaden (Axial XL), another ACCase inhibitor, may control certain biotypes resistant to Hoelon, yet not other Hoelon-resistant biotypes. This inconsistent response to Axial XL makes it difficult in identifying resistant problems for this species.

Resistance to ALS inhibitor herbicides has been reported as a major problem in other wheat production regions of the United States, but not in Kentucky or neighboring states. The fact that sulfonylurea herbicides, which are ALS inhibitors, are widely used in Kentucky makes it important that growers be on the lookout for problems with ALS resistance.

The potential for weed resistance to develop increases with repeated use of herbicides that have the same site or mode of action. Therefore, monitor herbicides used in all rotational crops and use production practices that prevent or reduce the potential for the development of herbicide-resistant weedy biotypes.

Herbicide carryover. Injury due to carryover of herbicide residues is a concern when growing wheat in rotation with corn and double-crop soybeans. Growers must use caution in selecting herbicides that do not persist in soil for long periods and cause injury to rotational crops. While wheat injury due to carryover of atrazine residues has not been a widespread problem in Kentucky, the atrazine label warns that the risk of injury may occur. Simazine is chemically similar to atrazine, but may pose a greater threat to carryover injury to wheat than atrazine. There is a significant risk of injuring wheat where clomazone (Command) was used the previous spring in other crops. See Photos 6-11, 6-12, and 6-13 for injury symptoms due to herbicide carryover.

Certain ALS inhibitor wheat herbicides persist in soil and injure double-cropped soybeans. Dry weather and high soil pH are conditions that prolong the persistence of many ALS inhibitor herbicides. Products that contain such active ingredients as chlorsulfuron (Finesse or Finesse Grass & Broadleaf), metsulfuron (Finesse), propoxycarbazone (Olympus Flex), or sulfosulfuron (Maverick) have potential to injure double-cropped soybean. It is important that growers consult labels for the required rotational interval and any recommendation on planting a Sylfonylurea Tolerant Soybean (STS) variety.

Harvesting restrictions. Most herbicides used in wheat have label restrictions regarding use of the crop as grain or for forage purposes. The EPA has established these restrictions to prevent illegal residues in the harvested grain or forage for livestock feed. When more than one product is included in the spray tank mixture, follow the label that is most restrictive.

Cleaning spray equipment. If spray equipment is not rinsed properly, herbicide residues can accumulate in the spraying system and dislodge in subsequent applications, causing injury to susceptible crops. Check the herbicide label for recommended procedures for cleaning equipment. The procedures may appear cumbersome but are often necessary to remove small amounts of herbicide that could injure other crops.

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Section 7. Disease Management

Donald E. Hershman and Douglas Johnson

Scouting for Diseases | How Pre-plant Decisions Affect Diseases | Variety Selection | Crop Rotation | Tillage | Seed Quality, Seeding Rate, and Planting Method | Planting Date| Nitrogen Fertility | Fungicide Seed Treatments | Foliar Fungicides | Disease Descriptions | Diseases Caused by Viruses | Diseases Caused by Bacteria | Diseases Caused by Fungi

Photo7.1 Fusarium head blight(head scab) is caused by Fusarium Graminearum and must be managed in Kentucky.

Disease management is a key component of high-yielding wheat production. In many years, it simply is not possible to produce high wheat yields without paying attention to matters related to disease control. Some diseases, such as take-all disease, barley yellow dwarf, and Fusarium head blight, must be managed proactively, before disease symptoms are evident. Other diseases, such as speckled leaf blotch, leaf rust, stripe rust, stem rust, and powdery mildew, can be managed successfully after initial disease symptoms have become evident. Generally, Kentucky producers place too much emphasis on disease control using foliar fungicides only. As a consequence, little attention is paid to implementing helpful non-fungicide disease control tactics. Most diseases are best managed through the use of multiple tactics, both proactive (e.g., crop rotation, delayed and/or staggered planting plates, use of resistant varieties of varying maturities, proper fertility, and application of seed treatment fungicides) and reactive (e.g., application of foliar fungicides and timely harvest). Leaving disease control to chance is a highly risky approach to producing high-yielding wheat.

Scouting for Diseases

For a variety of reasons, few Kentucky wheat producers place much emphasis on scouting their wheat diseases. Time and labor constraints (for do-it-yourselfers), the cost of hiring a crop consultant, and indifference to the need for scouting rank among the top reasons why this is the case. However, scouting is essential for an integrated approach to managing diseases. First, scouting helps build an on-farm database that can be used to select appropriate disease management tactics for future crops. Second, scouting helps you make the best possible fungicide use decisions, which frequently results in the decision NOT to spray a fungicide.

Research and experience over the past 20 years suggest that fungicides are not helpful or needed in about two out of every five or six years. Low disease years are most often associated with extremely dry and hot weather following flag leaf emergence. Applying fungicides in low disease years is a waste of time and money and is not good for the environment. Effective crop scouting can help you avoid making unnecessary fungicide applications and will make your wheat operation more profitable and sustainable in the long haul.

Effective crop scouting takes time, experience, and patience, but is not difficult. The Kentucky Integrated Pest Management (IPM) Program offers annual scout trainings, as well as multiple scouting resources (http://www.uky.edu/Ag/IPM/ipm.htm). In addition, there are numerous other training opportunities held throughout the year, and there is an inexhaustible supply of wheat disease and scouting information available on the Internet. Take advantage of all opportunities to learn how to scout for, and identify, the most common wheat diseases on your farm. At first, scouting for diseases may seem daunting, but only a few diseases have the potential to seriously reduce crop yield, and these tend to occur at specific times during the season, not all at once. The University of Kentucky operates two Plant Disease Diagnostic Laboratories to help with disease identification. Pest problems must be identified accurately before embarking on any pest management program, especially those that involve the use of a pesticide. For more information on submitting samples for diagnosis, contact your local county Extension office.

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How Pre-plant Decisions Affect Diseases

Most Kentucky wheat producers have their total disease management program in place once the seed is in the ground. By that time, decisions have been made regarding the length of time since the last wheat crop (crop sequence), tillage method and seedbed preparation, variety selection (maturity, disease package, yield potential, etc.), seed quality (germination, vigor), seed treatment, planting date, seeding rate, seeding method, and fall fertility. Individually and collectively, these decisions play an important role in determining which diseases might develop, their severity, and their potential impact on crop yield, test weight, and grain quality. Because pre-plant and planting decisions are so important in the management of wheat diseases, you need to understand how they influence disease development.

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Variety Selection

Decisions relating to variety selection are possibly the most important decisions you can make in managing diseases. Every commercially available wheat variety has a unique disease package and this information is usually very easy to come by for most soft red winter wheat varieties. Excellent resistance is not available to manage some diseases, and it is hard to find high-yielding varieties that have decent resistance to all disease threats. Nonetheless, which and how many varieties are planted on your farm will determine the potential for certain diseases to develop. Failure to consider the ramifications of variety selection in managing diseases is a costly mistake made by many producers. It is best to select two or three high-yielding varieties with the greatest level of available resistance to the most common diseases on your farm. To do this, you must have some idea about the disease history of your farm (see above section on scouting). If you don't have access to historical disease information for your farm, talk with your county Extension agent, farm supply dealers, local crop advisor, and/or neighbors. This information may not be as good as actual data from your farm, but it is far better than basing decisions on no information. It is important to plant more than one variety for this key reason: it is common for a single disease to severely damage a single variety. However, when multiple varieties are planted, the risk that a disease will wreak havoc on all your wheat acres is significantly diminished. In addition, planting more than one variety, especially when different maturities are represented, can help with the logistics of harvesting and planting doublecrop soybean.

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Crop Rotation

Few wheat producers in Kentucky give much thought to the influence of crop rotation on diseases. Our normal production systems rarely include planting wheat in the same field in consecutive years. This is good in that planting wheat in alternate years (or even less often) helps in the management of wheat pathogens that survive between wheat crops in wheat residue and/or are short-lived in the soil in the absence of a host crop. One such disease is take-all. In fact, crop rotation is the only practical way to control take-all disease. Rotating crops also can reduce infections by certain windborne foliar diseases, such as the diseases that make up the leaf blotch complex (speckled leaf blotch, Stagonospora leaf blotch, and tan spot). It should be noted, however, that favorable effects are frequently compromised, or even negated, by spores blowing into fields from neighboring fields or from fields that are many miles away.

Most wheat in Kentucky is planted no-till following corn. Corn is generally considered to be a good non-host crop to grow in rotation with wheat because the two crops have few diseases in common. However, there has been some concern that planting no-till wheat where corn was planted the previous season, significantly increases the risk to Fusarium head blight (FHB, head scab). FHB also attacks corn (causes stalk and ear rot) and readily survives between seasons in corn stubble. Planting wheat behind corn does not significantly enhance the FHB threat in Kentucky. Results of multi-year research trials, disease surveys, plus many years of observations, all point to the same conclusion: weather, not local tillage regime, determines if FHB will be serious enough to reduce yields and grain quality or not. This is because when weather conditions favor FHB, so many FHB spores are produced and blow into fields from both local and distant sources, that the role of in-field spore production is relatively unimportant. Under conditions favorable for FHB, disease severity can be slightly elevated in no-till fields. As a result, levels of deoxynivalenol (DON), an undesirable mycotoxin usually associated with FHB, can also be elevated. Nonetheless, tillage regime will never be the factor that determines whether FHB will be severe in a particular field or not.

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Tillage

In continuous wheat systems such as are common in the Great Plains Region, tillage hastens the breakdown of residue that harbors certain wheat pathogens. This can help reduce levels of some soil-borne and foliar diseases caused by fungi. However, in southern states, like Kentucky, where wheat is planted every second or third year in a field and soil conditions favor residue breakdown, most of the residue is deteriorated by the time the next wheat crop is planted. Thus, local tillage regime has little impact on diseases that develop from one wheat crop to the next. Implementing community-wide or regional tillage programs might be beneficial, but this approach is impractical.

See the above section on crop rotation for a discussion on the limited impact of tillage on FHB.

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Seed Quality, Seeding Rate, and Planting Method

Seed quality, seeding rate, and planting method can each affect stand establishment and development. Excellent seed germination and seedling growth are required for sufficient stands and maximum yields. High-quality seed treated with a broad-spectrum fungicide and good planting techniques (especially depth) foster good stand establishment. Excess stands, however, encourage foliar and head diseases by reducing air circulation and light penetration into the canopy later in the season. Calibrate your equipment to achieve sufficient, but not excessive, stands (see Section 3 Cultural Practices for more information).

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Planting Date

The trend in recent years has been to plant wheat earlier than is recommended for a given area. The desire to achieve high yields and the logistics of planting large acreages appear to be the main factors behind this trend. The problem is that early-planted wheat (defined as wheat planted prior to the Hessian fly-free planting date) is at greater risk of damage caused by barley yellow dwarf (BYD), wheat streak mosaic (WSM), take-all disease, and Hessian fly than is later-planted wheat. In addition, early planted wheat may also encourage leaf rust and stripe rust infection in the fall and this can increase the risk that one or both disease will carry through a mild winter and into the spring. If logistical considerations cause you to plant some of your wheat acres prior to the fly-free date for your area, make sure those acres have been well-rotated, that volunteer corn (which is green bridge for WSM) in and around the field has been killed, and plant a variety that can tolerate some BYD. You might also target these acres for a seed applied or fall foliar insecticide treatment (See Section 8 Insect Pests.). Finally, make sure you scout your early-planted acres for signs of leaf rust and/or stripe rust in the spring so as to not miss hotspots which could lead to a more general infection later in the season.

Planting all your wheat acreage prior to the fly-free date is extremely risky and is not recommended under any circumstances.

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Nitrogen Fertility

Too much nitrogen in the fall can encourage excessive fall growth that can increase your problems with BYD and most foliar diseases caused by fungi, but especially powdery mildew. Increased problems with BYD are often the result of an extended period of aphid activity (aphids transmit BYD virus) when stands are dense in the fall. The same situation encourages infection and overwintering of foliar fungal diseases, such as leaf and stripe rust, powdery mildew, and leaf blotch complex. Excessive spring nitrogen results in lush stands that promote disease in a manner similar to that associated with excessive seeding rates.

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Fungicide Seed Treatments

Seed treatment fungicides are used on nearly all wheat seed purchased in Kentucky. Stands and yields are not always improved when fungicide treated seed is planted, but the cost of fungicide and treating is relatively low compared to the potential benefits. Think of seed treatments as a form of low cost crop insurance; it is there when you need it.

Getting and keeping a good stand is a key component of high-yielding wheat. Typically, achieving excellent stands is not that difficult in Kentucky as long has high quality seed is used, and planting date and planting method are consistent with University of Kentucky recommendations. We have conducted a great many seed treatment fungicide tests over the years, and we rarely see a significant impact on spring stands, tiller counts, disease control, or yield. Occasionally, we see significant stand improvements in the fall, but these rarely carry over into the spring.

Seed treatment fungicides play a significant role in Kentucky wheat production. Many times, one or more factors are compromised at planting and in the absence of a seed treatment fungicide, yield and quality could be compromised. For example, dry soil conditions in early fall frequently cause a delay in planting as producers wait for soil moisture conditions to improve. Under these circumstances, it is not uncommon for wheat to be planted well after the recommended planting date for an area. Often soil conditions in November become hostile to germinating wheat and young seedlings. Under these conditions, germinating seed and young seedlings need the benefit of a seed fungicide. Even when planting date is optimal, stands can be compromised if seed are planted too deep or too shallow, if planting equipment is not properly calibrated and functioning, or if soil conditions turn cool and wet earlier in the fall than normal. In these cases, seed treatment fungicide may help you attain and retain acceptable stands that can produce a high yield.

Table 7-1. Activity of Common Seed Treatment Fungicide Active Ingredients*.
Fungicide Activity
Carboxin Modest control of general seed- and soil-borne pathogens; excellent control of loose smut.
Difenoconazole Moderate control of general seed- and soil-borne pathogens, very good control of Fusarium seed rot and seedling blight, and excellent control of loose smut. Minor control of early powdery mildew and rust and good control of seedling blights caused by Stagonospora and Septoria.
Fludioxonil Provides excellent control seed borne Fusarium as well as several soil borne pathogens, with the exception of Pythium.
Imazalil and Thiabendazole Similar to thiram and captan except for much improved control of Fusarium seed rot and seedling blight.
Mefenoxam and Metalaxyl Provides protection from Pythium for a limited time following seeding. Other classes of seed and soil-borne pathogens are not controlled.
Pentachloronitrobenzene Provides protection from Rhizoctonia for a limited time following seeding.
Tebuconazole Similar to difenoconazole, except provides no control of fall powdery mildew.
Captan, Maneb, Thiram Moderate activity against many common seed- and soil-borne fungi.
Triadimenol Similar to difenoconazole, but provides excellent control of fall powdery mildew and very good control of fall infections of leaf rust or stripe rust. In high mildew areas, can often be used as a replacement for foliar fungicide sprays for mildew in early spring (up to head emergence . Very good control of Fusarium seed rots and seedling blights. Excellent control of loose smut).
Triticonazole Provides excellent control of smuts and very good control of seed borne Fusarium and several soil borne pathogens with the exception of Pythium.
*Consult with your chemical salesperson and/or ag supply dealer for product trade names. Most commercially-available seed treatment products are comprised of multiple active ingredients.

Another significant role of seed treatment fungicides is to assist with stand establishment when seed planted has reduced percent germination and/or vigor. For example, stocks of high germination seed are usually very limited in the fall following a big FHB year. In these years, growers frequently have to settle for seed with lower than desired germination rates (e.g., 70%). As long as seed is within acceptable tolerances for both germination and vigor, certain fungicide seed treatments can be the difference between achieving acceptable stands or not. This does not apply to severely damaged seed that may contain a lot of tombstones (dead seed) or has suffered serious mechanical damage.

Historically, diseases like loose smut used to be serious disease problems in both wheat and barley, but this is no longer the case. Like near eradication of polio in the human population, it is now very rare for smuts and related diseases to cause significant damage in most wheat producing states. Good seed production practices and certification standards have played a major role in helping to achieve this status. However, the regular use of certain seed treatment fungicides capable of eradicating the smut fungus in seed, has also been extremely important. The increase in occurrence of smuts would be all but certain if growers quit using seed treatment fungicides, many of which are highly effective against smut.

There is currently a very long list of seed treatment fungicides available for use on wheat. The vast majority of newer products are effective at very low use rates and consequently can only be applied by certified applicators. Hopper box treatments are still available, but their use has been considerably reduced in recent years. Some fungicides have a broad spectrum of disease control activity and others have very specific uses.Table 7-1 lists some of the most commonly used products and the diseases they control. Contact your local farm supply dealer for more specific information.

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Foliar Fungicides

The role of modern foliar fungicides is to manage certain common diseases caused by fungi. Target diseases include leaf rust, stripe rust, stem rust, powdery mildew, speckled leaf blotch, Stagonospora leaf and glume blotch, and tan spot. Certain fungicides also suppress FHB. Other diseases, like take-all and all diseases caused by viruses or bacteria, are not controlled by fungicides.

Since the first printing of this publication 1996, foliar fungicide use in Kentucky has gone mainstream. By 1996, only 30 percent of producers had ever applied a foliar fungicide to wheat. At present, fungicides are used by most producers interested in achieving high yields. There is no doubt that producers will at least recover the cost of fungicide and application in most years. However, fungicides are not needed every year. Unfortunately, the current trend is to apply fungicides on a calendar or growth stage basis and not according to actual need. Scheduled applications, while easier to plan for and implement, are in direct opposition to established good farming practices. Fungicides should certainly be used when needed, but there are many good reasons to keep the sprayer in the barn in some years. The best and most sustainable approach is to base fungicide spray decisions on results of field scouting and after considering other production practices that impinge on a crop's risk for disease.

Regardless of how fungicide use decisions are made, it is important to understand what fungicides do and do not do. Their main role is to protect crop yield potential from losses caused by specific fungal diseases. Fungicides vary in their effectiveness against these target diseases (Tables 7-2 and 7-3). Fungicides do NOT give a yield bump. Rather, they protect yield potential that is already built into the crop. This may seem like a minor point, but it is actually quite important. If you understand this principle, you will appreciate why fungicides do not always result in higher yields compared to untreated crops.

Table 7-2. Fungicide Efficacy for Control of Wheat Diseases.
The North Central Regional Commitee on Management of Small Grain Diseases (NCERA - 184) has developed the following information on fungicide efficacy for control of certain foliar diseases of wheat for use by the grain production industry in the U.S. Efficacy ratings for each fungicide listed in the table were determined by field testing the materials over multiple years and locations by the members of the committee. Efficacy is based on proper application timimg to achieve optimum effectiveness of the fungicide as determined by labled instructions and overall level of disease in the field at the time of application. Differences in efficacy among fungicide products were determined by direct comparisons among products in field tests and are based on a single application of the labeled rate as listed in the table. Table includes most widely marketed labled products and is not intended to be the list of all labeled products.
Efficacy of fungicides for wheat disease control based on appropriate application timing.
Fungicide(s) Powdery mildew Stagonospora leaf/glume blotch Septoria leaf blotch Tan spot Stripe rust Leaf rust Head scab Harvest restriction
Class Active Ingredient Product Rate/A (fl. oz)
Strobilurin Azoxystrobin 22.9% Quadris 2.08 SC 6.2 - 10.8 F(G)1 VG VG E E2 E NR 45 days
Pyraclostrobin 3.6% Headline 2.09 EC 6.0 - 9.0 G VG VG E E2 E NR Feekes 10.5
Triazole Metconazole 8.6% caramba 10.0 - 17.0 --3 --3 --3 --3 E E G 30 days
Propiconazole 41.8% Tilt 3.6 Ec
PropiMax 3.6 EC
Bumper 41.8 EC
4.0 VG VG VG VG VG VG P 40 days
Prothioconazole 41% Proline 480 SC 5.0 - 5.7 --3 VG VG VG --3 VG G 30 days
Tebuconazole 38.7% Folicur 3.6 F4
Embrace 3.6 L
Monsoon
Muscle 3.6 F
Orius 3.6 F
Tebucon 3.6 F
Tebustar 3.6 F
Tebuzol 3.6 F
Tegrol
Toledo
4.0 G VG VG VG E E F 30 days
Prothioconazole 19%
Tebuconazole 19%
Prosaro 421 SC 6.5 - 8.5 G VG VG VG E E G 30 days
Mixed mode of action Metconazole 7.4%
Pyraclostrobin 12%
Multiva
TwinLine
6.0 - 11.0 G VG VG E E E NR Feekes 10.5 and 30 days
Propiconazole 11.7%
Azoxystrobin 7.0%
Quilt 200 SC 14.0 VG VG VG VG E E NR 45 days5
Propiconazole 11.4%
Trifloxystrobin 7.0%
Stratego 250 EC 10.0 G VG VG VG VG VG NR 35 days
1Efficacy categories: E = Excellent; F = Fair; G = Good; NR = Not Recommended; P = Poor; VG = Very Good. Efficacy designation with a second rating in parenthesis indicates greater efficacy at higher application rates.
2Efficacy may be significantly reduced if solo strobilurin products are applied after stripe rust infection had occurred.
3Insufficient data to make statement about efficacy of this product.
4Generic products containing tebuconazole may not be labled in all states
5The pre-harvest interval for Quilt is under review by EPA and may be adjusted to consider a growth stage restriction.
This information is provided only as a guide. It is the responsibility of pesticide applicator by law to read and follow all current label directions. N0 endorsement is intended for products listed, nor is criticism meant for products not listed. members or participants in the NCERA - 184 committee assume no liablity resulting from the use of these products.

The bottom line is this: If disease pressure is great enough to reduce crop yields, then fungicides may help protect the crop from potential losses. However, if disease conditions are light such that no or nominal yield loss is possible, than applying a fungicide would not result in either a yield or economic advantage.

Table 7-3. Preliminary Estimates of Fungicide Efficacy for Stem Rust of Wheat and Barley.
Preliminary estimates are based on available data. We have more data for Tebuconazole and Propiconazole than for other products. When products have only been evaluated in a few studies the efficacy ratings are based in part on product efficacy against other cereal rust diseases.
Fungicide(s) Stem rust
Class Active Ingredient Product Rate/A (fl. oz)
Strobilurin Azoxystrobin 22.9% Quadris 2.08 SC 6.2 - 10.8 E*
Pyraclostrobin 3.6% Headline 2.09 EC 6.0 - 9.0 E
Triazole Metconazole 8.6% caramba 10.0 - 17.0 E
Propiconazole 41.8% Tilt 3.6 Ec
PropiMax 3.6 EC
Bumper 41.8 EC
4.0 VG
Prothioconazole 41% Proline 480 SC 5.0 - 5.7 VG
Tebuconazole 38.7% Folicur 3.6 F 4.0 E
Prothioconazole 19%
Tebuconazole 19%
Prosaro 421 SC 6.5 - 8.5 E
Mixed mode of action Metconazole 7.4%
Pyraclostrobin 12%
Multiva
TwinLine
6.0 - 11.0 E
Propiconazole 11.7%
Azoxystrobin 7.0%
Quilt 200 SC 14.0 E
Propiconazole 11.4%
Trifloxystrobin 7.0%
Stratego 250 EC 10.0 VG
*Efficacy categories: E = Excellent; VG = Very Good. Efficacy designation with a second rating in parenthesis indicates greater efficacy at higher application rates.
This information is provided only as a guide. It is the responsibility of pesticide applicator by law to read and follow all current label directions. N0 endorsement is intended for products listed, nor is criticism meant for products not listed. members or participants in the NCERA - 184 committee assume no liablity resulting from the use of these products.

Deciding whether or not to apply foliar fungicide should involve a couple of steps. First, catalog your use of production practices that favor disease development (or not). Doing this gives you a way to assess your disease risk and the concurrent potential for a fungicide to give an economic result. Carefully consider the following:

Scout the wheat at critical stages for 1) incidence and severity of fungal diseases targeted by foliar fungicides, 2) crop yield potential, and 3) to determine if some other pest or disease has compromised crop health to the point where apply a fungicide is not prudent. For obvious reasons, crops with low yield potential are not good candidates for fungicide application. Typically, fungicides applied during or immediately following head emergence give the best yield response when disease pressure is sufficient to reduce yield. Fungicides applied for FHB suppression, however, must be applied at early anthesis (beginning of flowering) for best results. This can create a tension if other diseases, such as leaf blotch complex, threaten the crop. Most of the time, this is not a serious issue and applications made for FHB also do an excellent job against other late-season fungal diseases. But the occasional situation develops when a producer may need to decide which target disease is the highest priority. Depending on the decision made, either FHB suppression or control of other leaf and head diseases could be compromised.

Periodically, fungicide manufactures probe the market to see if ultra-early to early applications of fungicides (i.e., stem elongation to flag leaf emergence) will be accepted and used by producers. Part of the lure in this approach is that many producers already apply herbicides and/or insecticides at early growth stages, so adding the fungicide is relatively inexpensive. In many cases, fungicide manufacturers recommend reduced fungicide rates when their products are applied early, so this sweetens the pot. In most soft red winter wheat states, early applications are not sold as a replacement for later applications, but, rather, in addition to later applications. In some parts of the country, like the Pacific Northwest, this strategy can pay off, as wheat frequently does have significant disease pressure prior to flag leaf extension. However, this is a rare situation in Kentucky. We tested early applications during the late 1980's, and again during 2007 - 2008, with little success. In most cases, disease pressure did not build up well until well after the fungicide was applied. In these instances, the fungicide was not there when it was needed. In other cases, disease never did build up, so the applications were not needed in the first place. Fungicide manufacturers frequently market early applications as a way to short circuit a disease epidemic before it gets started. This sounds good, but in most instances, things it don't pan out the way the early application programs are sold. All things considered, there appears to be little justification for applying any foliar fungicide prior to flag leaf extension in all but the most rare cases in Kentucky.

Finally, the relatively new strobilurin class of fungicides is being sold by some fungicide manufacturers as a means of reducing the impact of certain crop stresses, in addition to disease control. While it is true that there is considerable laboratory and greenhouse evidence that this is true, how this translates to field conditions is less clear. Strobilurin fungicides do frequently elicit and effect called the greening or stay-green effect. This so-called greening effect is pointed to by some as visual evidence that plant health is being improved. This is debatable. In fact, the greening effect is often not associated with higher yields. Furthermore, many wheat growers have taken to experimenting with various combinations of triazoles and reduced rates of strobilurins as a way to avoid, or at least reduce, the greening effect. The effect tends to delay harvest, which also results in doublecrop soybeans being planted later up to a week later. If the crop is harvested sooner, harvest must proceed at a slower pace and grain drying is often necessary, which increases the cost of production. The bottom line is that there is a mixed response to the greening effect and it should not be assumed that the greening effect is necessarily a good thing.

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Disease Descriptions

The following are general descriptions of the wheat diseases most common in Kentucky. Diseases are listed seasonally. More specific information on each disease is available through your county Extension office. If you are using picture sheets to help identify a disease, be aware that many diseases look similar and can be confused with one another. The University of Kentucky staffs two plant disease diagnostic laboratories to assist you, at no charge, in identifying plant diseases.

Diseases Caused by Viruses:

1. Barley Yellow Dwarf (BYD)

Occurrence. Greenup through late milk.

Symptoms. Primary symptoms include plant stunting, reduced tillering, and yellow to red-purple discoloration of leaf tips and margins. Affected plants may have a unusually erect, spiked, appearance. Symptoms can occur in the fall or spring, but are most common in the spring on the top two leaves of the plants. Foliar symptoms are frequently accompanied by secondary bacterial infections. These infections are visible as brown spots and streaks on BYD-symptomatic plants. Infected plants frequently occur in random, small groups. Large portions of fields or entire fields can be affected in severe cases.

Damage. BYD reduces grain yield and test weight.

Key features of disease cycle. Barley yellow dwarf virus (BYDV) is transmitted from infected grasses into wheat and barley by several species of aphids. In Kentucky, the bird cherry-oat aphid and, to a lesser extent, the corn leaf aphid are the most important vectors in the fall. In the spring, overwintered bird cherry-oat aphids and English grain aphids are the most important vectors. Regardless of the aphid species, winged adults immigrate into wheat fields from neighboring and distant sites, feed, and deposit live young on plants. The migratory behavior of winged vectors is the reason why initial BYD symptoms are often seen along field edges and in randomly occurring spots. Typically, the young aphids deposited by winged migrant adults develop into wingless adults that produce more offspring over several generations. These wingless aphids, in turn, producer a small number of winged aphids which fly locally and a larger number of unwinged offspring that gradually spread in fields by crawling from plant to plant.

BYDV is transmitted to wheat through the feeding activities of both winged and wingless aphids. Aphids acquire the virus by feeding on diseased plants for as little as 30 minutes. BYDV cannot be transmitted from adult to young aphids. For this reason, the percentage of winged aphids originally carrying the virus into a field is an important piece of the picture. This percentage can vary greatly from field to field and from season to season. Although you can never tell which aphids are carrying BYDV and which are not, having knowledge of seasonal aphid activities can help you assess the potential for BYDV to occur.

Photo7.2 Barley Yellow Dwarf Yellow Reaction.
Photo7.3 Barley Yellow Dwarf Purple Reaction.

Fall infestation. The numbers of aphids arriving in the fall depend largely on two factors: general growing conditions the preceding summer and when the first hard frost occurs in relation to wheat seedling emergence in the fall. Normal or greater rainfall during the summer usually benefits the aphid population. In drier summers, fewer aphids are produced due to reduced host plant quality. For the same reasons, a greater proportion of BYDV-infected host plants die due to the extra stress.

Crops that emerge long before a hard freeze have a greater potential for aphid infestation (and exposure to BYDV) than those emerging after a hard freeze. The fly-free date, which is used to control Hessian fly infestation, is based on that principle and works well as long as the freeze occurs when expected.

Winter survival. Aphids arriving in the field during the fall continue to move, feed, and reproduce as long as temperatures remain above about 48°F. Mild temperatures or insulating snow cover during cold spells, usually results in significant survival of the aphids during the winter. Harsher weather results in greater mortality. BYDV-infested aphids that survive the winter months are a primary source of BYD increase in the spring.

Spring infestation. The English grain aphid has a spring flight and arrives about the same time that winter wheat is greening up and the overwintering bird cherry-oat aphid becomes active, in early spring. The numbers of winged adults of the English grain aphid depend on the same factors that determine survival of the bird cherry-oat aphid. Good conditions for survival should produce larger spring flights and, possibly, increase the movement of BYDV within and among wheat fields. Because of this timing the English grain aphid is less likely to be important in the movement of BYDV.

Management. Plant after the Hessian fly-free date. Plant wheat varieties tolerant or moderately resistant to BYDV. Limit BYDV infection by controlling aphids with insecticides if aphids reach treatment threshold within 30 days after planting in the fall, or in early spring (See aphid threshold levels in Section 8 Insect Pests.). The greatest probability for the successful use of insecticides exists when the following criteria are met: the crop is planted prior to the fly-free date or first killing frost; drought stress the previous summer was not widespread; there is an extended period of mild weather in the fall; there is a mild winter or good snow cover during cold periods; there is an early, mild spring; at least ten aphids per row foot are observed in the crop; the crop is at the stage prior to flag leaf emergence; and there is high crop yield potential.

If the aphids-per-row-foot level is reached in the fall or spring, it is an indication that at least some of the above criteria have been met. If this aphid level is reached in the fall especially within 30 days of seedling emergence, it may be advisable to make an insecticide application. If it turns cold after the application, wait and scout again in the spring. If the fall and / or winter is mild and winged aphids continue to arrive in the field, continue to scout. It is possible that a second fall application might be needed to achieve acceptable BYD control. Regardless of what was done in the fall, a spring application may be needed if greenup is early and the aphid treatment guideline is reached prior to flag leaf emergence. Failure to make the necessary spring applications may negate any gains associated with fall applications.

Keep in mind that the above aphid treatment guideline is not chiseled in stone. In some years, the aphid thresholds may be too low and in other years too high. Herein lies the difficulty when attempting to control BYD indirectly using insecticides; the system is not perfect. However, until our understanding of BYD epidemiology and aphid biology is enhanced by new research, aphids-per-row-foot treatment guideline is the only one available with any experimental basis.

2. Wheat Soil-borne Mosaic

Occurrence. Symptoms are most prominent from green-up through stem erection, but plants may remain permanently stunted.

Symptoms. Leaves of infected plants exhibit a mild green to prominent yellow mosaic. Small green islands and short streaks may be evident on an otherwise yellowed leaf. Infected leaves may be somewhat elongated and have rolled edges; tillering of plants is commonly reduced. Wheat soil-borne mosaic can occur throughout fields, but is usually most severe in poorly drained or low areas in fields. Symptoms are most prominent early to mid season when day temperatures are between 55°F and 70°F. Symptoms tend to fade somewhat as the weather warms up, but in severe cases, plants can remain permanently stunted.

Damage. Yield is reduced.

Key features of disease cycle. Virus is transmitted by a soil fungus, P. graminis, that is common throughout Kentucky. Infection can occur in the fall, winter, or spring, but autumn infections lead to the most serious problems. High soil moisture favors infection.

Management. Plant resistant wheat varieties. Delay fall planting operations past the Hessian fly-free date to limit fall infections. Improve internal and surface drainage of fields where problems exist. Avoid crop production practices that encourage soil compaction.

3. Wheat Spindle Streak Mosaic

Occurrence. Greenup through flowering.

Photo7.4 Wheat Spindle Streak Mosaic.

Symptoms. Symptoms are highly variable, depending on the wheat variety and growing conditions. Foliar symptoms appear as random, yellow to light green dashes running parallel with the leaf veins. Early in the spring, the dashes may have a nondescript appearance. With age, however, some dashes are pointed at one or both ends, hence the name spindle streak. Spindles may have an island of green tissue in their centers. Plant stunting and reduced tillering can be associated with severe infection by the virus. Symptoms usually appear during the period the crop should be greening up in early spring. Symptoms are frequently uniformly distributed across fields and usually fade as temperatures warm in mid spring. During cool springs, symptoms may be evident throughout the season.

Damage. Yield is reduced.

Key features of disease cycle. The virus is transmitted to wheat in the fall, winter, or early spring by the soil fungus Polymyxa graminis. The onset and degree of symptom expression can be highly variable in a field from one year to the next, even though P. graminis and the virus are present at relatively constant levels. This is related to the time of year wheat becomes infected and the range and consistency of winter and early spring temperatures. Disease is favored in wet soils, although excessive moisture is not required for severe disease to occur.

Management. Same as wheat soil-borne mosaic.

4. Wheat Streak Mosaic (WSM)

Occurrence. Greenup through late milk. Infections evident before heading will have the greatest impact on crop yield. Severe infections, however, are rather rare in Kentucky and usually only occur in the year following drought conditions when abandoned corn and or soybean fields exist in the vicinity of emerging wheat (fall).

Photo7.5 Wheat Streak Mosaic.

Symptoms. Leaves turn pale green to yellow and leaves exhibit white to cream colored parallel streaks of varying lengths. Plants may appear flaccid when symptoms develop during stem elongation to flag leaf extension. Severe infections can be evident across an entire field, or symptoms may be evident in hot spots, especially near field edges. Symptoms are frequently confused with those associated with barley yellow dwarf (because of leaf yellowing) or wheat spindle streak (WSSM) or wheat soil-borne mosaic (WSBM; because of the streaks which are produced). However, a side by side comparison of these diseases indicates notable unique features associated with each disease. Specifically, BYD does not show streaks, and WSSM and WSBM do not show characteristic yellowing of leaf tissue.

Damage. Yield and test weight are reduced. Fields showing extensive foliar symptoms prior to flag leaf extension are frequently destroyed and replanted to either corn or soybean. Plants exhibiting symptoms after flag leaf extension may not have full yield potential, but an acceptable yield can often be produced as long as other stress and diseases are not a factor.

Key features of disease cycle. WSM virus (WSMV) is transmitted through the feeding of wheat curl mites. This pest is not an insect but a mite, more closely related to ticks and spiders. The mite (and therefore the virus) requires a green bridge of volunteer wheat or corn (another host crop) that grows in late summer allowing the mite to survive in large numbers until the next wheat crop emerges in the fall. Mites are deposited from near or distant sources into wheat during the fall or spring. Mites that carry the virus feed on plants and spread the virus.

Management. Varieties differ in susceptibility to WSMV, but because the virus occurs so infrequently that seed companies usually cannot provide reliable WSM ratings. Thus, it is best to assume that all soft red winter wheat varieties are susceptible to WSMV. The best and most reliable means of managing WSM is to eliminate volunteer wheat and corn from your farm for a period of 30 days before wheat emerges in the fall. This break in the green bridge will greatly reduce the potential for WSM to occur. However, in years where volunteer wheat and/or corn are common on a regional basis, be aware that mites can be spread from distant fields and deposited on your farm, sometime in significant quantities. See Entfact-117 for more information.

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Diseases Caused by Bacteria:

1. Bacterial Streak/Black Chaff

Occurrence. Flag emergence through grain fill.

Photo7.6 Bacterial Streak .

Symptoms. Leaves will develop water-soaked streaks of varying lengths that eventually turn necrotic (brown). Severely diseased leaves can die, but this is not typical in Kentucky. Infected heads will have glumes with black streaks that follow the glume veins. Black chaff is easily confused with a genetic discoloration of glume veins that is typical for a small number of varieties. Genetic symptoms will be very uniform, whereas black chaff will have a more random occurrence and will almost never involve all the heads in a field.

Damage. Test weight reduction.

Key features of disease cycle. Xantomonas translucens is seed and probably soil-borne in Kentucky. The disease is rather rare in Kentucky, but when it does occur it is usually seen along field margins where leaves receive more wounding from dirt blowing, or in patches where dust devils may have occurred. In some cases, bacterial streak is seen after leaves sustain some freeze damage. The causal bacterium is unable to directly infect plants and requires a wound in order to gain entrance into tissue.

Management. None.

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Diseases Caused by Fungi:

1. Fusarium Head Blight (FHB, Head Scab)

Occurrence. Early milk through maturity.

Symptoms. Individual spikelets or groups of spikelets turn cream to white on otherwise green heads. Entire heads may become diseased when extended periods of warm, wet weather occur during flowering and early grain fill. Salmon-colored patches of fungal growth frequently can be seen at the base of infected spikelets. Infected spikelets often fail to develop grain, or grain is extremely shriveled and of low test weight. Shriveled grain may have a pinkish discoloration.

Photo7.7 Wheat head scab.
Photo7.8 FHB Effect on Seed.

Damage. Low test weight, shriveled grain is produced in diseased heads. Germination and viability of seed and milling qualities of grain are also reduced. Scabby grain is usually contaminated with mycotoxins, especially deoxynivalenol (DON), which affects feed and food uses. Grain with extremely high DON levels (>5ppm) may not be marketable in some regions.

Key features of disease cycle. In Kentucky, the FHB fungus, Fusarium graminearum, overwinters primarily in corn stubble. Spores are produced in stubble when temperature and moisture requirements are met. When conditions favor spore production and release, spores blown into fields from remote or local sources and/or are splashed onto nearby heads. If spores are deposited on heads when conditions are warm and moist and wheat is in the early flowering to early grain fill stages, heads can become infected and the characteristic disease symptoms will be evident after a 5-7 day latent period. Most fields, in most years, escape serious infection because conditions do not favor spore production and /or flowering and grain fill do not occur during warm, wet weather. Epidemics occur when extended periods of disease-favorable weather occur while much of the Kentucky wheat crop is in flower.

Management. Nature provides the best management by limiting disease-favorable conditions during crop flowering. Moderately resistant varieties are now available and these will perform reasonably well as long as disease pressure is limited. Certain triazole fungicides (see Table 7-2), applied when the crop is in early flowering, can provide additional suppression of FHB and DON. However, do not expect greater than 40-50 percent control compared to a non-treated crop. Crop rotation and tillage have little effect on FHB because of the widespread occurrence of the causal fungus in Kentucky. This is related to the nature of corn production in Kentucky. Specifically, corn is grown in relatively small, widely-scattered fields across most grain-producing regions of the state. Consequently, when conditions favor spore production and dispersal, there are so many spores of the FHB fungus blowing around, that anything that is done on an individual field basis has only a minor impact on FHB/DON. Planting different varieties that flower at different times may reduce the overall incidence of FHB in a moderate to light disease year.

2. Glume Blotch

Occurrence. Early milk through maturity.

Symptoms. Infected glumes and awns develop gray-brown blotches, usually starting at the tips of glumes.

Damage. Infected heads develop low-test-weight, shriveled grain. Seed quality can also be reduced and this can result in problems with stand establishment if a high percentage of diseased or infested seed are planted.

Key features of disease cycle. Spores of Stagonospora nodorum blow to or are splashed onto wheat heads. Spores originate from diseased foliage (see leaf blotch complex) or infested wheat stubble. Infections occur during periods of extended wetness, especially when nighttime temperatures are warmer than normal.

Management. No highly resistant varieties are available. Plant moderately resistant varieties and high-quality, well-cleaned, disease-free (e.g., certified) seed. Control foliar and head infections on susceptible varieties with fungicides applied prior to the appearance of widespread symptoms. Avoid nitrogen excesses and deficiencies, which encourage glume blotch.

3. Leaf Blotch Complex

(Stagonospora leaf blotch, speckled leaf blotch, and tan spot)

Occurrence. Stem erection through late dough.

Symptoms. Foliar symptoms of speckled leaf blotch, caused by Septoria tritici infection include brown, elongated rectangular lesions with irregular margins. Lesions have numerous pinpoint, black specks (pycnidia) throughout. Pycnidia are most evident in the morning following heavy dew or after rain. Symptoms usually start in the lower leaves and move upward. Lesions are often first found at the tips of leaves.

Stagonospora leaf botch, caused by Stagonospora nodorum, is evident as lens-shaped, tan-brown lesions of varying sizes with regular borders that are frequently surrounded by a yellow halo. Young lesions have a dark brown center. Lesions of various ages contain light brown pycnidia, but these are difficult to see without the aid of a hand lens. Infections can occur very early in the season, but are most evident just prior to and after heading. Infections start in the lower leaves and move to the upper leaves and heads (see glume blotch). Symptoms become evident seven to ten days following infection.

Photo7.9 Stagonospora leaf and glume blotch.
Photo7.10 Speckled Leaf Blotch.

Tan spot, caused by the fungus Pyrenophora tritici-repentis, looks very similar to Stagonospora leaf blotch, except that lesions do not coalesce as readily, so they tend to remain more discrete and are frequently very numerous. They are lens-shaped like Stagonospora leaf blotch, but they lack pycnidia. Instead you will usually see (with a 20 x hand lens) a weft of fungal growth. These are called conidiophores and are the structures on which new fungal spores are produced.

Damage. Yield and test weight are reduced.

Key features of disease cycle. S. triticiand, S. nodorum, and P. tritici-repentis overwinter in wheat stubble of previously diseased crops or on infested seed. Spores are produced during wet weather and are either splashed or wind-blown onto leaf surfaces. Infection of plants by S. tritici is greatest during cool to moderate temperatures. Infection by S. nodorum and P. tritici-repentis can occur over a wide range of temperatures, but are favored in the mid to late stages of crop development. The fungi that cause leaf blotch complex can occur individually in a crop or at the same time, even on the same leaves.

Management. Plant resistant varieties and high-quality, well-cleaned, disease-free seed that is treated with a fungicide (e.g., certified seed). Avoid excessive seeding rates as well as nitrogen deficiencies and excesses. Protect the upper two leaves and heads of susceptible varieties with fungicides. Crop rotation and tillage of infested wheat stubble may help in leaf blotch management, but neither provides a high degree of control.

4. Leaf and Stripe Rust

Occurrence. Seedling emergence through late dough.

Symptoms. Leaf rust is initially evident as pinpoint, yellow flecks on upper leaf surfaces. After about one week, flecks develop into orange pustules, each containing many thousands of spores. Many things can cause wheat leaves to fleck, so flecks are a good indicator of leaf rust only when at least some mature pustules are also visible. Leaf rust pustules usually form in random patterns, primarily on the upper surfaces of leaves. Stripe rust appears in linear rows, of varying lengths, of bright yellow-orange pustules that are oriented with leaf veins. Symptoms can also develop on glumes.

Damage. Yield and test weight are reduced. Indirect losses associated with crop lodging can occur when rust is severe.

Photo7.11 Wheat Leaf Rust.
Photo7.12 Wheat Stripe Rust.

Key features of disease cycle. Both rust fungi can overwinter in Kentucky, but more commonly spores are blown into Kentucky from the south. With leaf rust, spores blow in and infect foliage during moderate to warm temperatures, and six or more hours of continuous leaf wetness. Leaf rust is a potentially explosive disease and requires just a short time to go from low to epidemic levels on a susceptible variety. Stripe rust has the ability to develop at lower temperatures than leaf rust, so it frequently can be found prior to head emergence. Symptoms are often first evident in hot spots 5-10 ft in diameter. From a distance, these affected areas will appear yellow. Close inspection of plants will reveal characteristic stripe rust lesions, with pustules. If left to develop unchecked, leaf or stripe rust can develop to the point where entire fields are involved.

Management. For leaf rust, plant resistant or moderately resistant wheat varieties. About half the soft red winter wheat varieties grown in the U.S. are susceptible to stripe rust. Unfortunately, many seed companies do not have good information on how their varieties will perform against stripe rust. Thus, it may be difficult to find varieties with known, acceptable resistance to stripe rust. For both rusts, avoid excessive stands, which tend to decrease air circulation and light penetration into the crop canopy. Protect the upper two leaves of susceptible varieties with foliar fungicides. Most modern foliar fungicides do an excellent job with managing rust diseases, but they must be applied BEFORE significant infection has occurred to perform acceptably. Crop scouting, thus, plays a central role in rust management.

5. Loose Smut

Occurrence. Head emergence through maturity.

Photo7.13 Loose Smut.
Symptoms. Floral parts of infected plants are transformed into a mass of black, powdery spores. Diseased tillers usually head out in advance of healthy tillers.

Damage. Seed infected with the smut fungus will produce smutted heads, with 100 percent grain loss being experienced by those heads.

Key features of disease cycle. Spores produced by diseased heads blow to and infect the flowers of healthy heads during rainy weather. Infected flowers give rise to infected grain. Infected grain develops normally, but harbors the loose smut fungus. The fungus remains dormant until the seed is planted and germinates. Infected plants appear to be normal, but develop smutted heads.

Management. Plant certified or otherwise high-quality, disease-free seed. Infections in seed can be eradicated by treating seed with various systemic fungicides. Many older and most new seed treatment fungicides are highly effective in controlling seed-borne smut diseases (see Table 7-1).

6. Powdery Mildew

Occurrence. Stem erection through maturity.

Photo7.14 Powdery Mildew.

Symptoms. White, powdery patches form on upper leaf surfaces of lower leaves and eventually can spread to all aboveground portions of plants. Patches turn dull gray-brown with age.

Damage. Yield and test weight are reduced, directly due to infection and indirectly due to harvest losses associated with lodging.

Key features of disease cycle. Fungus persists between seasons in infested wheat stubble and in overwintering wheat. Spores infect plants during periods of high moisture (not necessarily rain) and cool to moderate temperatures.

Management. Plant resistant or moderately resistant varieties, and avoid farming practices that favor excessively dense, lush stands. If necessary, protect upper leaves and heads of susceptible varieties by using foliar fungicides.

7. Take-all

Occurrence. Stem erection through maturity.

Symptoms. Infected plants appear normal through crop greenup, but eventually become stunted and uneven in height, with some premature death of tillers. Tillers that head out are sterile and turn white or buff colored. Affected plants easily can be pulled out of the soil because of extensive root rotting. A shiny black discoloration is evident under the leaf sheaths at the bases of diseased plants. Infected plants can occur individually, but more typically occur in small to large groups. Entire fields or large portions of fields can be diseased in severe situations.
Photo7.15 Take-all Crown Symptoms.
Photo7.16 Early Take-all.

Damage. Diseased plants yield little or no grain.

Key features of disease cycle. The take-all fungus survives from season to season in infested wheat and barley stubble and residue of grassy weeds. Infections are favored in neutral to alkaline, infertile, poorly drained soils.

Management. Allow at least one year (preferably two years) between wheat (or barley) crops. Soybeans, corn, grain sorghum, and oats are acceptable alternative crops. Maintain excellent control of grassy weeds and volunteer wheat in fields that are part of your farm's wheat operation. Fertilize fields and lime fields according to soil test recommendations. Do not allow fall or spring nitrogen deficiencies in the small grain crops. Improve surface and internal drainage of fields.

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Section 8. Insect Pests

Douglas W. Johnson and Lee Townsend

Field Scouting | Key Factors | Insecticide Management | Major Pests | Aphids | Wheat Curl Mite | Armyworm and Fall Armyworm | Cereal Leaf Beetle | Hessian Fly | References

Photo8.1 Lady beetle C-7, is a beneficial insect that feeds on aphids. Proper identification is insects is critical for pest management.

Under favorable conditions, several insects can cause significant yield loss in wheat. They can reduce plant vigor by removing sap or lower yields indirectly by feeding on leaf tissue. Some feed directly on grain heads or clip plant stems so that the grain falls to the ground. Fortunately, the probability of severe infestations is relatively low if sound management practices (crop rotation, planting after the fly-free planting date, and judicious nitrogen use) are followed. Early detection, correct identification, and assessment of pest problems allow appropriate management decisions to be made. Regular field monitoring is the best means of having the information needed to follow the recommended treatment guidelines. The small grain insect scouting calendar (Figure 8-1) indicates when pests are most likely to be present in a field.

Field Scouting

Fig8.1 Small grain insect scouting calendar.

Field scouting procedures differ among the key pests. However, look at three sites in fields up to 20 acres in size, and add one site for each additional 10 acres. For example, there would be five locations in a 50-acre field three sites for the first 20 acres and two more for the additional 20. The samples should be collected from randomly selected sites away from field edges and waterways.

Use the appropriate sampling method to collect information that can be compared directly to the treatment guideline for that specific pest. For example, the need for cereal leaf beetle control is determined by the average number of adults and/or larvae per stem. In some cases, methods for a pest may change during crop development. For example, aphid control prior to emergence of the flag leaf is based on the average number of aphids per foot of row. A rating system is used for these insects after the head emerges. (See IPM-4).

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Key Factors

Planting date, weather, particularly temperature, and nearby sources are key factors that influence pest activity. Planting after the Hessian fly-free date reduces the potential for damage from cereal aphids, fall armyworm/a>, Hessian fly and wheat curl mite in the fall. A killing frost before or soon after planting eliminates large numbers of these pests so that they are not present to move into the fields once the plants emerge. Nevertheless, abnormally long, warm falls or early springs favor aphid reproduction and can allow damaging numbers to develop from just a few individuals. The third key factor is volunteer or cover crop small grains that emerge well before recommended planting dates. These early plants can serve as a source of aphids, Hessian fly and wheat curl mite.

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Insecticide Management

Photo8.2 A greenbug(left) and a bird cherry oat aphid(right). Bird cherry oat aphids, common in fall, are dark green with red band across the end of the abdomen.

Insecticide applications are valuable in quickly reducing pest infestations that could reduce yield or quality. Read the label before purchasing and applying any pesticide. Use the lowest rates consistent with the severity of the infestation and size of the insects present. The label may recommend low rates for light to moderate infestations or for insects in the early stages of development; high rates may be needed for severe infestations or pests in later, more damaging stages.

Use selective insecticides when possible to minimize the effect on beneficial species that may be present. If using tank mixes, read the labels of all products in the combination. For example, sulfonylurea herbicides should not be applied with or near to the time that organophosphate insecticides are used. This combination can cause a variety of problems from temporary plant injury to yield reduction. (See ENT- 47).

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Major Pests

Aphids

Photo8.3 Parasitized aphids. Note the tan color compared to the green healthy aphids. Tiny wasp emerge from these mummified aphids and sting healthy ones.

Corn leaf aphids and bird cherry-oat aphids are the most common fall species. Adults and nymphs can appear any time after plant emergence and can move barley and cereal yellows viruses into the crop, resulting in Barley Yellow Dwarf disease. The bird cherry-oat aphid is the most important vector.

English grain aphids are most abundant during spring and early summer. Infestations in grain heads can cause shriveled, lightweight kernels. Occasionally greenbugs can be found; fortunately this destructive species is relatively rare in Kentucky. (See Entfact-121 and Entfact-150).

Occurrence. Aphids may be found any time after plant emergence.

Description. Aphids are small, soft-bodied, pear-shaped insects. Color varies from green to blue to yellow. Their piercing-sucking mouthpart looks like a small tube arising from under the head.

Damage. Aphids can cause two types of damage. 1) direct damage by sap removal and 2) indirect damage by injecting a virus (primarily barley yellow dwarf virus) into the plants. Damage due to direct feeding is usually confined to the head filling stage and causes low test weights. Fall BYDV infections cause stunting and yellowing to purpling of the plants and can result in severe yield loss.

Always be on the lookout for new aphid pests. Currently, feeding by aphids in Kentucky produces little visible damage. If you see aphid-infested plants that are dead or dying, or that have tightly rolled leaves and/or severe yellowing, collect the aphids and have them identified. The yellow sugarcane aphid and Russian wheat aphid currently are not present in Kentucky but are potential major pests.

When to scout. In the fall until temperatures remain below 45°F and again in the spring when temperatures regularly exceed 45°F.

How to scout. Scout in the fall and in the spring before leaf emergence (Feekes 8, Zadoks 37). Examine three separate 1-foot lengths of row at each location. Look over the entire plant, especially near the soil line. Count and record the number of aphids on each 1-foot section of row, then calculate the average. This sampling is for making decisions relative to movement of BYDV. Label these records as Counts.

Table 8-1. Rating based on number of aphids per head.
Rating - Description Number of Aphids
0 - none none
1 - slight <50
2 - moderate 50 - 100
3 - severe >100
The examination is for direct damage done by aphids to grain test weights. Label these records as "Ratings"

After heads have emerged in the spring, examine 10 grain heads at each scouting location for aphids. Record a rating of infestation based on the number of aphids per head (Table 8-1).

Economic threshold. In the fall when estimating risk of BYDV, consider a control if aphid counts average three (3) or more per row foot during the first 30 days post planting. An average of six (6) or more aphids per row foot from 30 to 60 days after planting, or ten (10) or more aphids per row foot thereafter, may justify a treatment (see Entfact-121). In the spring during head fill when using the rating scale for direct aphid damage, consider a control if an average rating of 2 (moderate) or greater is recorded.

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Wheat Curl Mite

Wheat curl mite is important only because it is the only known vector for Wheat Streak Mosaic virus . The mite (and therefore the virus) requires a green bridge of volunteer wheat that grows in late summer and allows large numbers of mites to survive until the next production wheat crop emerges. (See Entfact-117).

Occurrence. Wheat curl mite can infest plants any time before frost. The pest is especially important in very early plantings and/or in the presence of volunteer wheat, known as a Green Bridge.

Description. Wheat curl mite is microscopic so it cannot be seen by the naked eye. Feeding causes leaves to roll up, giving an onion leaf appearance. Mites can be seen by carefully unrolling the leaves, and examining it with a 10X hand lens.

Damage. Feeding by wheat curl mite produces indirect damage resulting from movement of wheat streak mosaic virus into the plant.

When to scout. There are no standardized scouting procedures for this pest.

Comments. There are no rescue treatments for this pest. All control is preventative.

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Armyworm and Fall Armyworm

Most armyworm feeding occurs from late May through early June. Damage starts at the leaf edge and progresses inward, giving a scalloped appearance. While this can reduce yields, the most serious losses occur when armyworms chew through stems and clip off the grain heads. (See Entfact-111).

In some years, fall armyworms can damage emerging stands of small grains in the fall. Damage is possible from early September until the first heavy freeze.

Annual and historic progression of both of these populations can be tracked on the IPM web page. (See UK-IPM).

Occurrence. Mid April to late May. Luxuriant or lodged vegetation in low, wet areas is especially susceptible to attack. Cool, wet springs favor armyworms.

Description. Larvae are greenish brown with a narrow stripe down the middle of the back and two orange stripes along each side. The yellowish head is honeycombed with dark lines. Armyworms are about 1.5 inches long when full grown.

Damage. Armyworms are primarily leaf feeders but they will feed on awns and tender kernels or clip off the seed heads. Infestations are more common in barley than in wheat. Armyworms may feed on oats, rye, and some forages.

Photo8.4 Fall armyworms feed on emerging tillers.
Photo8.5 True armyworms feed on the leaves and may clip awns.

When to scout. Mid April through maturity.

How to scout. Visit each field at least once a week.

First, check field margins and lodged grain. If armyworms are present, begin surveying in the standing grain. Armyworms feed during late afternoon, night, and early morning. They may be hidden under debris on the ground when you are in the field during the day.

Sample 4-square-foot areas at locations throughout the field using the number of sites determined by the "Field Scouting section. Walk at least 30 paces into the field before sampling. Pick spots randomly and look at the leaves for signs of chewing damage. Armyworms feed from the edge of the leaf in toward the mid rib. Examine the ground for dark fecal droppings and look for the larvae under surface litter or in soil cracks. Note average larval length. Walk to the remaining locations, and repeat the process.

Record. Record the number of worms present in each sample. Note the average length of the armyworms in each area.

Economic threshold. An average of 16 .5 to 0.75 inch long armyworms per 4 square-foot sample.

Comments. Armyworms longer than 1 inch may have completed most of their feeding. If the grain is nearly mature and no head clipping has occurred, controls are not advised. Warm spring weather favors parasites and diseases that attack armyworms. Note on your scouting report the percentage of worms parasitized or diseased.

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Cereal Leaf Beetle

More of a problem on oats than wheat, overwintering adults can be seen on the leaves from early April until mid-May. Their distinct yellow eggs are laid from mid-April until late May; the larvae are active and feeding from late April through mid-June. Both adults and larvae remove long, narrow strips of tissue from the upper surface of plant leaves, producing a distinct symptom of long, white scars. (See Entfact-107)

Occurrence. April to maturity.

Description. A shiny black beetle with red legs and thorax, approximately 0.5 inch long. Larvae are pale yellow and soft bodied. They glue pieces of trash and leaf on their backs as camouflage.

Damage. Adults and larvae chew long, narrow strips of tissue between veins.

Photo8.6 Cereal leaf beetle larve produce long white streaks on the upper leaf surface. The light yellow, grub-like larvae are covered with brown waste material.
Photo8.7 larvae of cereal leaf beetle and leaf damage from cereal leaf beetle feeding.

When to scout. April until maturity.

How to scout. Check 10 stems per sample site for larvae or adults.

Record. Record the total number of larvae and adults found on the 10 stems examined at each sample site. Calculate and note the average number per stem.

Economic threshold. Controls may be warranted when there is an average of more than one larva and/or adult per stem.

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Hessian Fly

In the past, fall infestations of this pest have severely damaged wheat by causing stand reduction. Both stand loss and lodging of the plants can be seen in the spring. You can reduce losses greatly by following the recommended fly-free planting date for your area and using resistant varieties. Check for the overwintering (flaxseed) stage on weakened seedlings (October through March) or for the small, white, maggot-like larvae in leaf sheaths during May. (See Entfact-101).

Occurrence. Fall and spring.

Description. The Hessian fly adult is a small, fragile gnat. The larva is a very small, white, legless maggot. The larval stage is damaging and may be found between the leaf sheath and stalk. About two weeks after egg hatch larval maturity occurs, and feeding ends. The outer skin darkens and hardens as the larvae enters its overwintering stage. This overwintering larvae (flaxseed) is the stage most often found if an infestation has occurred. This is a small, brown, seed-like case, usually found at the base of the plant between the leaf sheath and stalk.

Damage. A fall infestation can result in stand loss and broken (lodged) plants. Spring infestations usually result in plants of reduced vigor and bad color. There are two generations per year.

Photo8.8 Hessian fly-infested plants (center) appear stunted. There is no stem elongation and the leaves are usually broad and green.
Photo8.9 The flaxseed or pupal stage of the Hessian fly can be found behind lower leaf sheaths of infested plants or below the soil line.
Photo8.10 Hessian fly adult.

When to scout. Survey fields one time after the first frost and from early spring until June.

How to scout. Look for thin, stunted, chlorotic patches in the field. Examine the base of these plants for presence of the flaxseed.

Record. Record the number of flaxseed found per 10 stems examined at each sample site. Note the presence of adults or larvae.

Economic threshold. There is no rescue treatment; however, preventive measures may be used to avoid future infestations.

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References