Seedling damage by early season insects is becoming apparent as the weather warms. White grub, wireworm, cutworm, stink bug, and corn flea beetle damage have been reported from various areas in the state. The good news is that weather conditions now are promoting rapid seedling growth that moves these seedlings quickly past the small vulnerable stages.
White grub damage can be characterized by uneven stands, yellowing or purpling of lower leaves, and wilting of the entire seedlings. White grubs are larvae of several species of scarab beetles, including Japanese beetle, masked chafers, and May beetles. Of those that attack corn seedlings, May beetle grubs are the most damaging. They attack just below the crown of the plants severing the first major roots. Once symptoms of damage begin to appear, there are no effective rescue treatments.
Wireworms attack the seed and seedling corn. Damage is characterized by uneven stands, skips in the row, deadhearting of young seedlings, and tillering of seedlings. A deadheart is where the older leaves of the seedling appear healthy, but the youngest leaves emerging from the whorl are dead. Wireworms often bore into the base of the seedling to feed on the growing point, killing the youngest leaves. As with white grubs, there are several species of wireworms that are common. These are the larvae of click beetles. Some of these species may have extended life cycles lasting two or more years. If you suspect wireworm damage to seedlings, dig up suspicious plants and look for a hole in the base of the stem near the growing point. This is where the attack usually occurs. Once symptoms occur, there are no effective rescue treatments.
Cutworms damage is recognized by most, but can be confused with damage caused by the Southern corn leaf beetle (See KPN #1019). With cutworms, the whorl or the young seedling has been completely cut off, Southern corn leaf beetle often leaves a small portion of the stem attached. Cutworms may carry some of the severed leaves or stem to its burrow where it can continue to feed on these during the day. Unlike wireworm and white grubs, there are very effective rescue treatments for cutworms. We recommend that a rescue spray be considered if 3 percent or more of the plants have been cut and 2 or more live cutworms are present per 100 plants. Generally, we are more concerned with small cutworms rather than large ones, as the large ones have already done their damage.
Stink bug damage is beginning to appear as distorted, poorly growing plants. Stink bugs feed with piercing-sucking mouthparts and remove plant juices. They also inject enzymes into the plants which dissolves plant tissue around the wound. They feed at the base of the stem just above the soil line. Leaves that emerge often have four holes in a row, but unlike holes caused by boring insects, these holes have margins that have been dissolved by stink bug enzymes and appear faded. Unfortunately, it takes days for the damage to appear, usually long after the stink bug has moved on. Stink bug damage is common around field margins but may extend into the centers of no-till fields.
Corn flea beetles are also common in many fields, but the damage is most minor and plants recover quickly as conditions that promote rapid growth return. Corn flea beetle damage appears as fine scratches on the upper surface of the older leaves. The pin-head size flea beetles are often found near the damage. While flea beetles can vector the bacteria that causes Stewart's Wilt, most field corn hybrids are non susceptible to this disease. Much of the corn in the state was treated with one of several seed treatments (Gaucho, Cruiser, or Poncho) which will protect seedlings from corn flea beetle. Field corn infrequently requires treatment for corn flea beetle.
The size of corn is often a critical factor in determining when it is safe to apply postemergence herbicides. Labels of postemergence herbicides often use plant height or growth stage (or both) when discussing timing of applications relative to corn growth. While this may sound simple, there is come confusion on how to determine height or growth stage of corn with respect to herbicide applications.
A common method for determining corn height is done by using free-standing plants. When checking individual plants, measure from the soil surface to the arch of the uppermost leaf that is more than 50 % emerged. Because of the variability among corn plants in the same field, it is better to get an average from several plants than relying on just one plant.
A temptation for some folks is to measure from the soil surface to the tip of outstretched leaves. The measurements by stretching leaves upward and measuring to the uppermost leaf tip often leads to a greater height than intended by the herbicide label.
The collar method is also used to determine the proper timing and method of application of many postemergence herbicides. Staging corn plants in their vegetative growth stage is usually done by counting the number of leaves that have visible collars. The collar is the part of the leaf that joins the leaf blade and leaf sheath and occurs as a discolored line. Collars are not evident until the leaves are well developed and emerged from the whorl; consequently, as you progress up the plant, count only leaves with visible collars and not the uppermost ones that are still in the whorl. For example, a plant may appear to have 5 leaves, but after close examination, it may have only three leaves with visible collars and would be considered in the V3 growth stage.
The first true leaf that emerges during seedling development is characteristically oval-shaped and is the reference point for counting leaves. Once plants reach stage V5 (5 leaves with visible collars), the leaf and ear shoot initiation will usually be complete and a small tassel is initiated in the stem apex tip (i.e. growing point). During tassel initiation, corn will be approximately 8 inches tall and the growing point will be just at or beneath the soil surface. Once plants reach V6, the growing point and tassel will be above the soil surface and the stalk elongation will be rapid. The growth of the stalk and nodal roots will eventually result in the tearing and deterioration of the lowest leaves, thus making it difficult to accurately determine the growth stage.
Staging corn plants that are beyond V6 is possible but may require some practice to become efficient. Dig a plant and cut the stalk lengthwise through the root area. Check for the first elongated internode, which is usually about one centimeter (0.4 inch) in length. The first node above this internode is generally connected to the 5th leaf. Once the 5th leaf has been determined, then use it as the reference point for counting to the uppermost visible leaf collar.
The use of drop nozzles can limit the risk of injury from certain herbicides, especially as the corn canopy develops. Directed applications help in some instances by keeping the herbicide from being intercepted in the top of the canopy where it can be funneled into the whorl and increase exposure to the growing point. This is particularly a problem with certain sulfonylurea herbicides. In cases involving contact herbicides such as Gramoxone MAX, the directed sprays must be fairly precise to limit the amount of contact with the corn plants.
The recommended timings for several postemergence herbicides used in field corn are summarized in the following table. Always check the product label for specific direction
Timing of Postemergence Herbicides Relative to Corn Growth Stage
HERBICIDE - RECOMMENDED MAXIMUM CORN HEIGHTS / GROWTH STAGE
Accent - Broadcast up to 20" tall or 6 collars (V6). Apply with drop nozzles when corn is between 20" to 36" tall. Do not apply when corn exceeds 30" tall or has 10 or more collars.
Accent Gold - Up to12" tall or 6 collars (V6), whichever is more restrictive.
Aim - Up to 8 leaf collar (V8).
Atrazine - Up to 12" tall.
Basis Gold - Up to12" tall.
Beacon - Broadcast between 4" to 20" tall. After corn is 20" tall or exhibits more than 6 collars (whichever occurs first) use directed applications. Apply before tassel emergence.
Callisto - Broadcast on corn up to 30" tall or up to the 8-leaf stage stage of corn growth, whichever occurs first.
Celebrity Plus - Broadcast 4" to 24" tall.
2,4-D - Broadcast before corn exceeds 8" tall. Use directed applications when corn is >8" tall and before tassel emergence.
Dicamba - At 1 pt/A rate: 5th leaf stage or 8" tall whichever is more restrictive.
[Clarity/Banvel, etc]- At 0.5 pt/A rate: 8 to 36" tall, if 6 leaf is emerging, or 15 days before tassel emergence. Use directed application if: 1) Corn leaves limit spray coverage of weeds, 2) Sensitive plants are nearby, 3) Tank mixing with 2,4-D.
Distinct At 6 oz/A rate: corn 4" to 10" tall; At 4 oz/A rate: corn 10" to 24" tall.
Equip - Broadcast on field corn from emergence up to 12" tall or through V4 growth stage, whichever is more restrictive. Use drop nozzles for applications when corn is 12" to 36" tall (V4 up to V8 growth stage).
Exceed - Broadcast between 4"and 30" tall. To limit injury apply, apply with drop nozzles when field corn is 20" to 30" tall or exhibits more than 6 collars V6 (whichever occurs first).
Expert (RR-corn ONLY) - Up to 12" tall corn.
Gramoxone Max - Apply only as a DIRECTED treatment after smallest corn is 10" tall. Do not apply broadcast over-the-top of corn.
Hornet WDG - Broadcast from corn emergence (spike stage) up to 20" tall or V6 stage, whichever occurs first. Use drop nozzles for corn 20" up to 36" tall.
Liberty (LL-corn ) - Broadcast on corn up to 20" tall or 7 collars (V7), whichever comes first. Apply with drop nozzles for corn 24 to 36" tall.
Liberty ATZ (LL-corn ONLY) - Up to 12" tall.
Lightning (Clearfield-corn ONLY) - Broadcast on corn up to 20" tall; Use drop nozzles if corn is >20" tall or has 6 or more collars (V6), whichever is more restrictive, or if the crop canopy prevents adequate weed coverage. Do not apply within 45 days of harvest.
Marksman - Through fifth leaf stage or 8" tall, whichever occurs first.
Option - Broadcast on corn up to 16" tall or through V5 growth stage, whichever is more restrictive. Use drop nozzles when corn is 16" to 36" tall.
Permit - Spike through layby growth stage.
ReadMaster ATZ (RR-corn ONLY) - Up to 12" tall corn
Roundup WeatherMA Or Glyphosate 4S (RR-corn ONLY) - Through V8 stage or 30" tall whichever occurs first.
Spirit - Broadcast on corn between 4" to 24" tall. Use drop nozzles when field corn is 20" to 24" tall or exhibits more than 6 collars (V6), whichever is more restrictive.
Steadfast - Apply to corn up to 20" tall or exhibiting 7 collars (V7), which is more restrictive.
Steadfast ATZ - Apply to corn up to 12" tall or exhibiting 7 collars (V7), which is more restrictive.
A severe case of the seedling blight phase of Stewart's wilt on field corn came into the diagnostic lab last week. Stewart's wilt is a bacterial disease that causes seedling blight in young plants and leaf blight in older plants.
Symptoms. The seedling blight phase results in stunting, wilting, and death of seedlings and young plants. The leaf blight phase results in long (2-10 inches), linear (1/8 to 1 inch wide) leaf lesions with very wavy margins. At first, lesions are pale green to yellow, but become light brown when they dry. Severely affected leaves are killed. Lesions of Stewart's Wilt are easily confused with lesions on hybrids carrying an Ht2 resistance gene to Northern Leaf Blight. To aid diagnosis of the leaf lesions in older plants , hold leaves to light and look in lesions for scratch-like feeding marks of flea beetle. Infection of seedlings causes rapid wilt and death.
Key features of the Disease Cycle. The bacterium overwinters in the body of corn flea beetle, which also spreads it. Because of our generally moderate winters, which allow survival of the flea beetle, disease pressure is usually high in Kentucky.
The serious outbreak diagnosed last week from Wayne County was somewhat unexpected because it developed in a hybrid of field corn, most of which have significant resistance. However, the corn in this field experienced very extensive flea beetle feeding, with feeding scar destroying about 75% of the leaf surface. Furthermore, an analysis I conducted using a well- known (among plant pathologists) predictive model indicates that disease pressure for Stewart's wilt this spring is high in all areas except Northern Kentucky, where it is moderate. Thus, it is possible that other new corn plantings are experiencing damage from Stewart's wilt. Resistant varieties are recommended for field corn plantings. For sweet corn, see the recommendations given in the Extension publication, ID-36, Vegetable Production Guide for Commercial Growers, 2004- 2005.
Last week's KPN reported forest tent caterpillars causing extensive defoliation in Trimble county. In a follow-up call Trimble Co Ag and Nat Res agent Mike Pyles indicated that the infestation includes an area of significant defoliation along the Ohio River from Madison, IN to Warsaw, KY. This may be due to FTC but several other caterpillars are feeding on trees now, too.
Several caterpillar species were feeding extensively on oaks and other hardwoods last week at sites in Menifee and Bath counties, according to Dr. Lynne Rieske-Kinney UK forest entomologist. These included the linden looper, oak besma, and halfwing and one of the green fruitworm caterpillar complex.
Most of the species encountered have only one generation each year and are near the end of their feeding period. The extensive defoliation seen at some sites will contribute to the stress that many trees already are under as a result of previous droughts.
Looper caterpillars are easily recognized by the reduced number of fleshy legs along the abdomen that causes them to move in a "looping" fashion as they crawl. Their shape and body color causes them to resemble twigs. Linden loopers are exceptions because of the lemon yellow stripe that runs along the sides of the body. The green fruitworm is a plump, light green caterpillar with light speckles over the body.
Annuals and herbaceous perennials transplanted into flower beds add color to many Kentucky landscapes. Unfortunately, when these plants fail to make good growth or die in the landscape, time and money are wasted and the anticipation of colorful splendor is lost. Last summer, some beds of petunias, impatiens, vinca, geranium, begonia, and many other bedding plants died out due to one or more root and stem rot diseases. Most parts of Kentucky had excess rainfall in late spring and early summer last year, just after flower beds were established, thus contributing to loss of plants in the beds. Root and stem rot pathogens, including Rhizoctonia, Thielaviopsis, Phytophthora, and Pythium were often involved, but sometimes abiotic influences made the disease problems worse. Although root rot fungi can be carried from the greenhouse to the landscape, some already living in the bed soil can attack healthy transplants.
Root rots and their effects. Root rot is a general or localized root decay, whether from infectious or non-infectious causes or from natural aging. Root rot begins when cortical cells (outer tissue of the root cylinder) become non-functional or die. As cell death continues, the root becomes discolored brown to black and appears decayed. The cortical cells readily slough off from the rest of the root, the central vascular cylinder (stele). Root branching and growth is curtailed, because dead root tips generate no new roots. In some cases, the root rotting pathogens continue to invade crown and stem tissues, causing crown rots. Significant root rot can occur on some landscape plants without obvious symptoms on the plant's above-ground portion. When an environmental stress is imposed on such plants, they may quickly die from the infection not previously noticed.
Rhizoctonia root and stem rot. Warm, humid weather and warm soils favor this disease which causes affected plants to turn yellow, wilt, and die in the landscape. Plant roots and crowns are infected and decayed by the fungus Rhizoctonia solani, a common soil-inhabiting pathogen. There is little that can be done for infected and decayed petunias or other plants. Before planting, be sure that organic matter in the bed is completely decomposed. If a particular bed had problems last year, try a different species of flower to transplant this year.
Black root rot. Many annual plants are susceptible to black root rot, caused by the fungus Thielaviopsis basicola. This disease has been appearing on bedding plant specimens in the Plant Disease Diagnostic Laboratory more frequently in recent years, especially in spring, and often again in fall. This soilborne fungus attacks many bedding plants including begonia, dianthus, gaillardia, geranium, pansy, petunia, phlox, primula, snapdragon, sweet pea, verbena, viola and many other plants. Roots of diseased plants may have black lesions or blackened root tips. Symptoms in the top of the plant may not appear until the plant is placed under stress and then plants may show yellowing, stunting, dead areas on the leaves, and occasionally wilting or death. Plants provided with excellent growing conditions (temperature, water, drainage, fertility), may show reduced or delayed symptoms. The fungus survives for many years in flower beds due to production of highly resistant chlamydospores.
Phytophthora root rot. This disease is favored by wet soils because the fungus has a swimming stage that aids in dissemination of the disease and because roots submerged in water often attract the fungus and are unable to ward off its advances. Some annual flowers or their cultivars are susceptible while others are tolerant, so expect to see some plants die while others survive under the same conditions. Phytophthora root rot often progresses to the lower stems and there it can girdle and kill the plants. Avoid over-watering annual beds. Soil temperature may have an effect on disease, for example Phytophthora parasitica, prefers warm soils, (in the range of 77 F).
Pythium root rot. This disease is also caused by water mold fungi. One of them, Pythium ultimum, is favored by cool soil temperatures (below 68 F), and by high soil soluble salts such as excess fertilization or leftover winter de-icing salts. Although extra water application will aid in leaching out salts, it must be done in advance of planting, because excess water also favors Pythium diseases. Diseased roots develop brown cortical tissues and the resulting root dysfunction causes stunting and chlorosis of the foliage. Most annual flowers are susceptible to Pythium root rot, especially if they have been exposed to environmental insults such as excess soluble salts.
Abiotic causes of root rot. Root rot may also occur when roots are injured by non-infectious factors such as flooding, drought, freezing, excess heat, excess fertilizer and soluble salts, and toxic chemicals in the soil. When roots are injured, non- pathogenic fungi and bacteria often invade these tissues and lead to a pathogenic root rot syndrome. Thus, when making a diagnosis of the problem, be sure that growing conditions that could have adverse results are known.
Disease management. A combination of cultural practices and avoidance of susceptible plants will be needed to reduce root and stem rot diseases in annual flower beds.
Cicada emergence for Brood X is underway in at least 13 counties across the Commonwealth. You can keep up with the reports by visiting the Entomology Department home page and clicking on Pest Alerts / News.
The following cicada calendar was comes from Cicada Watch 2004 - a web page developed by Dr. Gene Kritsky of College of Mount St. Joseph. It provides a general idea of how the season should unfold.
The first confirmed detection of West Nile virus in Kentucky (May 13) involved a dead dove in Ohio county. Birds and bird- feeding mosquitoes are key players in the epidemiology of this disease. West Nile virus has been detected in at least 138 species of birds in the US. Crows and jays appear to be particularly susceptible, often becoming ill and dying; while most infected birds of other species survive. After WN virus builds up in the bird population, it then can "spill over" into the humans, horses, and other mammals. Water from spring rains that has pooled in low spots or accumulated in tree holes or artificial containers provides prime mosquito breeding sites. When this water is coupled with warmer temperatures, mosquitoes can flourish. Inspect your property regularly for standing water and empty or drain them as feasible. Important vectors of WN virus breed in discarded tires, empty cans and buckets, and roof gutters. Three important steps to protecting yourself are
The Asian tiger mosquito (Aedes albopictus) was accidentally introduced into the US and has spread rapidly throughout the country. It is a dark black mosquito marked with distinctive silver bands on the legs and a stripe down the center of the head and thorax.
First found in central Kentucky in 1987, this species now occurs over most of the state. The larvae or wrigglers can survive in very small accumulations of water (as little as 1/4 inch deep) in artificial or natural containers. Discarded tires are a common breeding site along with aluminum drink cans, rain gutters, and vases. The life cycle from egg to adult can about 2 weeks.
The Asian tiger mosquito has several generations each year and feeds on humans, mammals, and birds. Consequently, it poses a serious potential public health threat. This aggressive day biter is most active in early morning and late afternoon and will enter structure to feed. Usually, adults stay within 100 to 300 yards of their breeding site.
The Asian tiger mosquito is associated with the transmission dog heartworm and is a potential vector of St. Louis and LaCrosse encephalitis. It has tested positive for West Nile virus in the field.
Samples containing one to several specimens of strikingly colored caterpillar hunter beetles have come in from several counties this week. The beetles are 1 to 1- 3/8 inches long with a purple/blue sheen on the sides of the head and thorax. The wing covers are metallic green with red margins, and have fine grooves running from front to back. In some places they are accumulating in large numbers - 100 or more!
These beetles have large jaws on the front of the head and could pinch the skin if handled but are not dangerous. As the name implies, they feed on a variety of caterpillars and are probably abundant as a result of high caterpillar populations in many of Kentucky's forests over the past few years.
Caterpillar hunters are fierce predators, often climbing trees in search of food. This member of the ground beetle family prefers cool, damp places to live, and so is often found on the ground under rocks, logs, leaves, bark, decomposing wood and other debris. The life cycle of the caterpillar hunter (egg, larva, pupa, adult) usually takes a year. Adults may live up to two or three years.
An outbreak of necrotic ring spot was diagnosed on Kentucky bluegrass last week. The occurrence of dry soils last week allowed symptoms of this root disease to manifest themselves. The disease appears first as bluish-green, wilted grass in patches one-half-foot to three feet in size. These patches quickly become brown as the wilted grass dies. Often, a tuft or patch of healthy turf is present in the center of affected patches, giving the turf a "donut" or "frog-eye" appearance. Below ground, roots of affected tillers have a light brown to dark brown decay. Symptoms are most severe in the driest parts of the landscape, such as on knolls or slopes. Although symptoms of necrotic ring spot generally develop in early summer, they can persist throughout summer until cool weather allows the turf to begin to recover.
Management Suggestions. Control thatch buildup. Avoid high nitrogen fertility, particularly in spring and summer. Irrigate to prevent drought stress. Although deep and infrequent irrigation is recommended for management of most turf diseases, light and frequent irrigation can promote survival after an outbreak of necrotic ring spot, since the disease results in a shallow root system. It may also help to apply this irrigation during the hottest part of the day. Maintain a mowing height no lower than two inches. Overseed affected areas with perennial ryegrass or renovate with resistant varieties of Kentucky bluegrass or with tall fescue.
I discourage the use of fungicides for control of this disease in almost all settings. However, for those interested, fungicide options are available in the Extension publication, PPA-1, Chemical Control of Turfgrass Diseases.
Recent samples diagnosed have included Stewart's wilt on corn; barley yellow dwarf virus on oat and wheat; powdery mildew on wheat; Sclerotinia collar rot, damping off (Rhizoctonia), Pythium root rot, growth regulator injury, Terramaster phytotoxicity and fertilizer burn on tobacco.
On fruits and vegetables, we have seen Mycosphaerella leaf spot on strawberry; leaf curl on peach; fire blight, scab and frogeye leaf spot on apple; Pythium root rot on muskmelon; and southern blight on tomato - which is very early for this disease to develop in Kentucky.
On ornamentals and turf, we have seen Phoma stem blight on vinca; anthracnose on liriope; black root rot on holly; scab and frogeye leaf spot on crabapple; anthracnose on ash and maple; Phloeospora leaf spot on mulberry; Ramularia leaf spot on persimmon; spring dead spot on bermudagrass; and red thread on fescue turf.
UKREC-Princeton, KY, April 16 - 23, 2004| Black Cutworm
| 10
| True Armyworm
| 10
| Corn Earworm
| 2
| European Corn Borer
| 2
| | |
Cam Kenimer reports the following True armyworms trap counts for Fulton County: April 21- 28 - 0 April 28- May 5 - 70 May 5 - 12 - 45
Current trap counts for Fulton County, Kentucky and click on "Insect Trap Counts".
For information on trap counts in southern Illinois visit the Hines Report. The Hines Report is posted weekly by Ron Hines, Senior Research Specialist, at the University of Illinois Dixon Springs Agricultural Center
NOTE: Trade names are used to simplify the information presented in this newsletter. No endorsement by the Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named.
Lee Townsend
Extension Entomologist