Inspector Findings in Kentucky

Volume 1 Issue 6

University of Kentucky - Department of Entomology

September 1997

Carl Harper -NURSERY INSPECTORS- Joe Collins

Reminder: Nursery license fees were due July 1. Most nurseries have paid but there are a few who still have not. Have you paid yet?
Last Issue
Pest Activity Seen in August
Look for Locust borers to begin appearing on goldenrod
Managing the Greenhouse Environment to Control Plant Diseases
Botryosphaeria Canker of Redbuds
Using Oils for Insect Control
Locust Leaf Miner
Maple Tar Spot is Visible, but not Very Damaging
Review of Rootball Specifications


Last Issue

This will be the last issue of Inspector Findings this year. We hope that you have found that the information to be useful to your operation.
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Pest Activity Seen in August

Redheaded pine sawfly Spindle gall-birch Japanese beetles
Mimosa webworm-honeylocust Bagworms Cercospora leaf spot-Honeylocust
Cottonwood borer Septoria leaf spot-River birch Apple scab-crabapple
Azalea lace bugs Spider mites-various plant spp. Hawthorn & quince rust
Powdery mildew-dogwoods, saucer magnolia

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Look for Locust borers to begin appearing on goldenrod

As the goldenrod begins to bloom this fall, you may see locust borers crawling around on the blooms. The adult beetle belongs to the family of long-horned beetles, so named for their long antennae. The beetles are feeding on the pollen of the goldenrod which is needed for oviposition.

Adult females lay eggs in and on the bark of black locust where the young larvae hatch and overwinter. In the spring, the larvae feed in the bark of black locust, and frass may be seen coming from wounds. In mid-summer the larva pupates in its burrow and emerges as an adult at the time of goldenrod bloom.

Black locust trees are highly susceptible to attack by this borer until they are 6 inches or more in diameter. As the bark thickens, larval survival is lower and the trees should escape further injury.
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Managing the Greenhouse Environment to Control Plant Diseases

By Brian Eshenaur and Robert Anderson*
Numerous plant disease problems can arise in greenhouse situations. These diseases can cause extensive damage if allowed to develop unchecked. Since plant diseases are strongly effected by temperature and humidity, the best way to combat disease is to manipulate the greenhouse environment. Unlike the weather outdoors, we can control the greenhouse environment.

Plant disease control in the greenhouse is generally more effective if the following aspects of the greenhouse environment are managed properly:

Humidity

High humidity levels encourage the development of many plant diseases. The relative humidity is usually 25-70% during the day in greenhouses and generally no problem. However, humidity levels are generally 90-100% during the night. During periods of rainy weather in winter, the relative humidity may stay near 100% for a number of days and nights.

Maintain adequate plant spacing. When plants are crowed together, disease development is encouraged by the high humidity in the canopy. Plants hung overhead reduce normal water evaporation and contribute to high humidity in the crop canopy.

Maintain air circulation during periods of high humidity. Most greenhouses are equipped with air circulation, fan-jet or horizontal air flow systems. These systems should operate continuously when high humidity occurs in the greenhouse, i.e., every night of the year and during rainy overcast days.

Ventilate the greenhouse to reduce internal relative humidity. Most winter evenings are cool enough to raise the humidity to 100% and cause considerable condensation in the greenhouse. This condensation can be reduced if the greenhouse is ventilated at dusk each day. In the late afternoon, turn on the ventilation fans to exhaust the warm moist air from the greenhouse. The warm moist greenhouse air is replaced with cool/cold, moist air from outdoors. When this outside air is heated in the greenhouse, it becomes much drier than the previous greenhouse air. This management practice greatly reduces the relative humidity in the greenhouse and reduces potential disease problems.

Watering

The risk of plant diseases is reduced when the foliage and flowers are kept dry. Most disease organisms need water on the plant surface for normal growth. Additionally, splashing water is the primary method of spreading disease from plant to plant.

Apply water only to the growing medium surface, when possible, rather than “showering” the whole plant. Water early in the day; do not water after 4 p.m.. except in summer.

Watch plants closely and water judiciously. Remember that with variations in light levels, temperature and humidity, the plants need for water will change. Watering practices should be attentive to these changes. Plants beneath overhead plants will not dry as quickly as neighboring plants in full sun.

Water thoroughly and do not water again until the growing medium is dry. Excess water in the growing medium weakens plants and makes them more susceptible to damping-off and root rotting diseases. Good drainage and the related good aeration of the growing medium is important to prevent root rot problems. Separate weaker plants that do not use water as fast as the strong plants; they will not be overwatered as easily.

Be sure to use a good growing medium. Professionally prepared media are generally the most effective and are recommended to commercial growers. When using mineral soil in a mix, be sure it is adequately sterilized. Never use more than 15% mineral soil in a growing media that will be used in containers less than 4” tall.

Sanitation

Keep the greenhouse clean and free of plant debris and outside soil.

Remove dead leaves, flowers, plant refuse and weeds from the greenhouse. Debris should be gathered regularly and discarded as soon as possible after collecting. Immediate removal is important because certain fungal pathogens can develop and produce spores on the plant debris. Weeds along walkways and under benches can harbor diseases that can be transmitted to greenhouse crops.

Non-sterile soil from outdoors should not be allowed into the greenhouse. Since many soilborne pathogens can be a serious problem if introduced into the greenhouse, the best control method is to exclude these pathogens from the greenhouse. This can be accomplished by using a soilless medium which is free of plant pathogens.

Surface sterilize all work surfaces and tools regularly to prevent accidental disease spread. Diseases are often transferred in the greenhouse through used pots, dirty tools, messy work areas, unswept floors, etc. Precautions should be taken to prevent this. Surface sterilization of tools and work surfaces in and around the greenhouse can be accomplished by washing with a dilute solution of household bleach (9 parts water to 1 part bleach) or the use of commercial disinfectants such as Greenshield and Physan.

*Dr. Anderson is Extension Floriculture Specialist. Brian Eshenaur was the plant diagnostician in the UK plant disease laboratory. He has since left this position to take another position in New York. His replacement is Julie Beale.
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Botryosphaeria canker of redbuds

Botryosphaeria canker is commonly found associated with annual or perennial cankers and dieback in wounded plants or in those stressed by drought, freezing, or defoliation. It also colonizes dying or freshly killed twigs and branches that it has not previously infected, such as those pruned from trees.

Botryosphaeria is the most destructive disease of redbud although it also attacks other woody plants. In landscapes and forests, it causes cankers and dieback of sweet gum, leaf blight and dieback of rhododendron, and cankers and dieback of dogwood, elm, linden, redbud and sycamore.

On redbud, multiple cankers of varying size develop on branches and sometimes on the trunk, eventually encircling and killing the infected parts. This process may occur during or between growing seasons. Leaves on some branches wilt, and on other branches the buds simply fail to open in spring. Cankers are usually centered on twig or branch stubs and sometimes become elongated. The killed bark becomes depressed, roughened, and darkened where fungal stromata break the surface. An old canker may be surrounded by a conspicuous callus ridge. The wood beneath a canker is discolored brown, and this discoloration extends above and below the canker.

For control of botryosphaeria canker, prune and burn branches showing cankers. Surgical excision of cankered tissue on the main stem is occasionally successful if all the infected bark and wood is removed. Periodic application of a copper fungicide during the growing season may help to prevent new infections.
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Using Oils for Insect Control

excerpt from Ohio State University Bulletin 504 “Insect and Mite Control on Woody Ornamentals and Herbaceous Perennials”.

The use of oil to kill insects and mites has been known to work since the 1700’s. However, the early use of oil usually resulted in killing the plants as well as the insects. Oil came into widespread use to control insects and mites after oil refining techniques were developed which would free the petroleum oils of unsaturated hydrocarbons, acids, and highly volatile elements. Oil is effective against insects and mites because it suffocates or causes cell membrane destruction of the pests that it hits as well as their eggs. Another advantage of oil is that no pest has been known to become resistant to its killing action.

At least three different types of oil are used for pest control: petroleum, summer or horticultural oil, petroleum dormant oil and citrus oil.

Petroleum summer or horticultural oil is a lighter weight oil applied during the active growth of a plant, when green plant foliage is present.

The dormant oil is usually defined as a heavier weight oil applied in spring prior to bud break or in the fall after leaf drop.

Citrus oil is usually added to other pesticide formulations such as soaps and botanical pesticides.

What makes the identification of oils confusing is the fact that summer oils can be used as dormant oils. However, do not use a dormant oil as a summer oil. The simplest method for identifying oils is to read the label. If the label only mentions usage on dormant plants, it is a dormant oil. On the other hand, if the label mentions using the oil on green, leafy plants during the growing season, it is a summer oil.

If you don’t want to rely only on the label instructions, there are three oil factors which you need to evaluate: 1) oil volatility, 2) oil viscosity, and 3) the unsulfonated residue rating.

Volatility is measured by the distillation temperature. This is the temperature that the oil comes out of heated crude oil at the refinery. A low distillation temperature produces a light oil. A high distillation temperature produces a heavy oil.. The lighter oils evaporate faster and thus have less of a chance to cause plant damage (phytotoxicity). The heavy oils may coat the plant and either smother the leaves or destroy some of the cells. The result is phytotoxicity. The distillation temperature is probably the most important number to look for on the label.

Volatility and Oil Usage
Distillation temp. Primary use Rate/100 gal
412°F Summer 1.5-3 gal
435°F Summer/Dormant 1.5-2 gal summer
3-4 gal. dormant
438°F Dormant 2-4 gal.

Viscosity is the flow rate or thickness of an oil. It is measured by the time it takes a volume of the oil to flow through a small funnel opening. The label may say the that the oil is a 60 second or 100 second oil.

The unsulfonated residue (UR) rating is an index of the quantity of oil free from unsaturated hydrocarbons. Look for oils with a minimum UR of 92%. Some oils are as high as 99% free.

Some of the better oil, especially summer oils, have distillation temperatures of 412°F and UR = 96%.

Even with the best oils, phytotoxity is always possible. However, this is the same as with standard insecticides. The following guidelines are recommended by most users and manufactures of dormant and summer oils:

1) Do not apply the oils when the temperature is below 40°F or above 100°F. If low humidity is accompanying the high temperature, oils have less of a chance of causing damage.

2) Do not apply oils if rain is a possibility or if the plant tissues are wet. The leaves must be dry and the oil must have a chance to evaporate.

3) Avoid spraying or getting drift on oil sensitive plants.

4) Apply the oil according to the label rates. Always go light, not heavy.

5) When using dormant oils or high rates of horticultural oils, do not spray when plant buds are fully open and shoot elongation is occurring.

6) Do not spray plants when the humidity is expected to remain above 90% for 36-48 hours.

7) Leaf drop in the fall is not a reliable method for determining plant dormancy. It is better to wait until after several light frosts.

8) Oil sensitive plants are: maples, hickories, black walnut, cryptomera, smoketree and many azaleas.

9) Plants tending towards oil sensitivity are: beech, Japanese holly, redbud, Savin junipers, Photinia, spruces and Douglas-fir.

Most of the oil labels contain a list of plants which are sensitive or tend towards sensitivity. Read them carefully. Most problems occur when oils are simply sprayed on everything in the landscape.

As with all pesticides, use oils only where needed. They do not need to be used as general cover sprays. Plants which have a history of aphid, scale and/or mite problems are the best ones to target.
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Locust Leaf Miner

By Monte P. Johnson, Extension Entomologist
It seems like most years we will see black locust leaves turning brown about this time of the summer. In some years, such as this one, it is quite noticeable. The culprit that typically is causing this unsightly problem is the locust leafminer, Odontota dorsalis. In its adult stage, this insect is a 1/4 inch-long beetle that is brownish-orange with a black stripe down the back and black legs, head and antennae. After overwintering as an adult, the beetles emerge and start feeding on black locust developing foliage, and soon will lay eggs on the undersides of the leaves. Flattened, yellowish-white larvae can be seen tunneling inside the leaves forming a mine that looks like an irregular blotch. After pupating in the mine, adults emerge and begin to skeletonize the undersurface of the leaves and will lay eggs for a second generation. The combined feeding of larvae and adults may destroy much of the foliage and result in seriously stressed trees. Other host plants listed for this beetle include false indigo, bristly locust, Sophora japonica, and golden chain tree. Adults will feed on a variety of foliage including dogwood, elm, oak, beech, cherry, wisteria, hawthorn as well as several herbaceous plants. If control measures are needed, a variety of insecticides are labeled for this pest, including Dursban, Lindane, and Talstar. Timing is important to control the adults and larvae before they enter the leaves.
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Maple Tar Spot is Visible, but not Very Damaging

by John Hartman, Extension Plant Pathologist
Tar spot is visible on maples in Kentucky now. Some people worry when they observe thick, crusty black splotches and spots on the leaves of their maple trees. The leaves of red, silver, and sugar maples may show symptoms and signs of either of two species of Rhytisma, cause of tar spot disease. Tar spot begins in early summer as light green or yellow spots on the leaves. The disease becomes very noticeable in late summer when the raised, tar-like fungal stroma develop on the upper surface of the leaves. R. acerinum, found more frequently in Kentucky, causes a black splotch approximately 1/4 - 3/4 inch across, while a related fungus, R. punctatum, appears as a cluster of tiny black spots.

New infections begin from spores produced on fallen leaves that were diseased last season. There apparently is no secondary cycle of disease during the growing season.

Withered leaves with many spots may drop from the tree, but normally they are too few and appear too late to affect the health of the tree. Thus, raking up and destroying diseased leaves, the source of primary inoculum for next year, should be all that is needed to manage tar spot. It is possible that one might worry if tar spot disease disappeared from the landscape. R. acerinum is reported to be sensitive to low levels of sulfur dioxide air pollution, thus tar spot is uncommon in certain urban and industrial areas.

Other species of Rhytisma may cause tar spots on tuliptrees, hollies, and willows. I have noticed that tar spot occurs in England on sycamore maple and in France on willows where it causes a fairly substantial blotch.
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Review of Rootball Specifications

The American Nursery & Landscape Association - “American Standard for Nursery Stock” and the Michigan State University web site
Fall is the time when many nurseries start to dig nursery stock, the majority of which are harvested by the balled and burlapped (B & B) method. B & B plants are dug and moved with the soil in place around the roots. Trees that are greater than 1 inch in trunk diameter and most evergreens are usually moved and planted B & B. The ball sizes in Table 1 are recommended by the American Nursery and Landscape Association. Trees that are difficult to move--such as beech, hickory, hornbeam, sassafras, sweet gum, tupelo, walnut and white oak--need larger root balls than trees that are easy to move. Trees growing in loose, well-drained soil, such as a sandy soil, will have more extensive or spreading root systems than trees growing in a hard, poorly drained soil such as a tight clay.

Cultural practices by the nurserymen, such as root pruning, irrigation, fertilization, root-ball configuration, and digging techniques, influence the percentage of harvested roots. Water stress, due to removal of most of the water-absorbing roots, is the primary cause of transplant failure. Most water absorption capability within a transplanted root-ball results from very small diameter roots. These fragile roots are the first to suffer from excess water loss in newly transplanted landscape plants.

A major advantage of "balled and burlapped" (B & B) plants is that soil types can be matched, thereby reducing any interface problems that might inhibit water movement between the rootball and surrounding soil of the landscape site. There is an acceptable, standardized formula for sizing rootballs, which is the American Standard for Nursery Stock. The diameter of the circle and depth of the trench are listed in Table 1.

Horticulture researchers have estimated that up to 75% of the roots may be lost when digging field-grown nursery stock which is a major disadvantage. The amount of roots harvested depends upon soil type, irrigation practices and root pruning during the production period. Plants moved B & B are subject to seasonal constraints. The most favorable seasons are when transpiration demand is low and root generation potential is high, such as in fall, winter and early spring. With the much-reduced root system, water is a critical element in the successful transplanting of B & B material.

Table 1. Root Ball Sizes for Plants Dug Balled and Burlapped.
Trees

Small

Flowering Trees
Minimum Diameter Height/Caliper Minimum Diameter
Caliper Ball Depth Ball Depth
Inches Inches Inches Inches Inches
1/2 12 9 2 ft. tall 10 8
3/4 14 11 3 ft. tall 12 9
1 16 12 4 ft. tall 14 11
1 1/4 18 14 5 ft. tall 16 12
1 1/2 20 14 3/4 in. cal. 16 12
1 3/4 22 15 1 in. cal. 18 14
2 24 16 1 1/2 in. cal. 20 14
1 3/4 in. cal. 22 15
2 in. cal. 24 16

Source: American Nursery and Landscape Association

Correct planting technique begins with the loading of the plant. Always protect the roots, stems and foliage during transport. The plant tops should be shielded from winds. Never pick up a plant by the trunk. Trees are particularly vulnerable to damage if growth has started, especially in the spring. B & B trees are very susceptible to this type injury because of the weight of the rootball. Lift plants from underneath the rootball with the appropriate equipment. If plants must be held or stored on the landscape site, it is best to place them in a location protected from the wind and sun. Do not let the roots freeze or dry out during this time. If the delay in planting is more then a few days, one should "heel in" B & B material by covering the roots with bark or some other mulch. Supplemental irrigation is critical for the nursery stock during the growing season.

For more information on correct ball diameters for shrubs and other plant material, consult the American Nursery and Landscape Association publication “American Standard for Nursery Stock”.
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