Pests to watch for in May
| Bronze birch borer | Dogwood borer | Potato leafhopper | Azalea lacebug |
| Pine needle scale crawlers | Holly leafminer Egg Laying | Euonymus scale crawlers | Flatheaded apple tree borer |
| Redbud leafhopper | Boxwood leafminer | Calico scale | Juniper scale |
| Lilac/Peachtree borer | Lesser Peachtree Borer | Twolined chestnut borer | Bagworm hatching |
Nursery Inspections
Our nursery inspections will be beginning again later this month. Along with inspecting nurseries, we will also be visiting some of the nursery dealers across the state.
Spring Diseases of Pines for Christmas Trees
by John Hartman, Extension Plant Pathologist
Scots pines and white pines are commonly grown as Christmas trees; occasionally trees in these plantations develop infectious diseases. Be looking now for symptoms of some of these diseases and be prepared to take action as needed.
Scots pine needle casts. There are three important needle cast diseases of Scots pines grown as Christmas trees in Kentucky. In general, the needle cast diseases caused by parasitic fungi begin with infections during moist periods;
needles then develop dead spots or bands a few weeks or months later. Within a few more months, needles turn brown and drop from the tree, leaving only the newest growth unaffected.
Since the weather last spring and early summer were very moist, infections of Naemacyclus needle cast caused by Cyclaneusma minus may have occurred. Growers should now be observing that needles are turning brown and dropping, indeed they have been turning brown and dropping over the entire winter. These dead needles are producing spores for new infections now, so if fungicides are to be applied, they are needed now through June.
Needles with the diseases brown spot needle blight caused by Mycosphaerella dearnessii and needle cast caused by Lophodermium seditiosum are not turning brown yet, but they do have brown spots on them. These needles are due to turn brown and drop in summer. Brown spot infections occurred last year in late spring and early summer while Lophodermium needle cast infections occurred in late summer and early fall. Growers need to know which of the three diseases is most important in their plantations so they can time their fungicide sprays to prevent infections by the needle cast fungi.
Needle cast disease management: a) Cut and remove severely infected trees. b) Remove and destroy live, infected branches on stumps of harvested trees. c) Shear healthy plantings first to reduce movement of fungal spores. d) Do not shear infected foliage during wet weather. e) Remove weeds from the planting to promote better air movement. f) Remove old diseased Scots pines from windbreaks or fence rows. g) Plant disease-resistant types, avoiding short needled French and Spanish varieties. h) If fungicides are needed, apply them at the proper time to get best results.
White pine root decline. Now is the time to observe early symptoms of white pine root decline, caused by the fungus Verticicladiella procera. As new candles are elongating, look for individual trees with delayed candle emergence. These trees are already infected and will soon begin to turn yellow, then brown as the trees die. Growers may be able to see that the shoot growth for the last year or two has been reduced compared to healthy trees. Clear away the grass around the base of the trunk and look for white patches of pitch or resin on the surface of the bark. Symptoms of discolored inner bark and cambium and even dark streaks in the wood may be revealed by cutting away the bark on the trunk. We have found in Kentucky that white pine root decline is often associated with wet areas in the field.
White pine root decline disease management. a) Remove and destroy infected trees. b) Grow white pines in good growing sites.
Controlling Insect Pests of Herbs
by Ric Bessin, Extension Entomologist
Unlike many bedding plants grown in nurseries, herbs are generally more resistant to insect pests. In fact, many herbs have been grown for years as companion plants to repel insect pests from more susceptible garden plants. Generally, herbs emit volatile compounds that reduce their attractiveness to many insect pests. However, herbs are not pest-free, and occasionally they do need some attention to control pests. This article will discuss cultural practices and available chemical controls to reduce the incidence of the most common insect and mite pests.
Slugs
Slugs and snails can become greenhouse pests when the humidity is high. Slugs are fleshy, slimy animals that feed mainly at night. They prefer cool moist hiding places during the day. Slugs rasp on leaves, stems, flowers, and roots. They produce holes in the leaves or just scar the leaf surface. Small seedlings are especially vulnerable to these creatures. Silvery slime trails are evidence of snail and slug infestations.
While little can be done to protect individual plants from slugs, there are some cultural practices that can be used under benches. This first step in reducing a slug problem is through sanitation. Old boards, plant debris, plastic, and used potting supplies provide ideal places for slugs to hide during the day. These items should be removed from the greenhouse. Slug baits containing metaldehyde may be used for slug control only underneath the benches.
Aphids
Aphids or plant lice are small, soft-bodied, sluggish insects that cluster together in colonies on the leaves and stems of the host plants. They are usually found on and under the youngest leaves. Aphids are the only insects that have a pair of cornicles, tubes that resemble exhaust pipes, on their abdomen. Aphids multiply rapidly; in greenhouses, each one is a female capable of giving live birth to daughters in about 7 days after its own birth. These asexually reproducing female aphids may be winged or wingless. Adult aphids can give birth to 6 to 10 young per day over their 20 to 30 day life span. Enormous populations can build up in a relatively short period.
Several species of aphids can be serious pests in the greenhouse. Control of aphids on herbs is more difficult than on ornamental plants because the only insecticides that can be used on herbs that is effective against aphids are insecticidal soap and pyrethrum sprays. For that reason, great care must be taken to avoid aphid problems. It is best to grow herbs in an isolated section of the greenhouse, free of other plants that may harbor aphids. If aphids are detected, two to three applications of insecticidal soap 3 to 4 days apart may be needed. Sprays should be directed toward the undersurfaces of the leaves.
Cabbage Loopers
The key to managing cabbage loopers is regular monitoring of the plants. Look for looper feeding on the edges of the leaves. Security lights left on at night near the greenhouse may attract some of these night-flying pests. If loopers are a problem, a spray containing Bacillus thuringiensis will control the larvae.
Spider Mites
Mites are sap-sucking pests that attack a wide range of greenhouse plants. These mites feed by piercing tissue with their mouthparts and sucking out cell contents. Two-spotted spider mites are light to dark green with two distinctive black spots on the abdomen. Eggs are spherical and clear when first laid. After hatching, the larva has three pairs of legs, but later stages will have four pairs. Males are smaller with more pointed abdomens than females. Heavy infestations of the two-spotted spider mite produce fine webbing, which may cover the entire plant. Generally, they feed on the undersides of leaves, giving the upper leaf surface a speckled or mottled appearance. Leaves of mite infested plants may turn yellow and dry up, and plants may lose vigor and die when infestations are severe. Females can lay 200 eggs, and during hot dry weather the life cycle may be completed in 7 days.
Mites can easily be moved to infested plants on clothing, so always examine infested benches and other hot spots last during greenhouse inspections. Often, it is better to discard infested plants than to attempt to control the problem with pesticides. If control is attempted, isolate the infested plants to reduce potential spread. In terms of sprays to control mites on herbs in the greenhouse, only insecticidal soap is available.
Thrips
Thrips are slender, tiny insects about 1/25 inch long and range in color from light brown to black. They have 4 wings, each fringed with a row of long hairs, which are held flat over their back. Plant feeding thrips cause economic damage when they infest the flowers, buds and young plants. Thrips feed by rasping the plant surface and sucking up the exuding sap. Heavily infested leaves have a mottled or silvery appearance. Female thrips insert eggs into slits in the leaf. Eggs hatch in 2 to 7 days. Nymphs feed similar to adults and molt 4 times during development. They are inactive during the last nymphal stage prior to becoming an adult.
Winged adults are carried into the greenhouse on contaminated plant material or fly in during the summer and continue to breed throughout the winter. Prevention of infestations through the use of screens on ventilators, inspection of new material entering the greenhouse, and weed control in the greenhouse will help to manage thrips. As with aphids, insecticidal soap and pyrethrum sprays are the only insecticides for use on greenhouse herbs that are effective against thrips.
Dogwood Powdery Mildew Fungicides
by John Hartman, Extension Plant PathologistIn recent weeks, there have been many questions posed by Extension Agents and landscape managers about dogwood (Cornus florida) powdery mildew disease.
During the past four years, we have seen an increase in the levels of powdery mildew of dogwoods in Kentucky landscapes. In contrast to dogwood anthracnose which is typically only found in the most heavily shaded landscapes, powdery mildew is affecting open-grown and partly shaded dogwoods. In addition to the white powdery growth of the fungus on the leaf surface, infected leaves exhibit marginal scorch, dead patches, reddish discoloration, yellowing, and premature defoliation. Most powdery mildews of landscape trees occur late in the summer and are therefore of little consequence, but dogwoods become infected in early summer. Since powdery mildew is often devastating relatively early in the season, dogwood growers need to know if fungicides can be used to assist in managing this disease.
Cultural practices. Cultural practices that enhance the health of dogwood trees are necessary for a fungicide spray program to be effective. If new dogwoods are being established, good site selection (with well-drained and well-aerated soil and a location exposed to morning sunlight and partial shade in the afternoon), good plant selection (healthy plants from a reputable nursery), and proper planting are essential. Good tree maintenance practices should include mulching to avoid unnecessary trunk and branch injuries from lawn mowers and string trimmers, irrigation (but not over-watering) during dry periods, pruning out dead and rubbing branches, and pruning back overhanging branches from nearby trees to allow better sunlight penetration and air movement. If dogwoods are being replaced, consider resistant dogwoods such as oriental (C. kousa) dogwoods. The C. florida cultivar 'Cherokee Brave' has partial resistance; all other C. florida dogwoods are susceptible.
Fungicides. If fungicide applications are considered, be sure that the dogwood is a valuable specimen and that spray equipment can provide good coverage. Effective fungicides for dogwood powdery mildew include: Banner MAXX (propiconazole), Bayleton (triadimefon), Cleary's 3336 (thiophanate-methyl), Eagle (myclobutanil), Immunex (the homeowner formulation of propiconazole), Immunox (the homeowner formulation of myclobutanil), and Rubigan (fenarimol). First sprays should be applied in early June and continued once every two to three weeks until mid-August.
A note on Immunex and Immunox: Immunex (propiconazole) is no longer being manufactured. Immunox, (myclobutanil), which has replaced Immunex, at first contained propiconazole, but the formulation has been changed so that it now contains myclobutanil. It is possible that all three products are in the garden stores, but new supplies would be Immunox (myclobutanil).
Two New Maples Resist Bugs, Cold
Breeders and nursery operators looking for a better maple tree may want to climb aboard the Red Rocket. Or, they can head for the New World. Red Rocket has fiery-red leaves and outstanding cold and disease resistance. New World is an orange-red maple tailor-made for city landscaping. Red Rocket thrives in USDA growing zone 3, where temperatures can go as low as -40°F. Columnar shape and cold resistance make Red Rocket an ideal line of defense against wind and weather around barns and livestock shelters. It would also work well as a shelter and screen around picnic areas and industrial sites. New World can thrive in zone 4, where temperatures can drop to -30°F. Unlike most maples-and somewhat like an American elm-New World sends its branches up, then out. This shady character, along with cold resistance, makes it ideal for city streets and residential neighborhoods in the Northeast and Midwest. Nurseries and breeders can request cuttings of the new cultivars from the U.S. National Germplasm System. Some wholesale nurseries may offer the trees by 2000. Alden M. Townsend, USDA-ARS U.S. National Arboretum, Glenn Dale, Maryland; phone (301) 344-4175, email nadt@ars-grin.gov/.
This article was published in the April 1998 issue of Agricultural Research, U.S. Department of Agriculture, Agricultural Research Service.
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Thistle Head Weevil Survey Update
by Lee Townsend, Extension Entomologist
Established populations of the thistle head weevil, Rhinocyllus conicus, were discovered in Fayette and Jessamine counties in 1988. Over the past 10 years, weevils have been collected and released at many new sites. These efforts along with natural spread, have extended the range of this beneficial weevil in Kentucky. Most recently, a cooperative program with the Animal Plant Health Inspection Service (APHIS) has allowed a great expansion of the survey and release effort.
We now have records of weevil establishment in the following counties- Anderson, Bath, Bourbon, Butler, Clark, Carroll, Christian, Estill, Fleming, Franklin, Gallatin, Hardin, Harrison, Hart, Henry, Jessamine, Lewis, Lincoln, Logan, Madison, Mercer, Montgomery, Nelson, Nicholas, Owen, Scott, Shelby, Simpson, Trimble, Warren, and Woodford.
Spring Plant Disease Progress
by John Hartman, Extension Plant Pathologist
During the weekend of April 18-19, prolonged leaf wetness set the stage for infectious diseases of fruit crops and of landscape plants. In addition, symptoms are now especially noticeable on plants injured by the cold temperatures we experienced March 12.
Apple and Flowering Crabapple Scab. Almost continuous leaf wetness for up to 24 hours (broken only by brief periods where leaves were not actually wet, but humidity was 100 percent) in many locations this past weekend has provided conditions essential for primary infections by the apple scab fungus, Venturia inaequalis. With temperatures averaging 45-50 degrees F, leaves become lightly infected with only 15 hours of leaf wetness. Most areas had 20 hours of leaf wetness, enough for moderate infections. Severe infection levels would have occurred with 30 hours of continuous leaf wetness. If normal temperatures prevail during the coming weeks, expect scab symptoms to appear in 14-15 days. Look for superficial, velvety olive-green leaf spots on apples and crabapples. The spots later turn dark with uneven edges.
Cedar Rusts. All three rust diseases caused by Gymnosporangium--cedar-apple, cedar-hawthorn, and cedar-quince--have been very active on cedars and junipers during the past week. The decorative bright orange show this spring has been spectacular; this follows the relatively wet July and August of 1996 when cedars which are now showing symptoms became infected. Given our temperatures this past weekend, only 6-7 hours of leaf wetness would have been needed for rust infections to occur on crabapple, hawthorn, and apple. Look for earliest symptoms of cedar-quince rust (bright orange spots) on hawthorn flowers after hawthorns are in full bloom. On apples, tiny orange spots of cedar-apple rust will appear on the upper surface of leaves in a few weeks.
Fire Blight. Primary infections of apples and pears occur only during bloom. Weather has generally been cool enough statewide that fire blight infections have not likely occurred. In some locations, there may have been one infection period; apples still in bloom are still vulnerable to primary infections until petal fall.
Flowering Pear and Crabapple Injuries. Cold March temperatures occurred just as flowering pears were about to bloom. Not only was the flower show cut off, it appears that for some trees at least, injuries to the cambium and phloem tissues caused a major dieback of twigs and branches. Some trees have about half of their branches dead, and on many of these, the only growth that is occurring is back on the trunk and main limbs. Most cultivars of flowering crabapples failed to produce a full bloom, and some lost twigs and small branches to the cold. In a few weeks, new growth will mask the crabapple twig dieback.
Juniper Injuries and Diseases. We are seeing many specimens of the low-growing junipers with significant browning over the foliage. In many cases, only the interior foliage is affected while tips are green, but in other cases, all foliage is brown. These symptoms are also likely due to cold injury. Bear in mind that there are also many junipers with mostly green foliage, but only the tips are now dying back. The cause of tip dieback (not whole plant or interior foliage) symptoms occurring now is Kabatina tip blight.
Pine Tip Blight. Recent prolonged wetness favored infections of Austrian and Scots pines by the tip blight fungus, Sphaeropsis sapinea. First new symptoms will appear next month as brown, stunted needles.
Leaf Burn and Dieback of Spreading Euonymus. Leaves of this semi-evergreen shrub turned brown soon after the March cold episode. Now, new leaves of some plants are only emerging from the base, indicating that stems were also killed.
Other Injuries. Plants infected with phytoplasmas are often regarded as more sensitive to winter cold injuries. Look for poor foliage development on ash trees infected with ash yellows disease. Leaves on mulberries with excessive witches brooms (a symptom of diseases caused possibly by phytoplasmas) are appearing only on the main limbs and the trunk, indicating that twig and branch tissues have died from cold temperatures.
Cankerworms
by Lee Townsend, Extension Entomologist
Most caterpillars have five pairs of prolegs (fleshy abdominal legs) including the pair at the tail end. The spring cankerworm has two pairs of prolegs and the fall cankerworm has three. This reduced number of legs causes the caterpillars to crawl with a looping or inch-worm type movement. When pale stripes are distinguishable along the sides of the body, the spring cankerworm has one per side and the fall cankerworm has three. The color of both cankerworms is variable.
The eggs of both species hatch in early spring, about the time tree leaves are unfolding. Cankerworms feed for three to four weeks, then either crawl or drop to the ground on silken threads and pupate in the soil. Fall cankerworms emerge as adult moths in late fall. The wingless females crawl up tree trunks onto branches, are mated by winged males, and then lay single-layered masses of flower-pot shaped eggs on limbs and trunks. The eggs are the overwintering life stage. Spring cankerworms overwinter as pupae and emerge as moths beginning in February. The wingless females crawl up tree trunks, are mated, then lay oval-shaped eggs in masses under loose tree bark.
Elm, apple, oak and many other fruit and shade trees are attacked by cankerworms. Entire leaves are eaten, leaving only the large veins. Generally most damage occurs about the time the leaves become fully developed. Trees may be completely stripped of foliage, some never having a chance to leaf out.
Cankerworm outbreaks sometimes occur two to three years in succession and then virtually disappear for a few years. If an outbreak can be anticipated, tanglefoot applied to tree trunks in a band two to four feet aboveground will prevent female moths from crawling up and laying eggs in the trees. This technique may be preferred to insecticide sprays especially when dealing with very large trees.
Usually it is simpler to use insecticides when defoliation first appears. Effective insecticides are Orthene, Imidan, Sevin and Bacillus thuringiensis (Dipel, SOK-Bt, Thuricide).