Pest Alert

July 15, 1999

Welcome to Pest Alert.

Categories for topics are as follows, H - Herbaceous plants and flowers, W - Woody Plants, V - Vegetables, F - Fruit and L - Lawns.

DISEASES

W,F APPLE SCAB continues to generate much concern. Many trees in some areas of the state are nearly defoliated. However, some crabapples have suffered little damage, attesting to the value of planting scab resistant cultivars. Apple scab lesions appear as small, irregularly shaped olive green spots, often located along veins in the leaves. Infected leaves will turn yellow and drop from the tree later in the season. It is too late to provide any effective suppression of scab on heavily infested ornamental trees but treatment should continue on bearing apple trees. Fungicides such as Captain and Benlate should be effective (read label for directions).

F,V DOWNY MILDEW has been seen on grapes and a few other plants around the garden. This disease will cause white powdery areas on the leaf undersides and leaf malformation. There really is no treatment for this disease once it appears since it is growing on the inside of the leaf. Bordeaux Mixture has been used to prevent this disease for years.

W DUTCH ELM DISEASE is increasing across eastern South Dakota. Several communities have reported losses to-date exceeding all losses from last year’s infections. Typical symptoms of the disease are curling and yellowing leaves that are falling now. The infected branches (those branches that have leaves curling and yellowing) generally show greenish streaks just beneath the bark. Due to the sudden onset of hot, dry weather, a number of infected trees are beginning to exhibit symptoms very quickly. It is possible for an infected tree to go from a healthy green appearance to almost completely defoliated in a week?s time. For more information regarding Dutch elm disease see the last issue of Pest Alert.

V EARLY BLIGHT on tomato, potato and eggplant continues to develop but has been slowed by recent hot and dry weather. This disease causes small spots that start out at about 3/8" in diameter on the leaves. The spots are also characterized by a target pattern of lines within the spots. Many spots may merge together to cause larger areas of the leaves to die but the spots will generally not cross a leaf vein. Like Septoria leaf spot, this disease overwinters on diseased plant debris from last year and is then spread to the lower leaves by splashing rain or irrigation water. Control is similar to that for Septoria leaf spot with Chlorothalonil (Daconil 2787) treatments beginning at the first sign of the spotting for best control.

F,W FIRE BLIGHT infections are now quite evident on trees and should be treated by pruning out any signs of the infection. The most susceptible plants include apple, pear, crab apple, mountain ash and cotoneaster. Dip the pruning saw or shears in a 10% bleach solution between cuts and thoroughly rinse and dry tools when you are finished. Blossom or spur infections can be numerous making pruning difficult. In severe cases, especially where other susceptible plants are in the same yard, consider completely removing the most severely infected plants.

V LATE BLIGHT, caused by Phytophthora infestans, continues to generate concern among gardeners but its progress has been slowed by recent hot and dry weather. This is a disease that can kill entire potato and tomato plants in a few days under cool, wet weather. A new variant of the fungus has developed and spread across North America which is more aggressive and is resistant to the fungicide that had been used most commonly to combat late blight.

Large blotches of dead leaf tissue are characteristic of late blight. The dead spots cross leaf veins, unlike any other disease of potatoes and tomatoes. Often the dead areas will be bordered by a yellow halo. On the underside of the leaf, a white, cobwebby growth may be present during wet weather. Dark, streaky lesions may also be present on the stems of either tomato or potato.

The disease can be controlled in the home garden by picking off diseased leaves and using fungicides such as chlorothalonil or mancozeb that are approved for use on garden vegetables. Severely infected plants should be removed from the garden. Many brands of fungicide with these active ingredients are available from garden centers and discount stores. Applications as often as every 7-10 days may be needed during periods of cool, wet weather.

V SEPTORIA LEAF SPOT is becoming more prevalent, causing the characteristic spotting of lower tomato leaves. Typical symptoms include small dark colored spots on the leaves, followed by yellowing and leaf drop. The disease overwinters on diseased plant debris from last season and is spread to new plants by splashing rain. The first diseased leaves may be picked off to reduce spread but fungicide treatments are usually the best means of control. Chlorothalonil (Daconil 2787) treatments should begin with the first signs of the disease and continue throughout the rest of the production season.

 INSECTS

F APPLE MAGGOTS should be emerging soon and begin laying their eggs on apple fruit. The "railroad worm" as it is called, is the larvae of a small insect that resembles a house-fly but has dark markings on the wings. The adult emerges from the soil and goes off in search of apples. The adult lays an egg just under the skin of the apple. The egg soon hatches into the maggot that feeds inside the apple. Infested apples will typically have a small black speck on the skin of the apple. The apple may also be "dimpled" where the egg has been laid. Small brown streaks will be seen in the flesh of the apple where the maggot has fed. Infested fruit, while unattractive, is still edible but has a much shorter storage life. Sticky red ball traps may be used to monitor for the emergence of the adults. Begin treatment when five adults have been captured on a sticky ball. Apply protective sprays of malathion, diazinon, Imidan or use a combination orchard spray to reduce infestations. Some people have reported good results by simply using 6 to 8 sticky traps in a few trees in a yard to reduce damage to tolerable levels without using insecticides.

W ASH-GRAY BLISTER BEETLES are beginning to appear across the central part of the state. The adults can be found feeding on the foliage of peashrubs and honeylocust. The adults are slow -moving beetles about a 1/2 inch long. While the beetles are slow, they generally are numerous on a plant and can often completely defoliate a tree or shrub in a week. The recommended treatment is methoxychlor (Methoxychlor), carbaryl (Sevin) or esfenvalerate (BugBGone) when the adults first appear. The name blister beetle comes from cantharidin, a substance the beetle can spray if bothered. The chemical can produce large blisters on the skin.

V COLORADO POTATO BEETLES continue to plague home gardeners with some of the first new larvae beginning to feed. Adults and larvae may be hand picked from the plant. Also, the bright yellow-orange egg clusters can be crushed when found on the undersides of leaves. Carbaryl (Sevin), esfenvalerate (BugBGone) and rotenone can be effective but there is a lot of potential resistance problems. Bacillus thuringiensis var. tenebrionis (M-Trak) is an option. There are no known resistance problems to it, but Bacillus is only effective against very young larvae. Keep potatoes well-watered to decrease susceptibility to defoliation.

W ELM LEAF BEETLES are feeding on elm leaves across the state. The 1/3-inch long larvae are yellow with two dark strips down their sides. This is the second generation of elm leaf beetles for the year. The first generation does most of the damage so usually controls are not initiated for the second or third generations. If the tree was severely defoliated by the first generation, however, it may be beneficial to the tree to reduce feeding damage from the second generation. Recommended controls include methoxychlor (Methoxychlor) or carbaryl (Sevin). Merit, as a soil or trunk injection will not be effective at this time, however, if applied late in the season, it will provide excellent control for all the next growing season.

F GRAPE PHYLLOXORA has been seen on grapes in many areas. This tiny aphid-like insect lays it eggs on grape leaves causing the leaf to develop a gall on the leaf underside. The young larvae develops inside the gall, eventually laying eggs. Once the eggs hatch, the larvae crawl out and start new galls on other leaves. While the galls are unsightly and can cause leaf malformation, the pest is usually not very damaging to the plant. No treatment is really feasible. Another more damaging form of phylloxera attacks the roots and can stunt and kill grape plants but this form of the pest is usually not a problem in modern grape varieties.

V,H,F,W GRASSHOPPERS are growing rapidly and feeding on many vegetables, flowers and turf. Grasshopper numbers may be large again this year. In most sites, the grasshoppers are still quite small. NOW is a good time to start controlling them. Some protection and control can be gained by spraying with Malathion, carbaryl (Sevin), Acephate) (orthene), chlorpyrifos (Dursban), esfenvalerate (BugBGone)and cyfluthrin (Tempo) on ornamental plants. Vegetables and fruit trees can be treated with sevin or malathion. The garden will be best protected by treating them with malathion, esfenvalerate (BugBGone), or carbaryl (Sevin) in combination with a treatment to areas bordering the garden with Tempo. Wheat bran baits containing Sevin can also be used in the garden. Remember, Tempo is NOT labeled for use on food crops. Re-spraying will be necessary as often as the grasshopper population continues to rebuild.

W MOUNTAIN PINE BEETLES are beginning to emerge as adults throughout the northern Hills. The brown to black beetles are about 6 mm long and have begun their mass attacks on trees. The beetles are emerging from trees that were killed earlier this year from last year’s attacks. These dead, infested trees are now turning yellow to red. The attacks will continue till about mid-August. It is now too late for effective treatment this year.

V POTATO LEAFHOPPERS appear to be numerous this year. These tiny green insects feed on a wide variety of garden plants. They can often be seen on the undersides of leaves. Feeding damage consists of yellow or white speckling of the leaves from the sucking of plant sap. In some cases, they may cause a "burning" or yellowing and curling of leaves since some plants react to the insects saliva that in injected into the plant during feeding. Treatment with malathion, carbaryl (Sevin) or diazinon can be effective.

W SPRUCE NEEDLEMINER populations still appear to be increasing, particularly in the northeastern part of the state. Many blue spruce trees have their lower branches completed infested with this small insect. The typical signs of an infestation are needles along the lower branches, particularly the tips, absent and instead, clusters or clumps of needles bunched together. If these clumps are closely inspected, the small hole at the base of the detached needles can be seen. Also, within the clump, can be found frass and a small reddish pupa. The adults, a small moth, will be emerging soon. Treatment should be started about 4th of July with either acephate (Orthene) or carbaryl (Sevin) sprayed into the needles. While earlier is better, it is still not too late to apply treatments.

V SQUASH VINE BORERS have been active across the state. These attractive bluish moths, which look like a wasp, deposit eggs near the base of squash, pumpkin, and occasionally melon vines. Newly hatched larvae immediately burrow into the stem where they continue to feed until they reach approximately one inch in length. This feeding eventually kills the vines, although it appears to happen quickly because the vines suddenly wilt. Vines must be protected from late June through mid August from egg laying adults and newly hatched larvae with an insecticide application to the base of plants. Dust formulations are probably best for this insect, including carbaryl (Sevin), rotenone, and methoxychlor, which is available as Ortho Vegetable Garden Insect Dust.

V STRIPED BLISTER BEETLES have been seen feeding on carrots, potatoes and other garden plants. These tan and black striped beetles are about 3/4" long and about 1/4" wide. They look like they have a very pronounced "neck" right behind the head. They can feed in fairly large groups, quickly defoliating plants. Care should be used around these pests since they can excrete a substance called cantharidin that can raise blisters on the skin. Most garden dust formulations should help control these insects, also the liquid formulations of carbaryl (Sevin), malathion and diazinon should work as well.

OTHER NOTES

V POOR FRUIT SET is a common concern among many gardeners on a number of vegetables. There have been many calls about tomatoes that look great but have not set any fruit, the flowers just fall off. Fruit set is a rather complicated process, dependant on many factors. Weather, specifically temperature and moisture conditions being of primary importance. If conditions are not right, fruit set will not occur. Plant development and nutrition levels in the soil are also important factors. Cucurbits are also affected by these same factors with the added dimension of having separate male and female flowers. Generally, the first flowers to be produced on cucurbits are male, with more female flowers developing as the season progresses. The general recommendation for all garden plants is to keep water stress at a minimum, do not over-fertilize and be patient. A healthy plant will eventually set fruit.

L STRESSED LAWNS. Hot, dry summer weather places cool-season turfgrasses, like Kentucky bluegrass, under extreme stress. Cool-season turfgrasses actually lose most of their root system this time of year, carbohydrate reserves are depleted, and shoot growth rate is dramatically reduced or stops. It takes less pressure from disease, insects or weeds to injure or kill turfgrass than at any other time of the year. Maintaining proper cultural practices can lessen the susceptibility of home lawns to damage from these pests. Summer is not a good time to fertilize home lawns. Summer annual weeds will better utilize nitrogen applied at this time of the year than turfgrass. Do not raise the mowing height in summer. The diminished root system will not be able to support the water needs of increased transpiration by leaves, particularly on hot, windy days. Do not apply herbicides to turf at this time of year. Weed control will be poor and phytoxicity to turf or other desirable plants in the landscape is possible. Irrigate deeply, but infrequently, when the turfgrass need it. Daily watering will enhance growth of annual grassy weeds such as crabgrass, goosegrass, and foxtail. Also, water only during the early morning hours to reduce the amount of time that moisture is present on leaf surfaces. This will help to reduce the incidence of certain foliar diseases. Cultivation practices, such as verification, should not be performed on turf during summer. Following these tips will help your lawn weather the stresses of summer until more favorable conditions return during the fall.

V,H,L,W WATER PROPERLY when the plants need it. Generally most lawns, gardens and flowers need about 1" of water per week. If your area has not been receiving this amount, supplemental watering may be needed. The important point to remember is to water thoroughly when you do water. Try to saturate the root zone of what you are watering. This means applying at least 1/2" at one time, preferably more like 1" should be applied at one watering. Then allow the soil to dry before watering again. You can tell when it is time to water by sticking your finger into the soil to see if it feels moist in the top 2" of soil. If it is dry, it is probably time to water. Watering from below is generally better than overhead. This can be done using soaker hoses or a drip system. Overhead watering tends to waste water and can promote disease problems if the foliage is allowed to go into the night wet.

W WHAT’S A DUTCH ELM? Many people around the state assume if their elm isn’t a Dutch elm, they will not have a problem with Dutch elm disease. Actually there is such a tree, released many years ago from a research station in the Netherlands, but it is rarely seen. The Dutch elm was also released because it was resistant to the disease, not susceptible. Most homeowners have either the American elm or the Siberian (often incorrectly referred to as the Chinese elm) elm in their landscape. The American elm has large leaves, usually between 3-6 inches long, while the Siberian elm leaves are much smaller, about 3/4-3 inches long. Both these trees can become infected with Dutch elm disease, but the Siberian elm is relatively resistant.