Pest Alert

August 4, 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. 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).

V BACTERIAL WILT. Typically infected cucumber, melon or squash plants or individual vines suddenly wilt during hot, dry weather and but may partially recover during the night. Infected plants may be diagnosed by cutting a stem, near the base of the plant with a very sharp knife or razor blade. Squeeze the cut ends and stick them back together. Slowly pull the stems apart again and watch for the presence of tiny, sticky strands that will adhere between the two cut surfaces. If these tiny strands are present, diagnosis for this bacterial disease is almost certain.

No treatment is possible for infected plants. The bacteria overwinters in cucumber beetles which then spread the disease to young cucumber plants the next season when they feed. Since cucumber beetle feeding was heavy this spring, lots of bacterial wilt is expected. Early season control of the beetles should reduce the spread of the disease in the garden. Remove Bacterial Wilt infected plants from the garden to reduce the chances of further spread of the disease.

V SQUASH MOSAIC and related mosaic diseases have been seen on squash and other cucurbits. These virus diseases cause a mosaic pattern of green, lite green and yellow streaks to develop in the leaves initially. A bit later in the season, the fruit will become distorted and discolored, typically exhibiting raised spots which may be a very prominent yellow color. Infected fruit may also have an off-flavor. The disease can be spread by insects like cucumber beetles and aphids and also by physical contact or mechanical injury among the plants. Once a plant is infected, there is no treatment for the disease other than to remove the plant from the garden to lessen the chances of spread to other healthy plants.

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.

V PURPLE BLOTCH OF ONION: Gardeners with a love of onions have seen a common disease developing over the past week or two. Purple blotch, a disease caused by an Alternaria fungus, has been seen causing dieback of the onion tops. Purple blotch can also affect garlic and leeks. The disease is expressed as dark spots on the leaves that may show a target pattern. Sometimes, if the day and night temperatures are not far apart, the ring pattern may not be as obvious. As the spots grow larger, they develop a brownish to purple color. The leaves will yellow above the spots and as the spots girdle the leaves, the leaf will die back above that point. Purple blotch is a common disease of onions in South Dakota during periods of high temperatures and very high humidity. In years with those conditions it is probably even more common than Botrytis diseases.  Purple blotch will be more severe as the season progresses and leaves age. Sweet Spanish onions are among the most susceptible onion varieties. Fungicide applications with chlorothalonil can reduce the spread of purple blotch.

V SEPTORIA LEAF SPOT is 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 are probably emerging now and will begin laying their eggs on apple fruit. The "railroad worm," as it is called, is the larva 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.

V COLORADO POTATO BEETLES continue to plague home gardeners. 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.

V,H,F,W GRASSHOPPERS are continuing feeding on many vegetables, flowers and turf. 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. Re-spraying will be necessary as often as the grasshopper population continues to rebuild.

F RASPBERRY CANE BORERS may be attacking raspberry plants in the garden. This adult borer chews two rings around the stem, near the top and lays her egg in the stem. Later, the top of the stem dies while the larvae begins feeding and tunneling in the stem. Best control is to prune out any canes that show this feeding activity.

V SQUASH VINE BORER DAMAGE is now readily apparent on squash and pumpkins. Examine vines that are wilting during hot days. Infested plants will have stem bases that are swollen and may have a wet sawdust-like material coming out of small holes in the stem. If the stem is cut open, it will reveal tunneling and a cream colored worm. The adult is an attractive bluish moth, which looks like a wasp. The female deposits 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. Once the plant is infested, one can try to "surgically" removing the larvae with a sharp knife, put the stem back together and cover with soil. The plant may be saved by encouraging roots to develop at other points along the stem by covering it with soil 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. High temperatures will also greatly shorten the viable lifetime of pollen, decreasing the chances of ovules in the fruit from being successfully fertilized. 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.

V POTATOES READY FOR HARVEST can be found in many gardens around the state. Many of the early varieties have already died down to the ground. In some cases this is because the plants have matured naturally, but in most cases the vines were killed by various diseases, with early blight being a prime factor. Many gardeners now have potato tubers that are at the soil surface and are being exposed to sunlight which will turn the tubers green. Potato plants should have soil pulled (hilled) up around the plants as they are growing to prevent tuber greening also to provide space for more tubers to develop. Potatoes can be allowed to remain in the garden soil for some time after the vines die down but can be subject to re-sprouting or decay if soils get too wet. Potatoes are best stored in a cool (40-45F) and humid location (80-90 %RH) for best results.