Pest Alert

June 15, 1999

Welcome to the second issue of the 1999 growing season. Spring is an active time of growth for plants and pests. Weather is playing a big role in that it has been unusually wet and cool for this time of year. While the cool conditions slow down pest development, the wet conditions foster a number of foliage diseases.

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 is now quite visible on infected leaves. Lesions will 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. There have been reports of some crabapples that have lost the majority of their leaves already! Recent rains and humid weather conditions have helped to foster scab development. 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 Captan and Benlate should be effective (read label for directions).

W ASH RUST calls are by far the dominant concern across the state. The rust disease results in distorted leaves with fuzzy, yellow-orange dots forming gall-like structures on the underside of the leaves. Sometimes these galls may become up to 1" long or more and up to 1/2" in diameter if on a small twig. These infected leaves will often fall prematurely. Controls are ineffective at this time as the disease is already in the plant.

W BLACK SPOT on roses has been reported in many areas of the state. Dark colored spots generally first appear on lower leaves. The disease can quickly spread to other parts of the plant during wet weather. The first diseased leaves can be picked off to reduce the spread. Mulching around the plant helps to reduce infection by covering up old leaf debris which acts as a source of the disease. Protective sprays of Funginex are quite effective.

W CERCOSPORA BLIGHT has been observed on eastern redcedars and Rocky Mountain junipers around the state. While infections usually occur in the spring, the symptoms for the new infections do not begin to appear until about mid-July or August. The typical pattern to cercospora blight is bronzing and dying of the interior branches. The two other common juniper diseases, Kabatina and Phomopsis tip blight, generally have the browning start at the branch tips and spread inward. Most of the spores for cercospora are released during the moist spring weather. Thus the best time for control is an application of Mancozeb in late May followed by a second application in early July.

H IRIS LEAF SPOT infections are also severe this year causing the characteristic tan colored spots on the leaves. Later, heavily infected leaves turn brown and die back from the tip. The best control for this disease is prevention. Thorough clean up of old leaves in the fall or spring before growth begins can significantly reduce new infections because the disease overwinters on infected leaves. Heavily infected leaf tips may be cut off. A protective spray of Captan or chlorothalonil will help reduce further infection.

H HOLLYHOCK RUST has been common again this year. Symptoms are numerous, small, raised, rust-colored spots on the leaves. Thorough cleanup of old plant debris in the fall helps to reduce the disease next season. Fungicide treatment now would be of little value.

W MAPLE ANTHRACNOSE is showing up on trees in the state. Like ash anthracnose, it causes fairly large, irregularly shaped brownish spots, typically along the margin of the leaf. However, these infected maple trees will generally not drop as many leaves as ash trees do when they are infected. Control strategies are similar to that of ash so treatment now will not be very effective. Maple anthracnose is usually not as much of a problem so preventative treatments are generally not needed. Young trees can be sprayed with a protective fungicide like chlorothalonil in the spring when new leaves are developing.

 INSECTS

W,F BLACKHEADED ASH SAWFLIES (Tethida cordigera) have been observed feeding on ash in central South Dakota. The larvae can be found feeding in colonies along the margin of the leaflets. The larvae are about 0.3-0.5 inches long, whitish with a yellowish tinge and a shiny black head. There is only one generation per year. Winter is spent as a prepupal in the soil. Adults emerge in the spring and lay their eggs in slits cut along the outer margin of the newly expanding leaflets. The sawfly is rarely a serious problem on ash on the northern Great Plains. However, an occasional tree may suffer significant defoliation. A heavily infested tree can be completely defoliated in a week. Carbaryl is the recommended treatment, but only when several colonies are observed feeding on a tree.

V COLORADO POTATO BEETLES continue to plague home gardeners. Carbaryl (Sevin) 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.  (Short AVI movie file of feeding.)

V CUCUMBER BEETLES have been actively feeding on young plants in many areas. These small voracious eaters can totally consume seedling pumpkins, melons, cucumbers or squash in just a few days. They also may inoculate the plants with bacterial wilt. In either case you will want to be on the watch for these small black and yellow striped beetles and use protective measures to reduce their damage. Floating row covers work very well to prevent the beetles from getting to the young plants. Also, treatment with Sevin or Rotonone containing products should be effective as well.

V,H,F,W GRASSHOPPERS have already been reported 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, sevin, orthene, dursban and 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 or 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 HONEYLOCUST POD MIDGE injury should be showing up across the state soon. The oblong, pod-like galls form on the new leaflets of honeylocust. Within each of these pods you can usually find several small, yellowish larvae. The larvae pupate within the pod after about 3 weeks and then emerge as small, mosquito-like flies. There are several generations per year. While the injury looks unsightly, it rarely results in any serious decline for the tree. Sometimes a few interior branches will decline and occasionally a young, stressed tree will die. Recommended control is carbaryl when the new foliage begins to expand and repeat every two weeks through June.

W,F PEAR- & ROSE-SLUGS have begun feeding on roses and on a wide range of plants but seem to prefer cotoneaster. The slimy looking pests are actually the larvae of an insect and not true slugs. They are usually light green to dark green and often have a dark colored dab of fecal material on their back. They feed on the underside of the leaves and cause a skelotonization type damage. They may defoliate plants in a few days when they are feeding in high numbers. Sevin, Malathion and Orthene are quite effective in controlling these pests but best results will be seen if the spray is directed on the undersides of the leaves.

V SQUASH VINE BORERS are becoming 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 sevin, rotenone, and methoxychlor, which is available as Ortho Vegetable Garden Insect Dust.

W UGLY NEST CATERPILLARS have been seen feeding on various plants in large numbers. While they prefer to feed on types of Prunus, they will feed on other plants as well. The cream-colored caterpillars feed in small groups, webbing leaves together into a loose-appearing, "ugly" nest, eating the leaves as the nest grows. In large numbers, they can defoliate small plants. Treatment is often difficult because the insects are fairly well protected in their silken nests. Small, individual nests may be pruned out or the plant may be treated with Sevin, Malathion or Orthene. The botanical insecticides that contain B. t. may also be effective on early infestations.

W WOOLY APHIDS are being found in elm and ash trees. These aphids feed in large numbers in a fairly concentrated area of the branch or tree. Generally, one will find the tip of a branch exhibiting curled, distorted leaves with an abundance of small, whitish insects on the leaves, especially on the curled undersides of the leaves. They feed by sucking plant sap but are generally not a serious problem. The easiest treatment is to simply prune off the infested branch tip and destroy it.

WEEDS

L THE NEW WEED B GONE is a three-way combination of 2,4-D, MCPP and dicamba. This improves the weed spectrum controlled. The "old" Weed B Gone is still available as a ready-to-use product containing 2,4-D and MCPP.

V TIME TO MULCH EARLY VEGETABLES. Remove existing weeds first. Avoid using mulching materials that may contain weed or grass seed. Do not use grass clippings for mulching if the lawn was recently treated with pesticides that could damage or leave illegal residues on the garden crop. Mow at least two times before using these clippings.

H,V,W RESIDUAL HERBICIDES. Avoid using residual herbicides for weeds in gravel drives, cracks in sidewalks or in alleys. Ingredients such as prometon will move with rain and the residual can be picked up by roots of non-target plants.

V DON?T TRY TO USE ROUNDUP IN ASPARAGUS now during the growing season. Roundup can be used in late fall after killing frost has stopped asparagus growth.

OTHER NOTES

V ASPARAGUS HARVEST SEASON is about to a close. Remember, the longer asparagus shoots are harvested, the greater the drain on the stored carbohydrates needed for the production of big thick spears. Therefore, it is important to allow lots of the shoots to grow up, mature and be allowed to remain on the plant until a killing frost. These ferny shoots manufacture the carbohydrates that are stored for next year?s spring shoots.

F GRAY MOLD ON STRAWBERRIES is a common problem. Clean harvesting of all ripe fruit will greatly help to reduce problems with botrytis gray mold. This very common fungal disease grows rapidly on over-ripe or damaged fruit and can quickly infest an entire fruit cluster under moist conditions.