Pest Alert

June 30, 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 severe in most areas of the state. 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,F BLACK KNOT ON CHERRY is a perennial problem in the Northern Plains. The disease occurs on wild plums and cherries but is most common on the Canada Red or Shubert chokecherry. It appears as a black, elongated gall on the twigs and branches. Generally the twig growth beyond the infection is killed. The "knots" are forming now but can not be controlled by an fungicide treatment at this time. The best management at this time is to prune out any new "knots". Lime sulfur sprays can be applied beginning in early may every 10 days until the terminal growth stops, typically June, however these applications are rarely effective.

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 DOTHISTROMA NEEDLE BLIGHT is showing up on ponderosa pines in the northern Hills. The infected trees have needles with yellow to tan spots. Typically the needle beyond the spots is dead while the base of the needle is still green. Black fruiting bodies can be found on the infected needles, but only after a rain. Within a day or two they disappear. This disease rarely kills trees and is not commonly found in the state. Treatment should begin in May so it is too late for control at this time.

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. On many trees the streaking can be found along the lower trunk meaning the disease has spread from the upper branches ? where it was introduced by the bark beetles as they burrow into the tree ? to the lower trunk. Once the disease enters the root system it can potentially spread to surrounding elms via root grafts. Control at this time is prompt removal of infected trees to prevent the spread to other elms. If the infected trees is near (within 20 feet or so) of other, healthy, elms the stump should also be quickly ground so that the roots coming out from the flare are separated. This will aid in reducing the possibility of root graft infection. Healthy trees, and trees showing only limited infection, about 10% yellowing leaves, can be injected with either Alamo or Arbortect 20S to prevent the spread of the disease. These chemicals do not kill the fungus but stop its movement. So once the treatment program is started, it should be continued for the rest of the tree?s life.

V EARLY BLIGHT on tomato, potato and eggplant is off to a good start this year. 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.

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 and is quite severe in many cases. Symptoms are numerous, small, raised, rust-colored spots on the leaves and stems. Thorough cleanup of old plant debris in the fall helps to reduce the disease next season. Fungicide treatment now would be of little value.

V LATE BLIGHT, caused by Phytophthora infestans, has been confirmed on tomato and potato plants. This is a disease that can kill entire potato and tomato plants in a few days under cool, wet weather. This is the disease that was at the heart of the Irish potato famine, 150 years ago. While late blight has occurred in the United States since 1992, the pathogen has become more important in the past 5 years. About that time a new variant of the fungus developed and spread across North America. This new form 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.

W MAPLE ANTHRACNOSE is showing up on trees in the state. Like ash anthracnose, it causes 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.

V SEPTORIA LEAF SPOT is already 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 resemble 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. 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.

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, carbaryl (Sevin), Acephate) (orthene), chlorpyrifos (Dursban) 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 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 will begin their mass attacks on trees now through mid-August. 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. If infested trees are within 1/4 to 1/2 mile of your mature (8 inches or larger) pines you may still want to consider treating the trunk with carbaryl (Sevin XLR) a specific formulation of Sevin for bark beetles. However, once the beetles are in the tree the spray is no longer effective. Spraying should be done by the 4th of July, if not sooner.

W,F,H SPITTLE BUGS are showing up on junipers across the central part of the state. The spittlebugs (Aphrophora spp.) are completely covering the tips of some redcedar shelterbelts. The "spit" is a frothy mass of spittle and air bubbles. If you look in the spittle mass you'll find the nymphs that are brown and about 6 mm in size. The nymphs do move several times during the growing season so it is possible to find "spit" and no insect. In about another two weeks the nymphs will become adults. The adults are also brown but are larger, about 10 mm long. Both the adults and nymphs feed on the sap from foliage and twigs of a number of woody plants and small fruits like strawberries. Heavy feeding damage usually results in brown and dying tips to the cedars. Control measures include carbaryl (Sevin) or malathion (Malathion) sprayed when the insects are detected. Light infestations, only a few spittle masses on the tree, can be controlled by a high-pressure stream of water into the spittle.

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 completely 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.

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 carbaryl (Sevin), rotenone, and methoxychlor, which is available as Ortho Vegetable Garden Insect Dust.

V VEGETABLE BORERS of several types are being reported by gardeners from around the state. Many plants are breaking off near the soil surface and have stems that are hollowed out. Holes in fruit are also common. The two most common are the stalk borer and tomato fruit worm, also known as the corn borer.

The stalk borer can be found boring into a variety of plants including sweet corn, peppers and tomatoes as well as nearly 200 other types of plants. These insects emerge from grassy weedy border areas and move into the garden where they begin feeding. They may grow to about 1 1/2 inches in length, are tan to brown with a distinctive dark colored "saddle" mark near the head end of the caterpillar.

The corn borer or tomato fruitworm is a small caterpillar that will reach a length of about 1 inch. It is normally grayish pink in color with a dark head and small spots on the top of each segment. It normally attacks sweet corn but will attack the stems and fruit of a number of different vegetable crops.

Control of these boring insects is difficult. No real treatment is possible after symptoms have occurred. Many of them will migrate into the garden from outside areas so good weed control around borders of the garden is important. Keeping grasses mowed or cultivated around the garden can help reduce damage. Protective sprays of carbaryl (Sevin) or malathion may reduce potential problems as well, especially if the garden has a history of these pests.

OTHER NOTES

V BLOSSOM-END ROT will soon arrive in many home gardens. This physiological disease is caused by a calcium deficiency in the fruit. The plant takes up less calcium than it should during times of rapid growth. This often happens during wet weather following a dry period. Since there is not enough calcium in the plant, the end of the fruit doesn't get enough to develop properly. Typically, determinant tomato varieties are more severely affected since much of the fruit develops at one time.

The best way to treat the problem is to discourage growth spurts from fluctuating soil moisture levels. Use a mulch to keep the soil moister, water during dry periods and try using semi-determinant and indeterminant tomato varieties. Applications of calcium are generally not effective since our soils generally have adequate levels of calcium.

V ROTTING SQUASH fruit generally means that the female flower was not adequately pollinated to allow for the development of the fruit. Without pollination, the fruit may survive for a couple days but then begin to rot from the blossom end. This is NOT like blossom-end-rot which is found in tomatoes and peppers. Extremely heavy, dense vine growth, wet conditions, odd weather patterns, excessive use of insecticides or just a lack of pollinating insects can increase the likelihood of this problem occurring. Generally the problem will disappear on its own. Also, warmer temperatures will promote the production of more female flowers to allow for more potential fruit than during cooler weather. Incidentally, bumble bees are much more effective pollinating insects for cucurbits than honey bees.

Cucurbits, in general, can also suffer from blossom-end rot, much like tomatoes and peppers. Treatment is the same - maintain even moisture conditions in the garden to facilitate even uptake of water and nutrients, including calcium.

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.