Pest Alert

Sept. 1, 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

V BACTERIAL BLOTCH OF WATERMELON has shown up in some plantings. This seed-borne disease first appears in the seedling stage but is most damaging when plants have 2-3 week old fruit. First symptoms are small, water-soaked spots which soon expand to cover the top surface of the fruit. Later, as the disease develops, cracks will develop which may exude a foamy bacterial ooze. Eventually melons decay internally. Best treatment is to start with clean seed, cull out infected seedlings and plants later in the season and rotate out of melons for 3 years if the disease has been a problem. Prompt control of volunteer seedlings and avoiding over-head irrigation is also helpful.

H,W BOTRYTIS GRAY MOLD (Botrytis cinerea and other related species) are appearing on many ornamentals. Gray mold is a disease of declining tissue that we see in the spring and fall during periods of cool weather with rain and long dew periods. The fungus that causes the disease is sometimes referred to as a mold because the fungus can appear very wooly on as the surface of affected plant organs. This fungus survives as black, BB-like resting structures that persist in the soil. Sanitation, the disposal of infected plant parts is critical to managing this disease. Several fungicides are also effective against gray mold. Chlorothalonil (Ortho, Acme, and Green Charm Multi Purpose Fungicide) is labeled for use on several ornamental plants that may have problems with gray mold, as is thiophanate-methyl. Thiophanate-methyl may have a long, difficult to pronounce name, but it is becoming more widely available as a fungicide for home garden use. It is sold commercially as Domain, Fungo, and 3336. Homeowner packaging of thiophanate-methyl is available from Green Light as Systemic Disease Control.

L PATCH DISEASE INJURY IN LAWNS has been widespread during the later part of this summer. Hot and dry weather put additional stress on lawns, making them more susceptible to the various disease pathogens responsible for the summer patch disease complex. While fungicides can be used to help treat these diseases, often the most important treatment is to examine the cultural practices used on the lawn and modify those to promote a healthier lawn. The three primary cultural practices of mowing, watering and fertilizing can seriously stress a lawn if done improperly. Thatch buildup is often a precursor to more serious problems later in the life of the lawn. If the thatch layer becomes more than about 1/2" in thickness, the lawn’s susceptibility to diseases and other pest problems increase. Thatch development is favored by: frequent, shallow irrigation - often 3 or more times per week; excessive fertilization - more than 3-4 lbs./1000 sq. ft. per year; improper mowing frequency - removing too much leaf tissue at one time; also, peat-based sodded lawns appear to be more prone to thatch problems.

Thatch management involves using more appropriate cultural practices such as: water deeply and infrequently - about 1" of water per week applied in one application per week; mow regularly so as not to remove more than 1/3 of the grass leaf area at one mowing; apply between 3 and 4 lbs. of actual nitrogen per 1000 sq. ft. per year with 2/3 of that amount being applied in the fall, generally split between two applications in mid- to late-Sept and mid-October. Apply remaining nitrogen in mid- to late-May and again in mid June if the lawn will be irrigated. Finally, use core aeration in the fall to reduce thatch buildup. If thatch layer is more than 1" in depth, core spacing may need to be as close as every 3" to be effective. Be sure there are at least 2 weeks of active growing weather left after aerification. Treatment can be repeated in early spring and again the following fall. Cores should be broken up over the turf to mix soil and microbes into the thatch layer. Vertical mowing or power-raking can also be useful but may not be as effective in treating a severe thatch problem.

L,V,W,H POWDERY MILDEWS are severe on lilac, phlox, roses, and Monarda. They also occur on many other hosts, including vegetables, especially pumpkins, squash and other cucurbits. Powdery mildews are easily recognized by the dusty, powdery appearance they cause on leaves of their hosts. When severe, powdery mildews can cause deformation of the leaves. Roses and other plants that have green stems may also be infected on that plant part. Powdery mildews will also colonize flowers and buds on rose, causing color breaks on flowers and bud abortion. During cool weather, most hosts can be treated with sulfur for powdery mildew control, but do not apply sulfur if the temperatures exceed 85oF. If temperatures are hot during application sulfur may burn the plants. Never use sulfur on sensitive crops such as apricot. They will always respond with a phytotoxic reaction. It is probably too late to do much to reduce the problem on vegetable crops. Most ornamental plants can also be treated with the fungicide Funginex for control. Be sure to check the label to see if the plant is listed on the label.

 INSECTS

W APHIDS are becoming noticeable on deciduous trees in southeastern South Dakota. Concerned homeowners are calling about their "weeping" trees. Black Walnuts, Norway maples and American elms are often heavily infested with aphids. Other trees, shrubs, vegetables and lawn areas beneath the trees may develop a black sticky appearance due to the honeydue and sooty mold that develops as a result of the aphid infestation. We are not recommending any treatments at this time. However, if a homeowner finds the "dew" objectionable or excessive defoliation (loss of 50% or more of the leaves) appears likely, the tree can be treated with malathion or orthene.

V,F PICNIC BEETLES OR SAP BEETLES are a problem in the garden and orchard as well as at backyard picnics and parties. The name comes from the fact that this insect is very prevalent at picnics and is attracted to various foods and beverages found there. There are several kinds of picnic beetles but the most common one is a small black beetle about 1/8 of an inch long with four small orange spots on the back. These beetles also burrow into vegetables and fruit such as sweetcorn, berries, and tomatoes. They are particularly attracted to fruit that is overripe or damaged in some way. Therefore the first management strategy against picnic beetles should be to keep berry and tomato patches picked on a regular schedule, removing any overripe or damaged fruit. Insecticide applications are also available, the most effective is diazinon or a dust formulation of Sevin could be used. However, be sure to read the label carefully to determine that the crop being treated is listed on the label, and be extremely careful to observe the harvest interval between application and harvest, as treatments for picnic beetles commonly occur during the period of fruit and vegetable harvest.

OTHER NOTES

F APPLE FRUIT DROP has been a big complaint this year. Fruit drop may occur any time during the summer but is more common in late May or June. Fruit drop at this time is common because the fruit that was not completely pollinated will often drop before it develops very much. Fruit drop later in the season usually results from some type of damage to the fruit like apple maggots, coddling moth, plum curculio, damage from disease like scab or powdery mildew or from mechanical damage like hail. There is no real treatment at this point other than to do a better job of preventing damage next season.

W COMMON BUCKTHORN. We have received lots of samples of this commonly found shrub this year. Common buckthorn, Rhamnus cathartica is identified as a shrub or small tree with small, dark green, elliptical leaves, and produces clusters of small black fruit. Many homeowners confuse the form with a crab apple. The twigs also end in a small thorn. Apparently it was a good year for fruit production and lots of would-be jam and jelly makers are wondering if the fruit is safe to eat and use. While the fruit is not considered poisonous, it does have laxative properties, (cathartic - evacuating the bowels) therefore, eating the fruit is not recommended. Birds will often eat the fruit and then "deposit" the seeds all over the yard. Consequently, buckthorn seedlings are frequent weeds in many yards.

W FALL COLOR OF DECIDUOUS TREES will be beginning sometime in September. Usually the first areas to color are in Roberts County and the upper elevations of the Black Hills. The brilliance of the display is dependent on the fall weather conditions. If we have a cool, sunny, dry (but not droughty) fall, color should be good to excellent. If instead we experience warm, cloudy and wet conditions the color will be much duller. Yellow fall color occurs as the two yellow pigments, carotin and xanthophyll are unmasked by the loss of chlorophyll. Red fall color is due to the formation of anthocyanin which results from the accumulation of sugars and tannins in the leaves. Our best red trees are Amur maple, sugar maple (with orange), serviceberry, and northern red oak. Some of our best yellow color occurs with Norway maple and birch. Fallgold black ash is noted for its golden yellow fall color, while Autumn Purple white ash and Autumn Blaze maple are noted for their brilliant reddish-purple and red colors, respectively.

W FALL IS A GOOD TIME FOR PLANTING. Many homeowners believe that landscaping has to be done in the spring. But fall is an especially good time to complete those landscape projects around the house. The typical fall combination of warm, moist soils and cooler air temperatures reduces transplanting stresses and speeds recovery. And added bonus, September garden center prices are usually at a seasonal low as managers try to reduce the stock to carry over the winter.

W FALL NEEDLE DROP WILL BE OCCURRING SOON. The fall coloring and loss of inner foliage is a natural occurrence of evergreen trees, including juniper (cedar) and arborvitae. The normal shredding of needles is most apparent on pines and spruce and results in numerous calls from alarmed tree owners. The critical factor is the age of the affected needle. Healthy pines retain their one- and two-year old needles while shredding their third. Healthy spruce retain their one- to six-year old needles while shredding their older ones.

L LAWN RENOVATION time is here. Aug. 15 to Sept. 15 is considered the best time of year to start a new lawn or renovate an old one by either seeding or sodding. The first step is to kill existing vegetation. One of the easiest ways to do this is to use a non-selective herbicide like glyphosate, sold as Round-Up, Kleen-Up, and a number of other trade names. Follow label instructions for treatment recommendations. The old grass and weeds should be actively growing and have several inches of growth for best results. Water the area a few days in advance to promote growth during dry weather. Do not mow for a couple weeks to encourage more growth. If a soil test has not been done in the area for several years, now is also a great time to gain some important information that could allow you to correct a problem that could cause reduced grass establishment or stand in the future. Once the old vegetation has been killed, you are ready to begin reworking the soil and adding any amendments that may have been recommended from the soil analysis. Be sure to select an appropriate lawn grass or mixture for your yard. Make sure to get good seed to soil contact when seeding. Be prepared to supply supplemental irrigation during establishment, keeping the seedbed moist for at least 3

W PROBLEMS WITH DEFOLIATED DECIDUOUS TREES. Foliage diseases are still expressing symptoms across the state. There are numerous trees that are almost completely defoliated from anthracnose, rust, and scab. The most common concern from tree owners is, "Is my tree going to die?" While there is no single correct answer, here are some important questions to consider.

How much defoliation has occurred? Obviously, the more foliage loss, the less food that is manufactured. However, most trees can tolerate as much as 50 percent defoliation in a single season with few ill effects. Defoliation of more than 50 percent usually results in refoliation. The refoliated tree is now out of phase with the seasons and the new foliage is under greater stress. It is generally less stressful for the tree not to put out new leaves.

How often has defoliation occurred? Trees, while not harmed by a single season of defoliation, can be severely injured if this continues for several years in succession.

When did the defoliation occur? The worst time for defoliation to occur is just after the new leaves have fully expanded (usually late May). The tree is growing rapidly at this time and is dependent on these new leaves to support the growth. Another critical time is late season. Defoliation late in the growing season can result in premature bud expansion. These partially expanded buds may be more susceptible to winter injury.

W WATERING is still a good idea for trees and shrubs. In many parts of the state we have had limited precipitation during August. Some people are concerned that watering during late summer will delay hardiness and increase winter injury. While this can occur if marginally hardy trees are provided with luxurious amounts of water during late summer this is the exception rather than the rule. Instead, providing trees and shrubs with water will usually not delay dormancy and can increase winter hardiness. How much water is enough? Here is the typical weekly water use for various diameter trees; a 1-inch diameter (diameter measured 4.5 feet above the ground) requires 20 gallons, a 2- inch diameter tree requires 35 gallons and a 5-inch diameter needs 125 gallons of water every week.