Pest Alert

August 16, 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).

W DUTCH ELM DISEASE is still showing up across the state with many communities reporting higher than normal losses. The reason for this increase is not so much that we are seeing an increase in infection, but instead an increase in the disease expressing symptoms. According to Mark Stennes, a Minnesota DED researcher, the wet weather we experienced earlier this year resulted in the formation of larger early-wood vessels (the "tubes" that transport water throughout the tree) which are conducive to the spread of the disease. Most likely, the trees that expressed symptoms earlier this season were infected sometime in the past year or so. The moist conditions allowed the disease to spread quickly through the trees this year.

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.

L RUST ON TURFGRASSES: Warm days and cool nights also favor the development of rust on many hosts including turfgrasses. The fungus that causes rust on turfgrasses blows in from southern states each year. Once it is here it can reinfect grasses area lawns. It is most common on newly seeded lawns where the grass may be left a bit longer during establishment. Young, tender plants also seem to be slightly more susceptible to the disease. Common bluegrass is somewhat susceptible, but the bluegrass cultivars Merion, Touchdown, and Windsor are very susceptible. Lawns stressed by low fertility, drought stress, or compaction, may also be more likely to have a rust problem. (Rust spores on shoes after walking in lawn.)

Control rust in turf by catching and removing your clippings. This is a form of sanitation that helps eliminate the pathogen from the site. Improve your fertility with a light nitrogen application. At this time of year a cut rate of about 1/3 of a pound of actual nitrogen will help with growth without causing too heavy a flush of growth. Fungicides are not usually needed unless disease is very severe and the weather favors continuous infection. If you choose to use a fungicide, most of the products labeled for turf application will help. Chlorothalonil, triadimefon (Bayleton, and Green Light Fung-Away) and mancozeb (Fore, and others) are all good choices for rust management.

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.

W BAGWORMS on arborvitae and junipers have appeared across the southeastern portion of our state. The "bags" appear as elongated masses of silk, twigs and needles hanging from the tips of arborvitaes and junipers. The bags range in length from 1 to 3 inches long. Inside these bags are the larvae. These will be pupating soon with the adults emerging in early fall. The female moth does not fly and instead appears maggot-like, lacking eyes, legs and antennae. The males have wings and can fly to adjacent plants in search of females. To prevent the females from expanding the populations on infested trees, remove and destroy the bags that are now present. Generally pesticides are not used to control the adult stage so the next opportunity for pesticides would be next June when the larvae have just hatched. (Note: Do not confuse this insect with Fall Webworm which makes larger webbed nests which contain many larvae.)

W FALL WEBWORM. The larvae should be approaching about an inch in length and their feeding damage and webbed nests will be highly visible. The larvae, which are either pale yellow with red heads and reddish-brown spots or yellow-green with black heads and a dark stripe, feed in groups in webbed nests constructed around leaves at the tips of branches. They are sometimes confused with tent caterpillars which occur earlier in the year and construct webbed nests in branch crotches. Fall webworm can be control with applications of Bacillus thuringiensis (B.t.) if the applications are made while the larvae are still small. Carbaryl and acephate may also be used but these will also kill the natural enemies of the fall webworm.

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. 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.

L DAMAGE FROM WHITE GRUB INFESTATIONS in lawns begins to appear this time of year. Symptoms include a general browning of the turf similar to severe drought stress. This is due to the severing and consumption of turfgrass roots by the final larval instar stage of certain Scarab beetles, such as May/June beetles, Masked Chafers, and Black Turfgrass Ataenius. With little or no roots, the turf cannot take up water and is no longer anchored in the soil. Large patches of turf can be easily lifted from the lawn. Oftentimes, damage from foraging animals such as skunks, raccoons, moles, and birds is more severe than actual grub damage. Rolling back the sod should reveal a cream-colored, C-shaped grub with a tan to brown head and three sets of prolegs. If populations are less than 8 to 10 per square foot for Masked Chafers or 3 to 5 per square foot for May/June Beetles, persistent watering may allow turf survival without the use of insecticides. If chemical treatment is warranted, Dursban or Diazinon can be applied now. Be sure to follow insecticide applications with 1" of water in order to move the product into the soil where the grubs are feeding. Since grub populations in the lawn are usually concentrated in certain areas, scouting for these areas and spot treating can reduce insecticide amounts. If populations are present this year, consider using one of two new products, Merit or Mach 2, next year. These products have low mammalian toxicity, are more environmentally friendly, and have season-long residual activity when applied in Spring. Don’t apply Merit or Mach 2 this late in the season since their mode of action affects growth regulation between larval instar stages, and won’t be effective.

OTHER NOTES

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 weeks.

V ROTTING, OFF-COLOR & MALFORMED TOMATO FRUIT has been common this year, especially in wetter areas of the state. In most cases the fruit rot is due to either Late Blight, Early Blight or Anthracnose. Late blight will cause fairly large areas of the fruit to look black but remain firm until the fruit ripens when it will soften and become watery. Early blight will generally attack the fruit at the stem end of the fruit causing darkened, rotting areas that have the distinctive target pattern associated with Alternaria infections. Anthracnose will cause small, circular, depressed areas to develop on the fruit which soon soften and spread through the fruit. Blossom-end rot has also been common. Here a calcium imbalance in the fruit results in a blackening of the blossom end of the fruit. This problem is usually more of a problem on the first fruit to ripen and if the tomatoes have not been watered properly. (See earlier issues of Pest Alert for more on blossom-end rot.)

Some gardeners have complained about fruit that does not ripen evenly and has hard greenish or whitish areas in the flesh. This is often a physiological problem called "persistent green shoulder". Related conditions described in some references include "internal browning" and "graywall". In most cases this severity of these problems will vary depending on the season and the variety of tomatoes grown. Some references also are made to involvement of certain bacteria or viruses. There is no treatment for these problems other than to try to provide good growing conditions, select good varieties and plant more than one to help insure a harvest.

Malformed fruit can develop from a variety of causes including exposure to chemicals like herbicides, unusual weather and the variety of tomatoes that are grown. One common problem is called "catface" which is usually the result of exposure to herbicides during flowering. The growth regulator effects of chemicals like 2,4-D cause uneven development of the tissue inside the fruit resulting in malformation of the fruit. Some varieties develop a nice smooth bottom and top to the fruit while others are known to have irregular shapes.

W SHEARING OF HEDGES should be completed within the next week or so. In response to late summer shearing, many shrubs will put out one more flush of growth. This late growth may not have time to harden in time for winter and may be injured by a late fall cold snap. Yews, for example, that are sheared after the Labor Day week will sometimes put out one more growth flush. This growth will be killed by subfreezing temperatures in October. The next spring the plant will exhibit numerous brown tips.

W,F WATERING is still a good idea for trees and shrubs. 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. Tree fruits should also be watered to provide adequate moisture to increase fruit size. Water-stressed fruit trees will have smaller fruit or may have fruit that develops large cracks, making it unusable.