May 15, 1998
Compiled by Vern Grubinger, University of Vermont Extension
(802) 257-7967

THE UVM PLANT DIAGNOSTIC CLINIC, coordinated by Ann Hazelrigg, is located at 235 Hills Building, UVM, Burlington VT 05405?0082. (phone 656?0493). The clinic identifies diseases, insects and weeds in all commercial crops. Their fee per sample is $15.00, but if the fee is a hardship simply request a waiver of the charge. Samples should be packaged in plastic bags in boxes or padded envelopes. No wet paper towels please! Send samples that are just showing the symptoms in addition to later stages of the problem if possible. Insects can be sent in clear alcohol or in plastic bags. Send sample early in the week so there is little chance of them sitting over the weekend at the post office. Large samples can also be sent on Vermont Transit buses. Just call and alert Ann and she will pick it up. Please provide background information about the problem as it helps with the diagnosis. Include your phone number so a follow?up call can be made if necessary. The clinic can also do virus testing of plants but these samples will be shipped to a lab specializing in virus detection, adding a fee of approximately $10.00.

GRAY MOLD IN STRAWBERRIES becomes a problem at harvest time, but if fungicide applications are to be effective they must be made during flowering. The gray mold fungus often attacks through dying, dead or injured floral parts such as petals or stamens. Since first infections typically occur as flower petals begin to naturally die, fungicides need to be in place during flowering. For information on labeled materials and rates see page 27 of your 1998?99 New England Small Fruit Pest Management Guide, or call my office for a copy. Illinois Extension suggests that successful gray mold spray control programs have 3 components: 1) Fungicide application intervals of 5 to 7 days unless conditions are very dry, then 10 day intervals. Applications should begin at 10% bloom and continue until flowers are gone. Preventing gray mold from attacking naturally dying flower petals and other floral parts is key to gray mold control. 2) Uniform application of fungicides using high gallonage (100 to 200 gpa spray volumes) Uniform application requires either high pressure with multiple nozzles pointed at each row or air blast equipment. 3) Use of different fungicides with different modes of action to avoid development of resistance.

CORN FLEA BEETLE is the primary vector of the bacterium which causes Stewart's wilt disease in sweet corn. Stewart's wilt is characterized by development of conspicuous streaking on the leaves and stalk due to plugging of vascular tissue. Early wilt infections cause reduction of yield by reducing stands due to death or stunting of infected seedlings. Seedlings which survive early infections remain stunted, tassel prematurely, and frequently produce no ears or nubbins. Very cold winter conditions cause high mortality to corn flea beetle populations. To forecast the likelihood of Stewart's wilt, sum of the mean monthly temperatures for December, January, plus February. Then likelihood is as follows: sum of 100+ = severe; 90-100 = moderate/severe; 85-90 = moderate; 80-85 = light; below 80 = absent. In the warmer areas of Vermont it is possible that Stewart's wilt may be a problem this season. However, Stewart's wilt has not recently been common in the state so not much inoculum is available for the beetles to pick up and carry through the winter. If you are concerned about the potential for Stewart's wilt, plant Stewart's wilt resistant sweet corn varieties whenever possible (see list in the New England Vegetable Management Guide). Delaying planting of susceptible varieties to "mid?season"can help because fewer beetles are present at that time.

* Tarnished plant bug nymphs found on raspberry buds. Flea beetles on brassicas (Burlington).
* Cabbage maggot fly eggs have been found on overwintering turnips. Crabgrass germinating early. Annapolis in bloom under row cover (Norwich).
* Cabbage root maggot eggs found on broccoli transplants 2 days after setting out, covering subsequent plantings (Hadley MA)
* Crabgrass germinated earliest ever seen. Tender annuals being set out (too?) early. Sales of bedding plants are early and brisk. Botrytis a problem in some flower houses due to cloudy weather and high humidity (E. Hartland)
* Flea beetles on uncovered cole crops. Thrips in greenhouse being kept at bay with consistent introductions of predator mites and sprays of BotaniGuard bio-insecticide (Shaftsbury)
* Thrips biocontrol failures in other locations ? going to sprays of Orthene or Avid.
* Phomopsis, fusicoccum cankers and winter injury on blueberries around the state ? pruning out of infected tissue underway.

TARNISHED PLANT BUG (TPB) is a major pest of strawberries and over 50 other crops including raspberries, peaches, apples, cabbage, broccoli, cauliflower, celery, snapbeans, cucumber, tobacco and alfalfa. TPB is a "true bug" with piercing-sucking mouthparts. The adults overwinter in protected areas such as leaf litter, plant debris, hedge rows and brush piles. They are 1/4 inch in length, greenish or brownish in color with a yellow triangle marking on the wing. Egg laying occurs at temperatures above 68 degrees F and lasts 10-31 days. The female lays approximately 5 eggs per day in grasses, broadleaf weeds (dandelion and chickweed), and in some crops like strawberry usually in early to mid-May. The eggs are inserted into the stems, petioles and mid-ribs of leaves, into buds, or tucked in florets of the same flower heads (e.g., composite weeds such as dandelion). The eggs hatch in 7-10 days. The first nymphs usually emerge during full-bloom period of mid-season flowering cultivars (e.g., Kent and Governor Simcoe). Nymphs undergo 5 instars (stages of development). The first to third instar nymphs are pale green and look like aphids. Nymphs can be differentiated from aphids by their more rapid movements, more robust legs and their lack of cornicles ("honey-tubes" on the "back end" of the aphid). Fourth and fifth instars show brownish features and lengthening wing pads. Nymphs develop into adults in 12 to 34 days. At higher temperatures these insects develop more rapidly.

Both adults and nymphs feed on and cause damage to strawberry fruit, but early?season nymphs are the most important economically on June?bearing cultivars of strawberry. Feeding can cause death of flowers and slight to severe deformation of the fruit. TPB feed on the achenes (seeds) or tissue just under the achenes. Feeding by TPB usually occurs in a cluster of seeds in one area of the fruit. The "fed upon" seeds abort and the growth hormone production in these seeds ceases. This prevents the normal development of the fruit. Generally the earlier TPB feeding occurs, the more severe the deformation/injury to the fruit is. Apical seediness is most likely to occur when the number of nymphs is above threshold and they are feeding during the petal fall period. Catfacing occurs to a lesser extent when TPB feeds during the achene separation period, when the seeds on the young green fruit have developed and the fruit is beginning to expand leaving space between seeds so that they are no longer touching. Feeding by nymphs during the achene separation to mature green stage produces marketable berries with only a slight deformation, mostly confined to the side of the fruit. At harvest, the presence of hollow and slightly brownish achenes in the deformed areas of the fruit is a good indication that TPB feeding has occurred. This damage is distinguishable from poor pollination by examining the size of the achenes. With TPB, the achenes are relatively the same size in the "deformed area" as compared to the "healthy area" of the fruit. On the other hand, fruit with poor pollination problems usually exhibit smaller achenes in the deformed area compared to the surrounding healthy area of the fruit.

Field monitoring should take place twice a week from the beginning of bloom until first pick. Keep observations for specific varieties separate, as later flowering varieties tend to have higher populations of TPB nymphs and potential injury. Monitoring is done by tapping of flower or fruit clusters into a shallow, white dish. The dish should be deep enough to capture the bug when tapping, but not so deep as to make it difficult to examine the nymph with a hand lens. If any TPB nymphs are present the cluster is considered infested. Under wet or windy conditions in the field, the tapping method may underestimate the true population of TPB nymphs on the flowers or developing fruit, so the field should be re?monitored in 1 to 2 days, especially if the number of infested clusters is just slightly below the treatment threshold. Sample at least 30 clusters; if less than 1/4 of clusters have nymph(s), no treatment is recommended.

Broadleaf weeds are alternate hosts and encourage the build?up of TPB populations. Good weed control throughout the season, in and around the strawberry field, can reduce overwintering TPB. Preferred weed hosts for TPB include dandelion, chickweed, wild mint, creeping charlie, and goldenrod. Weedy ditches, fence?lines with scrub vegetation, woodlot areas, and alfalfa fields are ideal feeding and overwintering sites for TPB. Early season mowing of alleys and field edges is important because the nymphs can not fly and remain in the ground cover and therefore are routinely killed by mowing operations. Mowing later in the season may actually force surviving adult tarnished plantbugs into the fruit canopy in search of food and egg-laying sites.

At the present time there is no economical biocontrol method available that will control TPB. However, there is some hope for the future. Promising areas include: a parasite known as Peristenus digoneutis, which is being researched but is not yet commercially available, and the fungal insect pathogen Beavaria bassiana, which is available for field use as Mycotrol, but is very expensive and not yet distributed in the northeast. In California, it has been reported that insecticidal soap will reduce certain plant bug populations. While promising, more research is needed to test its efficacy and ensure that it is non?phytotoxic to strawberries and then obtain appropriate registrations.