Plant Pathology Infobytes

April 26, 1996
Tomato Spotted Wilt Could Be A Problem This Year

Gardeners who've had first hand experience with tomato spotted wilt are aware this virus disease can be tough to handle. Tomatoes, peppers, and other vegetables are susceptible to tomato spotted wilt, and unfortunately, it looks as if this disease could be a problem for gardeners this season.

Control measures for tomato spotted wilt are limited, since varieties of tomatoes which are resistant to the causal virus are not yet available. The same is true for peppers, which are also frequent spotted wilt virus targets.

One of the major factors responsible for widespread spotted wilt problems this season is likely tied in with weather conditions earlier this spring. Cool temperatures and infrequent rainfall in March and through much of April allowed early buildup of spring weeds and thrips (tiny insects) which feed on weeds.

How do weeds, thrips, and spotted wilt connect? The virus responsible for spotted wilt persists from year-to-year in weeds such as chickweed, bindweed, mallow, nettle, and several others weed species. During the course of feeding on the weeds, thrips pick up the spotted wilt virus and later carry it tomatoes, peppers, and other vegetables. The virus is introduced into these plants during thrips feeding activities, and after seven to fourteen days, tomato plants develop symptoms of tomato spotted wilt disease.

Tomato plants infected by spotted wilt disease develop distinct symptoms. Look for distorted foliage near the top of plants. This foliage tends to curl down and back toward the main stem. As a result of blighting and dieback of young shoots, top growth of diseased plants is slowed down and takes on a bunchy appearance.

The most distinctive symptoms of tomato spotted wilt are found on individual leaflets, where small (pinhead size), black, circular, dead spots appear. Typically, this strain of the virus causes a "blackening" of the tops of plants. Dark streaks frequently appear on stems and leaf petioles. Occasionally, infected leaves take on a bronzed appearance, or less frequently, veins on the undersides of leaves of stunted plants develop a purple color.

Symptoms on bell and hot peppers resemble those caused on tomato plants, but peppers take on a more "bushy" and stunted appearance. Identification of tomato spotted wilt on peppers is more difficult and may require the assistance of your County Extension Agent.

Virus-infected tomato and pepper plants yield poorly, producing sm- all and unsightly fruit, or no fruit at all. Tomato fruit from infected plants may develop irregular yellow blotches.

Tomato spotted wilt has a wide host range and affects at least 166 species in 34 plant families. In addition to tomato and pepper, other susceptible garden plants include eggplant, lettuce, peanut, English peas, Irish potatoes, and spinach. A closely related strain of the tomato spotted wilt virus affects many types of ornamental plants.

What's the procedure for controlling spotted wilt? Unfortunately, spotted wilt resistant varieties are not available, and control measures for the disease are limited. However, the following procedures should reduce the amount of spotted wilt on tomatoes:

1. Remove and destroy tomato plants, or other affected garden plants, showing spotted wilt symptoms.

2. Keep down weeds in and around gardens to reduce the movement of virus-bearing thrips from weed hosts.

3. Thrips may be suppressed on tomatoes by applying either 57% malathion or 25% diazinon at the rate of 2 teaspoonfuls per gallon of water. These insecticides are also approved for thrips control on peppers and other vegetable crops. Check product labels for recommended rates and harvest restriction which apply for these products.

It is possible gardens could be reinfested following treatment because of the movement of thrips from wild hosts. This means following treatment continue to observe plants for thrips presence.

4. Research indicates the use of shiny mulch (aluminum color) materials around susceptible vegetables may cut down on thrips infestation, thus limiting chances for tomato spotted wilt virus transmission. Apparently, light reflected from the mulch surface interferes with the ability of thrips to locate the host plant.

The technique works best if highly reflective mulch is used. A suggested approach is to spray or paint black plastic mulch with aluminum paint after the mulch has been installed. Best results are obtained if the technique is used early in the season at the time of transplanting. Please note the reflective mulch should extend well beyond the transplants to afford protection as the plants increase in size.

Before going to the trouble of painting black plastic, check with your local garden supply dealer for availability of pre-silvered mulch. This type mulch is already available for commercial vegetable producers, and your local dealer could have this product in stock.

For additional information on disease and insect control in the home garden check with the County Extension Office.


Written by the late Dr. Frank Killebrew

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