Tomato Spotted Wilt - Bad Things Happen to Good Tomatoes

Plant Pathology Infobytes

May 12, 1999

Tomato spotted wilt virus disease is appearing in many home garden plantings. The causal virus attacks and causes disease in a wide range of 166 plant species, but tomatoes seem to be a favorite host. Spotted wilt infected plants exhibit a range of symptoms on foliage and fruit. However, the primary damage occurs on fruit which become discolored and develop irregular yellow blotches.

Blotched areas range in size from about one-fourth to occasionally one inch, or larger, in diameter and tend to be circular in appearance. Affected fruit are worthless, since they fail to ripen properly and are unfit for fresh consumption or processing.

Early symptoms of the disease show up on tomato leaves, which generally have many small, dark colored spots on the youngest foliage. Infected leaves often develop a bronzed appearance, and the tops of plants sometimes take on a "bunchy" appearance since top growth of infected plants is slowed by the disease.

Tomato spotted wilt diseased fruit are found on plants which were infected earlier in the season by the virus, as a result of thrips feeding. Thrips are tiny insects which pick up the virus from infected weeds and carry virus particles to tomato plants, which become infected during thrips feeding activities. In addition to tomatoes, peppers (bells, hots, and other types) are susceptible to this disease.

How do you handle tomato spotted wilt virus disease? First, determine if you have a spotted wilt problem. Collect samples (leaves from tops of suspect plants are best) and bring them by your county Extension office for a free diagnosis.

Gardeners with tomato spotted wilt can't do much about the disease this season, but the following procedures may be useful in plantings carried out later this spring or in the fall garden.

Remove and destroy tomato plants, or other affected garden plants, showing spotted wilt symptoms. This will cut down on the chance for movement of the causal virus from diseased to healthy plants.
Keep down weeds in and around gardens to reduce the movement of virus-bearing thrips from weed hosts.
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, that plants should continue to be checked for thrips presence.
Research has indicated 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 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 your county Extension office.

Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.