Plant Pathology Infobytes

July 7, 1998
Tomato Spotted Wilt Responsible for Blotchy Tomato Fruit

As a result of unfavorable climatic conditions, this has been a tough season for growing tomatoes. In addition to the dry weather, gardeners are running into another tomato problem -- spotted wilt virus disease -- which is being reported in many gardens.

The tomato spotted wilt virus attacks and causes disease in a wide range of 166 plant species, but tomatoes seem to be a favorite host. Tomato spotted wilt virus infected tomato fruit exhibit 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.

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? There's nothing that can be done for infected plants, but the following procedures may be useful in a fall garden or next season.

  • Remove and destroy tomato plants, or other affected garden plants, showing spotted wilt symptoms.
  • 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 continue to observe plants 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, check with the County Extension Office.


Written by the late Dr. Frank Killebrew

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