By Allison Matthews

MISSISSIPPI STATE -- Tomato spotted wilt virus is a persistent and growing problem for commercial tomato producers and home gardeners, but new resistant varieties are available for this growing season.

Alan Henn, Extension plant pathologist at Mississippi State University, said spotted wilt is a common strain of tospovirus that is becoming a more costly threat to many crops in the South and the rest of the nation.

"The threat has grown from being a controllable problem that may cause a low percentage of losses to a threat that could easily wipe out a significant portion of a tomato planting, if not cause an entire loss," Henn said.

Spotted wilt has an incredible number of hosts. Various strains of the virus can thrive in all types of crops, ornamentals, weeds and virtually any other plant that contracts the virus. The spread of spotted wilt is hard to control because it is carried by thrips, very tiny insects that can feed on an infected plant when they are young and carry the virus for the rest of their life. Thrips spread the virus to other plants with each feeding.

"It only takes about 30 minutes of feeding to move the virus, so even with a good pesticide coverage, it is quite possible that the thrips would not be killed before they had the chance to spread a tospovirus," Henn said.

Spotted wilt and its sister virus, impatiens necrotic spot, are unique because of the wide range of plants they affect. In Mississippi, spotted wilt has the greatest noticeable impact on tomatoes and peppers, Henn said.

Typical characteristics of the virus are dark spotting on the leaves and stems of a plant. Rings that resemble a bull's eye also may appear on the fruit, leaves and stems. The virus usually will not kill a plant immediately, but the plant will begin to decline, and it may produce misshapen or bumpy fruit.

Henn encouraged tomato growers to use resistant varieties of tomato plants while they are available. He said these varieties may lose their resistance within a few years because similar plants with single gene resistance have not shown long-term success in the past. Henn said plants lose their resistance because the spotted wilt virus has many strains with different genetic combinations. A plant that is resistant to one strain of spotted wilt may become susceptible to infection by another strain in a few years.

"Absolute resistance would absolutely be the best strategy possible against the virus. Unfortunately, it just hasn't been a Resistant tomato plant varieties BHN 444 and BHN 555 by seed company BHN Research will be available this year at the retail level. The BHN 444 variety is a table tomato, and BHN 555 is heat tolerant to produce fruit through August.

Henn recommended keeping garden areas cleared of weeds and other unwanted plants that could serve as hosts to thrips. Also, buy all garden plants from a reputable greenhouse to avoid purchasing plants that already may be infected with a tospovirus.