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Tomato Spotted Wilt Virus in Tomato Fields
This article was published originally on 7/15/1994This virus, abbreviated TSWV, has become a serious problem for some Iowa tomato growers in 1994. While most fields we have seen have only a few symptomatic plants, some fields have more than one-third symptomatic plants. Because plants showing symptoms this early in the season seldom produce marketable fruit, this means a yield loss of at least one-third. According to unconfirmed reports, some TSWV-stricken fields have been plowed down. Some questions and answers about this situation:
What are the symptoms of TSWV?
Symptoms vary depending on plant age, environmental conditions, and other factors. Plants infected early in the season tend to be stunted, often dramatically, and may fail to set any fruit. These symptoms are often the first ones that catch a grower's eye. Typical foliar symptoms include yellowing and a bronze or purplish cast, especially in the upper leaves. Eventually, the discolored leaves wilt and die, at which point TSWV infection is often mistaken for bacterial or fungal diseases. If plants become symptomatic after fruit set, immature fruit sometimes show green or brown, concentric rings with raised centers. More often, though, typical fruit symptoms include slight malformation or brown, sunken spots that can be mistaken for symptoms of bacterial spot.
Because of the ambiguities invloved in diagnosing TSWV on the basis of field symptoms, it's a good idea to send plants to ISU for a confirmation. Send entire plants, including fruit to the Plant Disease Clinic, 323 Bessey Hall, ISU, Ames 50011. We can usually make a rapid diagnosis at no cost to the grower.
Why is TSWV more severe than usual in 1994?
The likely answer is that populations of thrips, the insect vector of TSWV, were unusually high this spring and summer. In many cases, the problem probably began in the greenhouse. TSWV can survive the winter in hundreds of plant species, including scores of common bedding plants and weeds. Several growers recall seeing populations of insects on their tomato transplants; it's likely that thrips were present in at least some of these cases. Thrips can pick up the virus from an infected plant and spread it repeatedly, every time it feeds on a new plant. Because there is usually a time lag of several weeks between TSWV infection and symptom development, growers can unsuspectingly transplant infected plants.
Thrips can continue to spread the virus in the field. If infected plants are present early in the season, the extent of the problem can continue to snowball. It's a tough problem to control in the field for several reasons:
In the greenhouse:
Year of Publication:
IC-467(18) -- July 15, 1994