Tomato Spotted Wilt Virus (TSWV) is a destructive virus disease that affects a wide range of plants including certain bedding plants, floral crops, and vegetables. It is now known that there are two different but closely related viruses that cause greenhouse symptoms. The most common virus, previously known as TSWV-I (Impatiens isolate), is now known as Impatiens Necrotic Spot Virus (INSV). This virus causes problems on plants such as garden impatiens, New Guinea impatiens, cyclamen, cineraria and gloxinia. TSWV-L (Lettuce isolate) is closely related to INSV and is found most often on dahlias, mums, and vegetables.
Both viruses have a very wide host range. Both are vectored by Western flower thrips and other thrips species.
Symptoms: TSWV-L and INSV can be difficult to diagnose because symptoms vary from plant to plant and according to conditions in the greenhouse. Some plants develop small concentric rings on the foliage or fruit. On other plants, leaf veins or stems become discolored. Flowers may show abnormal coloration. Plants may show symptoms of stunting. Some plants may be infected yet show no visible symptoms.
Because visual diagnosis can be difficult, suspect plants should be tested for the virus. The presence of the virus can be confirmed by a serolgical test. Samples can be submitted to the Plant Disease Clinic, 323 Bessey Hall, Ames, IA 50011, for testing. A fee of $10 per sample is charged to cover the cost of testing.
Control measures: There is no cure for infected plants. The first step in control is to discard diseased plants. The safest practice is to discard the entire block of plants in which symptomatic plants were found, whether or not the rest of the block has developed symptoms yet. Carefully inspect plant materials entering the greenhouse for possible TSWV symptoms and for the presence of thrips. Regular inspection of plants in the greenhouse is also important. Monitor and control thrips in the greenhouse. Eliminate weeds in and around the greenhouse because they can harbor thrips and/or the disease. Consider using indicator petunias, such as 'Calypso', 'Super Blue Magic', or 'Summer Madness' to monitor for TSWV/INSV and thrip feeding injury. These indicator plants are useful because they develop symptoms 2-3 days after exposure to thrips carrying the virus.
This article originally appeared in the February 10, 1993 issue, pp. 10, 1993 issue, pp. 12-13.