Tomato Spotted Wilt Virus
Tomato Spotted Wilt Virus (genus Tospovirus; family Bunyaviridae) is an important pathogen of many ornamental, greenhouse and agronomic crops worldwide. At present, the host range of tomato spotted wilt virus includes over 1000 different plant species in 82 botanical families encompassing both monocotyledonous and dicotyledonous plants. Some of the more common hosts are tomatoes, peppers, celery, lettuce, eggplant, peanuts, and many weeds such as field bindweed and curly dock. This disease is especially damaging in the ornamental and vegetable greenhouse industry.
Symptoms of tomato spotted wilt virus are incredibly variable and heavily dependent on the host, viral isolate, plant stage at the time of infection and environmental conditions. However, there are two fairly common symptoms for which this disease was named. First, the young leaves turn bronze and subsequently develop numerous small, dark spots. Second, the leaves often droop on the plant, creating a wilt-like appearance (Figure 1). Other symptoms include die-back of the growing tips and dark streaking of the terminal stems. Affected plants may develop a one-sided growth habit or may be stunted completely. Plants that are affected early in the growing season often do not produce any fruit, while those infected after fruit-set produce diseased fruit with striking symptoms, including chlorotic ringspots, raised bumps, uneven ripening, and deformation (Figure 2) (Figure 3). Infected plants produce poor quality fruit and reduced yield.
TSWV is transmitted from infected plants to healthy plants by at least nine species of thrips. Thrips are tiny (approximately 1/16th of an inch) winged insects that feed on plants through sucking mouth-parts. Thrips transmit the virus in a persistent manner, which means that once the insect has picked up the virus, it is able to transmit the virus for the remainder of its life. The virus is not passed on from adult to egg; however, progeny that develop on infected plants will quickly pick up the virus and be effective disease vectors.
Controlling this disease is difficult. The wide host range, which includes many perennial ornamentals and weeds, enables the virus to successfully over season from one crop to the next. Additionally, efforts to control the insect vectors in agricultural fields has had little effect on TSWV. This is likely due to the fact that large populations of thrips may fly or be blown into treated fields from non-treated areas nearby.
Controlling thrips is somewhat more effective in greenhouse situations. In greenhouses, however, growers should take care to avoid repeated sprays of similar insecticides, as thrips are able to build up resistance to commonly used insecticides in a relatively short time. Rotating the insecticide class is the best approach to insect control.
Control of thrips may be obtained with pyrethroids, carbamates, chlorinated hydrocarbons, organophosphates, and soaps. Insecticides are most effective when applied in the morning, when the thrips are most active and the chance for plant damage is reduced. Pesticide regulations change frequently, so check with your local county extension service for information on available insecticides.
While elimination of disease may not be possible, the incidence and severity of the disease may be reduced by using cultural practices such as starting with virus-free plant material, removing all infected plants (once virused, there is no cure for the diseased plant), controlling weeds, and rotating crops. Some studies also have shown that the use of reflective mulches under plants may help to reduce infection. In greenhouses, it may be possible to greatly reduce the number of thrips entering the greenhouse by covering doors and air intakes with a fine mesh (400 mesh) cloth. Efforts are underway to breed cultivars with good horticultural characteristics that also exhibit tolerance to the virus.