In recent years, we have seen the introduction of a new tomato disease. Tomato spotted wilt virus (TSWV) was unknown to our area 10 years ago. Nowadays, TSWV turns out to be the culprit in about 50 percent of the tomato problems I diagnose for local growers.
Symptoms of TSWV are numerous and varied. 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. 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. 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 mouthparts. 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 overseason 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.
While elimination of TSWV 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. 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.
There are no indeterminate resistant varieties available. However, there are a few determinate varieties available. Floyd Anderson, a local producer, has had good results with the variety é─˙Ameliaé─¨. It was amazing to me to see tomato plantings in Floyd's field where the Amelia's looked wonderful while nearby susceptible varieties looked devastated. Even though determinate varieties only bear for a short time, those who have been plagued with TSWV may want to consider planting a few Amelia's. Several plantings could be made if you want to extend the time of bearing.