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. 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. Thrips tend to hide in crevices in flowers and young foliage, where spray penetration is often poor. Clean up all weeds in and surrounding the houses. Remove any symptomatic plants. Do not accept any shipments of transplants with thrips populations or symptoms.