Pathogen: Tomato Spotted Wilt Virus (TSWV) Tospovirus
Symptoms: Symptoms include numerous small brownish ringspots (see photo 1 below), that may be so prevalent that the leaves exhibit a bronzed appearance, purpling and upward rolling of leaves and stunting of leaves and plants. Fruit symptoms (see photo 2 below) can vary; yellow ringspots are the most commonly recognized, but brown, necrotic sunken areas are also produced.
1. Leaf symptoms of TSWV 2. Fruit symptoms of TSWV
Disease cycle: This virus is spread by insects called thrips. Infected plants cannot be cured and should be removed to help prevent spread to uninfected plants.
Management: Cultural practices to help reduce TSWV infections include garden sanitation, weed control and reflective mulch. Sanitation measures start with purchasing tomato transplants that are free of thrips. These tiny insects have elongated bodies and are very active. After planting, weed control around the garden will remove alternate host plants for both thrips and virus. Destroy weeds around gardens, particularly dandelion, annual sowthistle, chickweed, buttercup, and plantain. These are some of the most important alternate hosts for both TSWV and thrips. Tomatoes can be planted into silver reflective mulch to repel thrips. Silver mulch will delay maturity of early plantings, however. If reflective mulch is not available locally or through garden‑supply catalogues, black plastic can be spray painted silver.
Minute pirate bugs (Orius insidious) and big‑eyed bugs (Geocoris punctipes) are very effective predators of thrips. These predatory insects are available for purchase from various insectaries. Do not use organophosphate or pyrethroid insecticides, as they will kill beneficial insects, such as these, that naturally control thrips.
There are no effective insecticides available to home gardeners that will control western flower thrips (the vectors) consistently enough to prevent infection by the virus. However, spraying tomato plants early in the season may help to control thrips to some degree. Various neem products, botanical insecticides that are effective against thrips, are available for home gardeners. Summer oil or insecticidal soap also may be used. Before using any crop protection materials in the home garden, please read and follow label instructions carefully.