Widespread occurrence of Tomato Spotted Wilt (TSW) in South Carolina during 2002 has caused considerable concern among our tobacco producers. Numerous producers in South Carolina have seen losses in early season plantings approaching 30-70% of their tobacco crop. County agent surveys estimate up to 20% of the tobacco plants in South Carolina were killed or severely stunted by TSW in 2002.
Scope: TSW occurs worldwide and has caused serious losses in Central Europe, Greece, Brazil and Argentina. More recently losses in the USA have increased dramatically in the southern production areas such as Georgia with sporadic occurrences in production sites further north. Recently (2002, 2006), the incidence and severity of TSW has increased in South Carolina. The incidence of TSW causing noticeable stand loss reached 25% of the production fields in 2001. Damage appears to be more severe when the winter and spring weather is dryer than normal. TSW has a wide host range (166 species in 34 plant families) and can be found in winter weeds. Symptoms of TSW will depend on the age of the plant and the environmental conditions during plant growth. Early infection immediately after planting can kill the plant rapidly, appearing like damping off. As the plant ages new growth contains typical centric necrotic rings and zonate necrotic spots on the young leaves. The bud will frequently be twisted. As the plant matures black necrotic streaks can be seen on the stem. Severe stunting occurs after infection. Infected plants typically do not increase in height after symptom expression. Early infected plants rarely produce harvestable leaves. Plants can be infected at any stage of development. However, in South Carolina infections typically occur early in the season with losses climaxing in mid to late May.
Control: TSW infections occur through wounds in epidermal cells caused by tobacco thrips. Generally insecticides have been ineffective in reducing virus transmission because very little time is required to transmit the virus. The insecticide may kill the insect but only after the plant has already acquired the virus. Thrips populations peak in April and May and then decline in June. This approximates the timing of TSW seen in South Carolina. Imidacloprid (Admire 2F and Admire pro) applied as a greenhouse tray drench and to a much lesser extend as a transplant water treatment has been shown to reduce TSW in field plantings in Georgia and in South Carolina. The reduction in TSW may not be directly related to control of the thrips. Newer aphid control materials such as Platinum also reduce TSW. In trials in 2002, Platinum appears to give similar suppression of TSW when compared to Admire. TSW control following Admire or Platinum treatment can range from 25% to 50% reductions in plant loss. Due to the random nature of infection across the field (no edge effects) stand losses as great as 10% generally do not result in yield reductions! As the number of plants killed increases above 10%, dramatic yield and quality losses can be expected. Plants that are bordered by missing plants pick up nitrogen normally used by competing plants. This results in uneven ripening across the field. Although the effect is not readily apparent from observing the field, there is a pronounced lack of uniformity in leaf ripening and is reflected in leaf quality.
Actigard is a new pesticide labeled for the control of blue mold on tobacco. Actigard works through stimulating the plants own defense mechanisms, commonly refereed to as systemic acquired resistance (SAR). Excellent blue mold control has been observed following Actigard application. Data suggests that Actigard will also reduce TSW. Use of Actigard in combination with Admire or Platinum is additive providing a better alternative to producers than Admire or Platinum alone. Both products used together can reduce losses 50-60% on a regular basis. However, the potential of plant injury exists with the use of Actigard . Formerly, Actigard was labeled through a special third party label due to the potential for plant injury. The present blue mold label specifies application when the plants reach 18 inches tall, generally to late for TSW control. If you are considering the use of Actigard in 2008 for TSW suppression, check with your county agent for details.
Expectations: Losses to TSW are generally most severe during the first month after planting in the USA. Although damage can sometimes be seen throughout the season our experiences in South Carolina suggests reductions in new occurrences after the middle of May. The potential losses to TSW in 2008 cannot be predicted, however, based on the historical losses in Georgia severe losses in South Carolina could occur in 2008. Farmers should carefully weigh the cost of control and expectations for disease reduction when choosing their disease control system.
Host Resistance: Although promising breeding material exists, no released variety is resistant to TSW. Host resistance will play a vital role in suppression of TSW in the future.
The following points should be considered to help control Tomato Spotted Wilt: