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Disease Management

The usual approaches to management of diseases caused by viruses are host resistance, vector control or avoidance. While natural plant resistance to TSWV and other tospoviruses has been identified in a number of agronomic and horticultural crops, virus isolates capable of overcoming host resistance have readily appeared. Considerable effort has been expended to develop transgenic plants that have virus-derived genes to confer resistance to tospovirus infection. Transgenic tospovirus-resistant plant species that have been developed include chrysanthemum, peanut, tobacco, and tomato (Figure 17). To date, these have not been used commercially.

Figure 17

Use of insecticides to control thrips populations in the field is usually ineffective. Contact insecticides generally do not reach where the thrips are located on the plant, and systemic insecticides do not act rapidly enough to prevent virus transmission. Reflective mulches placed over plant beds prior to planting can disorient thrips or impair thrips feeding. Mulches have shown some utility in disease management, but results have not been consistent between different crops. Although elimination of thrips in the field is not practical, it is possible to reduce thrips populations in greenhouses. Screening air intakes and using double-door entries have helped reduce incidence of TSWV by preventing the entry of viruliferous vectors into the greenhouse. While avoidance or exclusion of thrips may be feasible in a greenhouse, it is not viable as a sole management practice in the field because of the wide host range of TSWV and the number of thrips species that transmit the virus.

An integrated approach that addresses many parameters that affect tomato spotted wilt development has been successful in mitigating tomato spotted wilt in peanut in the Southeastern United States. Peanut variety, planting date, plant population, insecticide application, disease history, row pattern and tillage were identified as factors affecting disease development. These factors are weighted to determine a "risk index" for TSWV in the crop. The grower obtains a low, moderate or high risk value that can be considered when implementing crop production practices (Figure 18; http://www.tomatospottedwiltinfo.org/peanut/riskindex.htm).

Figure 18

Indicator plants have been useful as an early warning system for the presence of infective thrips in certain ornamental crops. This strategy was most useful in identifying significant sources of infective thrips that could be removed as a preventive measure. In one California cut flower operation, the identification and removal of TSWV sources using indicator plants resulted in a reduction of virus incidence from 74% to 1%. Thus, crop specific integrated approaches for spotted wilt management will likely have to be developed. Continuing to elucidate the epidemiology of TSWV and to develop and deploy crops with natural and genetically engineered resistance may result in more effective management of tomato spotted wilt in the future.