Investigation of In Season Progression of Tomato spotted wilt virus and its Management in Flue-cured Tobacco Fields of North Carolina

Abstract

Tomato spotted wilt virus continues to be a serious problem on tobacco production in the southeast United States. The current control recommendation calls for acibenzolar-S-methyl, which is known by growers in North Carolina to have unacceptable phytotoxicity. In an attempt to reduce the amount of phytotoxicity while retaining similar levels of control as the current recommendation, on-farm tests were conducted in North Carolina to refine the application rate, timing and method of acibenzolar-S-methyl application, with or without an insecticide. The lowest disease incidence and highest phytotoxicity were recorded in treatments consisting of greenhouse drench applications of imidacloprid or thiamethoxam and acibenzolar-S-methyl, as compared to the untreated control. TSW severity and temporal progress results suggest that certain treatments delay the onset of symptoms. Furthermore, supplementary greenhouse experiments showed that timing the clipping relative to the timing of applications of the aforementioned chemicals had no effect on phytotoxicity. Both the order in which the chemicals were applied and applying the second chemical at varying times relative to the first had no consistent effect upon the phytotoxicity measured in the field. Overall, applying acibenzolar-S-methyl with an insecticide in the greenhouse provided the best control among the treatments tested. The temporal and spatial distribution of Tomato spotted wilt (TSW) was studied in a total of 25 naturally infested fields in 2006, 2007, and 2008. Sections within the fields were monitored that ranged in size from 76 x 11 meters to 137 x 10 meters. TSW incidence was measured on a weekly basis. TSW temporal progression data was fit to a logistic regression model with cumulative degree days and past TSW field history as explanatory variables. Analyses revealed that both variables are significant explanatory factors of the temporal progression of TSW on tobacco. Cooling degree-days were calculated with four different bases, and a base temperature of 10°C, corresponding to thrips development, resulted in the best fitted model. TSW spatial distribution was investigated in 8 locations using universal kriging interpolations on TSW incidence at 5 and 9 weeks post transplanting. Although isolated disease clusters were present, the overall spatial pattern was random, which has been previously observed with TSW incidence in other crop systems in southeast US. These findings suggest that when thrips move into a field infections occur randomly. Further work is needed in order to determine the dispersal of thrips within a field, which will help refine management practices.