Benefits of Applications of Chlorothalonil for the Control of Black Mold of Tomato.

Chlorothalonil was applied 2, 4, 6, 2 and 4, or 4 and 6 weeks prior to tomato harvest in fields in four counties in California over a 3-year period to assess the economic benefits of chlorothalonil applications, if any, on the reduction of black mold caused by Alternaria alternata. The percentage of visually infected fruit was significantly reduced across all four sites in 2 of the 3 years. At one location, the incidence of black mold was reduced over 50% in all 3 years of the study by a single application of the fungicide. Overall, the mean yield of marketable fruit was 79.8 metric tons per hectare. An average of 5.1% of the fruit harvested in all locations and years had visible symptoms of black mold. Generally, the most effective treatment was a single application of the fungicide 6 weeks before harvest, with no further benefit from a second application of the fungicide. Based on the cost of the fungicide and its application, a single dose of chlorothalonil applied to plots in Solano County, where the highest incidence of disease occurred, would net the grower $160.47 return per hectare. The application of fungicide resulted in significant economic benefits in tests in two of the counties, where the disease is historically a problem.