FUNGICIDE TIMING FOR OPTIMUM MANAGEMENT OF GUMMY STEM BLIGHT EPIDEMICS ON WATERMELON

During the fall of 1991 and 1993, four fungicide-application intervals were evaluated on watermelon for control of gummy stem blight, caused by Didymella bryoniae. Chlorothalonil combined with benomyl was applied full season (six times), early season (first half of the season, three or four times), late season (last half, two or three times), or not applied. During both years, early- and full-season applications reduced areas under disease progress curves compared with no-fungicide, and late-season applications gave less control than full-season applications (P less than or equal to 0.01). Slopes of disease progress curves changed over time in relation to fungicide applications and amount of healthy tissue remaining. In 1993, yields of marketable fruit were lower in all plots that received three or no sprays than in plots that received six sprays. In two additional experiments, areas under disease progress curves were lowest for chlorothalonil plus benomyl during both the fall of 1991 and the spring of 1993 compared with weekly applications of chlorothalonil alone, mancozeb, or alternating chlorothalonil with mancozeb. A minimum of six fungicide applications are required to manage gummy stem blight adequately on watermelon grown in disease-conducive environments.