Evaluation of combinations of chlorothalonil with azoxystrobin, harpin, and disease forecasting for control of downy mildew and gummy stem blight on melon

The foliar diseases downy mildew, caused by Pseudoperonospora cubensis, and gummy stem blight, caused by Didymella bryoniae, are frequently observed on melon (Cucumis melo ssp. melo cv. Athena) grown in the southeastern United States. Four combinations of chlorothalonil with or without azoxystrobin or harpin were applied weekly or according to the Melcast melon disease forecaster in field experiments in South Carolina in the autumn of 2002 and spring of 2003. Control treatments received water only or mancozeb followed by chlorothalonil. All nine fungicide treatments reduced areas under disease progress curves for downy mildew and gummy stem blight plus downy mildew compared to the water control. Chlorothalonil applied according to Melcast scheduling usually did not differ from chlorothalonil applied weekly. Azoxystrobin alternated with chlorothalonil was less effective than chlorothalonil alone when both diseases were rated together. Applications of harpin had no effect on either disease, but increased yield of fruit by 10% to 12% in one of two years compared to treatments that did not receive harpin. Growers could start the season by using Melcast scheduling to apply fungicides for gummy stem blight; these applications would also protect the crop initially from downy mildew. If downy mildew appeared during the season, the spray schedule could be switched to weekly intervals.