Effect of Protectant Fungicide Application Schedules on Gummy Stem Blight Epidemics and Marketable Yield of Watermelon

Abstract

The watermelon cultivar Royal Star was grown in fall 1996, spring and fall 1997, and spring 1998 and treated with 2.52 kg a.i./ha of the protectant fungicides mancozeb or chlorothalonil. Spray application schedules used in the experiments included two initiation times, sprays every 7, 10, or 14 days, and two termination times. Severity of gummy stem blight, caused by Didymella bryoniae, was high in fall 1996 and 1997, moderate in spring 1997, and low in spring 1998. In each experiment, fungicide applications reduced the area under the disease progress curve (AUDPC), percent leaf area diseased at the end of the season, time to reach 25% disease severity, or all three disease measurements relative to the nonsprayed control. In the 1996 and both 1997 experiments, 7-day spray intervals provided more effective disease control than 14-day intervals. In general, initiating sprays early reduced gummy stem blight compared with delayed sprays, but spray termination times did not affect AUDPC. In both fall experiments, fungicide applications increased yield of marketable fruit over the no-fungicide control. A 7-day spray interval increased marketable weight compared with a 14-day interval only in fall 1996. Weight of marketable fruit did not differ among treatments in either spring experiment. Differences in disease control among treatments often did not correspond to differences in marketable yields.