Xylogone ganodermophthora (Xg) is an ascomycetous fungus that causes yellow rot on cultivated Ganoderma lucidum. Previously, we reported in vitro antifungal activities of a Xg culture extract against several watermelon pathogens. In 2014, we conducted greenhouse experiments to evaluate the control efficacy of a water extract of cultured Xg on watermelon powdery mildew (WPM). The test material (stock solution, ca. 4,000 mg/ ml) was prepared by an autoclaved Xg culture in water at a ratio of 800

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... of culture per 6 liter of water, and then filtering it through filter paper. Six foliar applications of the solutions (diluted 100-and 1,000-fold) significantly suppressed the formation of conidiophores and conidia. The inhibitory effect of aqueous potassium phosphonate solution on the disease and its phytotoxicity was tested. Phytotoxicity on watermelon plants was observed at concentrations of 1,000 and 2,000 mg/ml as irregular brownish spots. The control efficacies against WPM were 91.9% at 2,000 mg/ml, 64.9% at 1,000 mg/ml, and 62.2% at 500 mg/ml.