A novel ascomycetous Xylogone ganodermophthora strain has antifungal activity. A composition includes the strain as an active ingredient for preventing plant diseases. A method for producing the composition includes culturing the strain, and a method for preventing plant diseases includes using the strain. The strain suppresses the growth of pathogenic fungi, including Phytophthora capsici, in plants. Therefore, the composition containing the strain, or a culture or extract thereof, as an active ingredient for preventing plant diseases has excellent antifungal activity and can thus be used as an environmentally friendly and pollution-free pesticide.