Occasionally, it occurs about mid September. Hot, moist weather encourages the pathogens. The pathogen overwinters on the surface of seeds, in infected debris, and in soil. It is commonly brought into fields on infected transplants. Warm, rainy weather favors rapid spread of Gray Leaf spot.
Gray leaf rot is a soil-borne disease caused by Stemphylium botryosum f.sp. Lycopersici, Stemphylium lycopersici, Stemphylium floridanum, Stemphylium solani, which enters the plant through the roots. The fungus grows up through the woody tissue and produces toxic substances that cause the plant to yellow, wilt, and die.
