Pythium Blight, caused by Pythium spp., occurs during hot, humid weather. Circular to irregular spots appear -- initially 1-6 inches in diameter. Early in the morning these spots have a greasy green or water soaked appearance, and the grass feels slimy or greasy. Later the grass in these spots dies and the dead grass becomes light tan and shriveled. The affected areas may have a matted appearance and a cottony white growth may develop on the dead leaf blades during wet periods. Grass blades at the edges of dead patches may have straw-colored borderless lesions. This lack of a dark brown border on lesions distinguishes Pythium lesions from the lesions of Dollar Spot. If the grass was mowed while wet, Pythium Blight may be spread by mowing and develop in irregular streaks and patches in the direction of mowing. Pythium Blight also tends to follow drainage patterns.
Pythium Blight is most severe with day temperatures of 84-95 F and night temperatures above 68 F. The disease is more severe on heavily fertilized cool-season grasses. Wet soil and thatch also favor the disease. A film of water on the leaves is necessary for infection.
In some areas, particularly in the northwest, a new mutated strain of pythium blight is causing damage when there are extended, cooler and wetter springs. The infection may go undiagnosed as being pythium blight because of when the symptoms show.
Control of Pythium Blight requires careful water management. Avoid excess water or watering late in the day when the plants may stay wet all night. Do not mow when the grass is wet, especially in hot weather. Avoid excess fertilizer, especially nitrogen fertilizer in hot weather. If the thatch is over 1/2 inch thick, remove excess thatch by dethatching. No fungicides for Pythium control are readily available to the homeowner, but several commercially applied products are very effective.