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Pythium blight, also known as grease spot and cottony blight, can be a highly destructive turf grass disease, especially on bent grasses and ryegrasses. Severe outbreaks can completely destroy the turf grass within a few days if weather conditions favor disease development.

Pythium blight first appears as small, irregularly shaped spots ranging from 1/2 to 4 inches in diameter. Leaves appear water-soaked in appearance at first, then shriveled. Diseased patches fade to a light brown or gray color. Groups of spots frequently join together. At times, the shape of the affected areas may resemble elongated streaks. Both the presence and pattern of these streaks seem to be determined by the flow or presence of surface water. With high humidity in early morning or throughout the day, diseased leaves may be covered with the white, cobwebby, mold like growth of the causal fungus. Pythium fungi may survive for long periods in the soil. In turf with a past history of Pythium blight, infected plant debris from the previous season or fungus spores in the soil are sources of infection. Disease development from the first infection centers occurs by growth of fungal mycelium and movement of spores from plant to plant. Under conditions favorable for disease development, Pythium blight can spread very rapidly. Primarily a warm, wet weather disease, turf blighting and disease development will be most rapid and severe at air temperatures from 85° to 95° F. As the air temperature approaches 95° F, destruction of grass stands can occur in a very short time.

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