Pythium blight

Pythium aphanidermatum

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

Symptoms and signs


Dense mycelium of Pythium aphanidermatum on tall fescue

Pythium blight first appears as small, irregularly shaped spots ranging from ½ inch 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, moldlike growth of the causal fungus.

Pythium symptoms

Symptoms of Pythium blight on creeping bentgrass putting green.

Disease cycle

Pythium fungi may survive for long periods in the soil. In turf with a 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. Pythium blight develops more rapidly when nitrogen levels are high, and more slowly under moderate or low nitrogen fertilizer programs.

Cultural control

In problem areas, satisfactory plant growth maintained through moderate fertilizer applications provides the least risk of Pythium blight damage. Kentucky bluegrass, the fine fescues, and tall fescue are less susceptible to Pythium blight than perennial ryegrass and bentgrasses. Providing adequate water and air drainage, especially where susceptible grasses are grown, can help control this disease.

Chemical control

In the northeast United States, bentgrasses and ryegrasses may need fungicide protection. Timing of fungicides is important. If continued hot, wet weather is expected, the first spray should be applied as soon as symptoms develop. On high value turf, such as golf course putting greens, tees, and possibly fairways, preventative fungicide applications may be justified.