Text adapted from Turfgrass Pest Management: A Guide to Major Turfgrass Pests in Oklahoma. Oklahoma Cooperative Extension Service.

CAUSE: Pythium blight (also known as grease spot, spot blight and cottony blight) is caused by species of the fungus Pythium. The fungi that cause this disease are called water molds, because excess free water is essential for their development and also weakens the turfgrass, favoring severe disease development. This highly destructive turfgrass disease can completely destroy an entire golf green within a single 24 hour period under favorable conditions.

HOSTS: Bentgrass, ryegrass and occasionally bermudagrass sustained under intensive culture.

SYMPTOMS: Pythium blight first appears as small, sunken circular patches from 1 inch to 12 inches in diameter during warm to hot, humid weather (Figure 1). The patches often resemble the early stages of hot weather wilt. Gray, cottony fungal growth may be seen in the infected areas during very humid weather along with greasy, water-soaked, blackened leaves that become matted (Figure 2). The water-soaked leaves often feel greasy and turn straw colored when the turf dries. The disease spreads rapidly along drainage patterns and can be spread by equipment (Figure 3).

DISEASE CYCLE: Species of Pythium can survive in soil and infected roots. They are common in thatch and are found everywhere. When conditions are favorable, the fungi begin rapid growth and produce mobile spores. These spores are very small and move rapidly in free water between soil particles. These spores germinate and begin infection when they come in contact with turfgrass roots.

FACTORS FAVORING DISEASE DEVELOPMENT: Pythium blight becomes very active when relative humidity is high and day and night temperatures exceed 85°F and 68°F, respectively. Excessive soil moisture, which occurs with overwatering, in low spots or in heavy soils favor disease development. Succulent turfgrass growth due to high nitrogen fertilization is also susceptible to disease development. Pythium blight and the root rot disease are likely to develop during extended periods of warm, wet weather. Young seedlings are very susceptible to these diseases.

CONTROL: Because Pythium blight requires the presence of free moisture on the leaves and/or in the soil, management of water is of utmost importance in controlling the disease. When establishing new turfgrass, provide good surface and subsurface drainage. Plant new, fungicide-treated seed if the weather is cool and relatively dry.Avoid overwatering the new planting and avoid watering late in the day. On established turfgrass, water early in the day so that the plant leaves are dry by nightfall. Water as thoroughly and infrequently as possible to provide adequate soil moisture.
Avoid excessive rates of nitrogen to prevent the development of lush, succulent growth. Such growth is very susceptible to infection by Pythium blight, especially when weather conditions are favorable for disease development.
Mechanical removal of thatch is recommended if thatch depths exceed 1/3 inch. Do not mow when the turfgrass is moist, especially in hot weather. Selectively prune trees and shrubs, arrange the landscape design to promote good penetration of light into the turfgrass canopy, and allow good air circulation.
Due to the potential for rapid development of this disease and loss of large areas of turf, golf course superintendents should consider a preventive fungicide program when hot, humid weather is forecast. Fungicides are required for controlling Pythium blight in Oklahoma.

Figure 2. -- "Cottony mycelium of Pythium sp. as it develops during periods of high humidity", Courtesy of C.S.S.A.

Figure 3 -- "Typical pythium blight 'streaks' caused by water drainage", Courtesy of C.S.S.A.