Pythium blight, sometimes called “grease spot” or “cottony blight,” is no longer only a disease of golf courses; it is becoming a problem in home lawns and other turfs. Pythium blight is cause by several species of Phythium fungi; however, the two most commonly associated with the disease are Phythium aphanidermatum and P. graminicola. These fungi are in a group known as “water molds” because they grow best under wet, saturated soil conditions. In turfgrass they survive in thatch and soil. All turfgrass species are susceptible to attack by Pythium blight fungi.
The two most important criteria for disease occurrence are poor soil drainage and a wet turfgrass canopy. Water-logged soils and a moist thatch layer, along with high relative humidity and day temperatures above 90°F with warm nights, provide and ideal environment for an outbreak of Pythium blight. The Pythium is spread when infected grass blades cling to shoes, mowers and other equipment, and when the fungus is moved by flowing surface water.
Early symptoms of Pythium blight are small, roughly circular, reddish-brown spots that suddenly appear in the turf. The pattern of symptoms may develop into streaks that conform to surface drainage channels, remain as random diseased spots, or form into larger areas where the individual spots have merged. The onset of symptoms is rapid during hot, humid conditions. When dew is present, infected leaves are characteristically water-soaked, slimy to the touch, and may contain a mass of fungal mycelium resembling a fluffy cotton ball. Sometimes the infected area will give off a fishy order. Pythium blight is most easily diagnosed in the early morning when the cottony mycelium is present.
No single control measure will provide complete protection against an outbreak of Pythium blight. Turf managers must employ a combination of good management, early disease detection, and preventive fungicide applications to avoid injury. Management techniques include providing good surface and subsurface drainage, fertilizing in a manner that does not stimulate lush growth, thinning adjacent landscape planting to promote air movement over the turf, and avoiding trafficking wet turf. Turf with a history of Pythium blight should be treated with a fungicide when humidity is high and day time temperatures are above 90°F and night time temperatures are above 70°F.
J. E. Watkins. (1997). Diseases of Cool Season Turfgrass. In F. Baxendale, Ph.D., & R. Gaussoin, Ph.D., Integrated Turfgrass Management for the Northern Great Plains (pp. 128-130). Location: Nebraska