Pythium Blight (Grease Spot)

Pythium blight, also known as grease spot, kills turf in small, roughly circular spots (2 to 6 inches) that tend to run together. Blackened leaf blades rapidly wither and turn reddish brown. Leaf blades tend to lie flat, stick together, and appear greasy. Roots may be brown and rotten.

When dew or high humidity is present, especially in the early morning, white, cottony mycelia can be seen in diseased areas. These areas may have an orange to purplish border or smoke ring. Diseased areas may "run" with the pattern of water drainage or mowing. The symptoms may appear rapidly under favorable conditions, sometimes killing large areas of grass within 24 hours. This is the foliar form of Pythium; the pathogen may also cause seedling damping-off or root rots.

All grasses are susceptible to Pythium blight, which can be caused by several different Pythium species. Certain Pythium species attack some grasses more preferentially, although the distribution of these species in California is not known at this time.

Infection and disease development is associated with daytime air temperatures above 86°F, night temperatures above 68°F and 15 or more hours of relative humidity above 90%. Lush grass with high nitrogen nutrition appear to be more susceptible to the disease as are young or germinating seedlings, so care must be taken during overseeding or establishment in hot weather. The fungus forms thick-walled sexual spores that enable it to survive in soil and plant debris for long periods.

Proper irrigation is the most important factor in the management of this disease, especially during periods of high nighttime temperatures.

Avoid overwatering; irrigate only when needed to a depth of 4 to 6 inches. Avoid mowing wet grass or applying high levels of nitrogen during hot, humid weather. Reduce shading and improve soil aeration and water drainage. Time establishment of new turf from seed or overseeding for periods when environmental conditions are not favorable for the development of this disease.

Due to the speed at which this disease can destroy grass, use fungicides preventively when environmental conditions favor Pythium development, or soon after symptoms are first evident. Apply a fungicide during overseeding or establishment if temperatures are high and the established area is being heavily irrigated. Fungicide treated seed is available.

Resistance has developed to mefenoxam for Pythium in a number of locations in the United States. Practice resistance management by alternating the use of fungicides from different chemical classes. In cases where mefenoxam no longer provides control, switch to a fungicide of a different chemistry.