Symptoms:

Damping-off and seedling diseases: Pythium fungi commonly cause seed rots and seedling death, resulting in the poor establishment of newly seeded lawns. Infected seeds fail to germinate. Infected seedlings are discolored, and many topple over at the soil line and die.

Pythium blight: This disease begins in mature lawns as small slimy or greasy looking spots. The spots may assume a reddish color if weather conditions become cooler and drier. When the turfgrass is wet or where air circulation is poor, the cottony, white, weblike mycelium of the fungus maybe observed. This disease can develop rapidly and extensively in poorly drained areas and is especially prone to following surface drainage channels. Other common names for this disease are "greasy spot" or "cottony blight," and it occurs most commonly in perennial ryegrass lawns.

Characteristic Pythium blight spots in turfgrass	Close-up of Pythium mycelium

Season:

Seed and seedling problems can occur at almost any temperature but are most common at temperature extremes, either high or low, when seed germination is inhibited. Pythium blight is a disease of hot, humid weather when night temperatures remain high, especially when accompanied by rain or excessive irrigation.

Disease Cycle:

Pythium fungi are "water molds" which produce microscopic swimming spores. These spores are attracted to living plants. Plants that are succulent from recent nitrogen fertilization are more susceptible to infection. The fungi grow well in soil and thatch in wet weather and survive as thick-walled resting spores during adverse conditions. Pythium species are present in every soil and will grow rapidly under conducive conditions.

Cultural Management:

Water is the most important factor in the development and severity of Pythium diseases. To avoid seed rots and seedling losses, care in preparation of a well-drained seed bed is extremely important. Newly seeded lawns may require frequent watering to hasten germination, but it is crucial to avoid overwatering and puddling in low spots.

In established lawns. water deeply, to a depth of 6 inches, only when necessary. Water early in the day so leaf blades will dry. Avoid watering in the late afternoon and early evening; do not water at night during hot, humid weather. Prune landscape plants to encourage air circulation. Avoid excessive nitrogen fertilizer, especially when hot, humid weather is expected. When Pythium mycelium is active, it is easily spread by feet and mowing. Mow turfgrass when dry to avoid spreading the fungus. Mow low or poorly drained areas last.

Chemical Management:

Turfgrass seed may be treated with a fungicide before planting, but this is not usually necessary with proper seedbed preparation, good weather conditions, and proper seeding density.

Pythium blight is not a common lawn disease in New England. Therefore, correction of drainage problems and attention to the other cultural practices described above are usually more effective and appropriate than fungicides. When necessary, several effective fungicides specific for Pythium diseases are available.

Adapted from the UMass Cooperative Extension, 2000.
Photos from the University of Wisconsin-Madison Turfgrass Diagnostic Disease Laboratory.