Pythium blight, also called cottony blight or grease spot, is a fungal disease of turfgrasses. All turfgrasses, warm and cool season, are susceptible to attack. This disease is most common during hot, very humid weather, especially in golf courses, less so in home lawns. The disease can spread rapidly, killing large areas of seedling or established turf in as little as a day during conditions of high temperature (80 degrees to 90 degrees F), high soil moisture, and little air movement. The disease can also occur at lower temperatures during cool (55 degrees to 65 degrees F) wet weather. When root and crown tissue is attacked, Pythium root and crown rot results. This disease occurs mostly during warm to hot weather. Wet, humid conditions favor the disease.

Symptoms and Diagnosis

Pythium blight is most readily recognized as small spots or patches of blighted grass that suddenly appear during warm, wet periods. In the early stages, the grass leaves may appear water-soaked, slimy (greasy), and dark. As the disease progresses, the leaves shrivel and the patches fade from green to light brown. When observing these patches in the early morning, cottony fungal growth can usually be seen on the foliage, although not always.

In many cases, these patches develop into diffused streaks that follow water drainage patterns or mowing patterns. These streaks are caused by the water or equipment picking up the fungus and spreading it along its path. Under favorable conditions for the disease development, these streaks may coalesce to form large areas of dead grass. If a sudden drop in temperature or humidity or the application of a fungicide halts the development of Pythium blight, distinct straw-colored spots develop. Grass blades may twist and collapse at the lesion.

Life Cycle

The pathogen survives over winter and periods adverse to disease development as spores associated with debris in the soil. It can be moved from one area to another by soil movement, by transporting diseased plants or plant parts, and by equipment, shoes, or surface water.

Damping-off, seed decay, or a seedling blight of turf grasses can also be caused by Pythium fungi. These fungi may also attack the plant roots and crowns, causing reduced growth, off-color, and thinning (Pythium root and crown rot).

Diseased plants serve as infection centers from which the pathogen spreads. Movement from these areas can be rapid in wet or humid, hot weather. High nitrogen fertility favors the disease on many grass varieties. Alkaline- or calcium-deficient soils also tend to favor the disease.

Integrated Pest Management Strategies

1. Fertilizer. Maintain grass growth by low to moderate rates of balanced fertilizers. Maintain soil pH in the neutral to slightly acid range. Test the soil every 3–5 years to understand and correct nutrient deficiencies.

2. Watering. If your lawn has the disease, do not water in the late afternoon or early evening. Generally speaking, water in the early morning.

3. Thatch. Remove thatch when greater than 1/2 inch.

4. Pruning. Selectively prune trees and shrubs growing near the area to improve air circulation.

5. Mowing. Increasing the mowing height and following other practices that promote good root growth will lessen damage from Pythium root rot.

6. Fungicides. A preventive fungicide program may be needed to stop the development of Pythium during extended periods of warm, humid weather. Applications of fungicides recommended for control of Pythium blights should be made to areas with a history of Pythium activity when conditions are favorable for development or when symptoms first appear. Repeat applications as necessary. Fungicide applications may be needed 7 to 21 days after planting to protect young seedlings and more often during high rainfall periods. Pesticides registered for use include chlorothalonil (Daconil), fosetyl-Al, mancozeb, and maneb.

Organic Strategies

Strategies 2, 3, 4 and 5 are strictly organic approaches. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 1.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against pythium. Not following the pesticide label before usage is a violation of federal law.