The Pythium Fungus


There are 15 or more species of fungi in the genus Pythium, a soil-borne or "water mold" fungi that are noted for the rot they cause in the roots and crowns of plants. Pythium fungi are a common problem in greenhouses and hydroponic systems. They strike vegetables, flowers and field crops, but they are especially pernicious as the cause of Pythium blight in turfgrass.


Pythium fungi are found in the soil as delicate mycelium, the branching, vegetative form of the fungus, and as round, thick-walled oospores, the sexual spores of the fungi. They spread from plant to plant by a cobweb of long, branching cells, favoring poorly-drained clay and other heavy soils. The fungi flourish in wet or humid weather with a temperature of 80 to 95 F and little air movement. The fungus can also appear in the fall, winter and spring during prolonged wet weather when temperatures range from 60 to 65 F. Pythium blight usually strikes dense, lush turfgrass. In extreme hot, humid weather the fungus can spread rapidly overnight, killing large areas of turfgrass.

Conditions for Growth

Pythium fungi first appear in low areas of gardens, fields and flower beds where the soil is wet and lingering dew forms early in the morning. When the air is saturated in the early morning, the water-soaked leaves of infected turfgrass collapse into mats held together by a cobweb of mycelium. Since the fungi need surface water to travel distances, their damage is limited in field crops. They are a special problem in greenhouses and hydroponic systems where water-carrying nutrients recirculate their spores. In a fungal scourge called "damping off," Pythium can kill thousands of crowded greenhouse plants in a few days.


The fungi cause brown to dark brown lesions on the tips of roots of vegetables, flowers and other plants. The roots lose their feeders and the outer surface of the roots peel away. The crowns of the roots may appear brown. The plants wilt during the day and may recover overnight, but they will eventually die. When Pythium blight strikes turfgrass, spots 1 to 2 inches wide appear. Early-morning leaves are dark, slimy and water-soaked. As they dry out, the grass leaves shrivel, forming reddish-brown to light tan mats. Blighted turfgrass clusters may merge to form streaks 12 inches wide or wider or irregular patches from 1 to 10 feet wide.


Growers with greenhouses need to clean their equipment. Treated, municipal water is usually safe; do not use untreated water from streams or rivers. Treat water in hydroponic systems with chlorine dioxide, hydroponic solutions containing copper, hydrogen peroxide or iodine. Horticulturalists at Purdue University Extension recommend preventative fungicides containing the active ingredients mefenoxam or propamocarb. Follow the label directions for proper use.

Grass Care

Pythium blight is especially damaging to annual and rough bluegrasses, creeping bentgrass and perennial ryegrass. Avoid sprinkling in the late afternoon or evening. Water infrequently, but to a depth of 6 inches or more during the summer or early fall droughts. Use slow-release nitrogen fertilizers during the spring or summer. Do not overfertilize. Avoid water-soluble fertilizers with high nitrogen content in hot weather. In the early spring or late summer, use an aerifier, power rake or vertical mower to remove thatch 1/4 to 1/2 inch deep. Do not mow wet grass if you can see cobwebs of mycelium.

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About this Author

Richard Hoyt, the author of 26 mysteries, thrillers and other novels, is a former reporter for Honolulu dailies and writer for "Newsweek" magazine. He taught nonfiction writing and journalism at the university level for 10 years. He holds a Ph.D. in American studies.