Root Rot & Pythium


Pythium root rot, which afflicts the roots of turf grass, flowers, shrubs and vegetables, has long been thought to be a water mold fungus. However, plant geneticists at Cornell University now say that based on DNA analysis, pythium is a kind of single-celled algae, an oomycete, related to downy mildew and Phytophthora.

Types of Pythium

There are 120 known species of parasitic pythium. Pythium aphanidermatum, P. irregular and P. ultimum do the most damage. Of these, P. aphanidermatum is the most aggressive and does the most damage. P. Ultimum does not have a swimming spore and so likes container mixes. It likes cool temperatures and is notorious for attacking poinsettias in the fall. P. aphanidermatum prospers at temperatures ranging from 95 to 104 degrees Fahrenheit. It produces zoospores in flooded soil and so spreads easily in recirculating irrigating systems. P. ultimum also produces zoospores that can move easily with irrigation water and can grow at temperatures between 34 and 95 F.


Pythium root rot causes a yellowing and thinning of turf grass. Grass or other plants might appear to be suffering from a lack of water when soil moisture is good. The roots looked water-soaked; the outer layer of roots look decomposed. Plants may wilt during the midday. Pythium causes stunting of vegetables and flowers and blackleg canker, a progressive blackening of the stems working up from the soil.

Conditions for Growth

Pythium develops in wet, humid conditions in cool, warm or hot weather. Pythium produces large numbers of zoospores in soil that is saturated with water. The zoospores don't have to go far to find another host root. The spread of pythium is limited by the surface water that zoospores need to travel distances. Particles in the soil filter and trap many zoospores. In hydroponic systems, pythium is recirculated in water that carries nutrients.The spread of pythium in hydroponic systems is difficult to control and causes severe damage to plants. In large greenhouse operations, thousands of plants can be infected in two to four days.

Cultural Controls

Avoid overwatering. Make sure your soil is well-drained. Avoid over-fertilizing. Remove and destroy infected plants. If you are preparing a plant bed, dig up the entire site and add pine bark or other organic materials to improve the structure of the soil and increase drainage.


Although it is technically an oomycete and not a fungus, pythium is treated with fungicides used to treat soil-borne fungal diseases. Horticulturalists at Cornell University recommend fungicides containing the active ingredients azoxystrobin or fosetyl-Al. There are signs that pythium may have experienced resistance to fungicides containing the active ingredients metalaxy or mefenoxam. There are no indications that any varieties of pythium are resistant to fungicides containing etridiazole. The rate and method of application differ with these fungicides; follow the manufacturer's instructions.

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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.