Numerous varieties of Pythium fungi, also known as water mold, inhabit almost all cultivated soils. They enter the roots of plants in wet soil where they can quickly cause black rot of the main root, sometimes moving up into the stem. The fungi attack the roots of shrubs, flowers and vegetables and are especially pernicious in lawns and turf.
Symptoms of Pythium root rot can appear anytime following the saturation of soil by heavy rainfall. Since roots infected by Pythium fungi don't absorb water well, the plants wilt and the leaves of older plants turn yellow. The rot begins on small feeder roots; the exterior of the roots will slip off easily. Irregular patches of yellow or orange appear on infected turf. The roots, crowns, stolons or rhizomes appear greasy and dark.
ELISA (enzyme linked immunosorbent assay) kits to test soil for Pythium fungi are available commercially.
Development and Spread
Some species of Pythium fungi reproduce by zoospores; zoospores swim in soil moisture in search of roots. Other species reproduce by oospores; oospores spread by the roots of infected plants, contaminated potting media and pots.
Pythium fungi like hot, humid weather with temperatures of 80 to 98 degrees F. In extended wet weather they will thrive at temperatures of 55 to 65 degrees F. They will appear if dew remains on turf for 14 hours or more. They like alkaline soils with a pH above 7 and soils deficient in calcium.
Pythium root rot ordinarily appears when soil remains saturated with water for long periods of time. Poorly drained soil, too much irrigation, heavy rainfall, and excessive buildup of thatch on turf can result in Pythium root rot.
Do not over-irrigate. Make sure your soil is well drained. Only use seeds, cuttings and plugs that are free of Pythium. Avoid splashing water between pots. Growing plants on elevated surfaces decreases the possibility of splashing spores from the local soil.
When rainfall is high, turfgrass should be mowed as often as necessary so that only 1/3 of the grass is mowed each time. University of Florida horticulturalists say applications of extra potassium in late summer-early fall are useful in controlling Pythium fungi.
Use new or sterile pots and potting media. Pythium fungi can survive on greenhouses flats, pots and soils in soil particles, dust or on the floor. Sterilize potting media by steaming at 140 degrees F for 30 minutes or heat to 140 degrees F for one hour.
Horticulturalists at the University of California, Davis, recommend treating Pythium fungi with granular formulations of fungicides containing mefenoxam before planting, or drenching the soil with a water-soluble formulation containing mefanoxam at planting. Mefenoxam is absorbed by the roots.
Apply a spray of a fungicide containing fosetyl-al to the foliage at the rate of 2 ½-to-5 pounds per 100 gallons of water; it will move downward into the roots. Apply a soil drench of a fungicide containing glioclyadium virens at the rates on the product label.
In addition to fosetyl-Al and mefenoxam, horticulturalists at the University of Florida recommend fungicides containing azoxystrobin, chloroneb, ethaxzol, and propamocarb either sprayed or lightly watered into the area of the roots according to the manufacturer's listed rate of application.