The numerous fungi species of Pythium that cause root rot, also called water mold, are found in nearly all cultivated soils. They thrive in moist conditions. Soil may contain several Pythium fungi at once; some are active in warm soils; others prefer cooler temperatures. Although organic methods and fungicides are available to prevent or treat Pythium root rot, the best way to control them is to make sure roots are now allowed to stand for long periods in water or moist soil.
Pythium root rot can strike any time of the year if the soil remains saturated for days or weeks. Pythium thriving in wet soil infects the tips of roots, causing black rot. The roots, rhizomes, stolons and crowns may appear dark and greasy. The rot may spread upwards into the stem at the crown.
If the soil dries, new roots may allow the plant to recover without showing symptoms of root rot. But if the soil doesn't drain well or the plants are watered too much, the rot will kill more roots. In time the plant may wilt, stop growing or even die.
Horticulturalists at the University of California, Davis, say Gliocladium virens, a parisitic fungus that uses Pythium as a host, is reported to be useful in suppressing the fungus.
Using a potting mixture containing 20 percent of composted pine bark will help control some root rots.
Steam potting mixes at 140 degrees F for 30 minutes. Solarize by putting the potting mixes under a double tent at 160 degrees F for 30 minutes or at 140 degrees F for an hour.
The University of California recommends fungicides containing the active ingredients mefenoxam or fosetyl-al. In addition, horticulturists at the University of Florida recommend fungicides containing azxystrobin, chloroneb, ethjazol and propamocarb. Follow the directions on the manufacturer's label. Contact your local agricultural extension agent for any restrictions that may apply in your area.
Identifying Your Pythium Fungi
Your local agricultural service may have ELISA test kits available to identify the specific form of Pythium that is causing root rot in your plants. ELISA stands for Enzyme-Linked Immunosorbent Assay, the technique used to identify pathogens. This is a necessary first step to identifying the best fungicide to use on your plants. Online stores also sell these kits.
Sanitation is critical for plants grown in greenhouses. Pythium fungi can survive in planting mixes, flats, pots, dust and soil particles on the floor.