Damping-off is the infection of young seedlings by a variety of different fungi, most often species of Rhizoctonia, Pythium, and Fusarium. The disease spores are spread by air, water, soil, and contaminated tools and pots. Excessively wet soil, excessively cool temperatures before germination or warm ones after, and densely populated seedbeds all favor the growth of damping-off fungi. Damping-off principally affects young seedlings. The development of secondary stem tissue creates a protective barrier that thwarts penetration by the fungus.
SYMPTOMS: Infected seeds fail to germinate and begin to decay, becoming soft and mushy, and turning brown to black. Seedlings that have emerged can be attacked just above or below the soil line. Slightly darkened, water-soaked lesions occur on the stems. The infected stem tissue may be colorless to dark brown. As the decayed area enlarges, it girdles the stem and shrinks, causing the seedlings to collapse.
CONTROL: Damping-off disease is easier to prevent than to cure. Once seedlings are infected, little can be done except to remove them before the disease spreads. Prevention starts with good sanitation. Always use clean containers and tools. Disinfect them, if necessary, with a 10% chlorine bleach solution (1 part bleach to 9 parts water) or soak them in hot, soapy water (160°F) for at least a half-hour.
Rather than sterilizing soil, use a soilless growing mixture containing peat, perlite, and vermiculite. While not sterile, strictly speaking, these media are typically free of fungus spores, provided they have not come into contact with open ground.
Some seed is pretreated with fungicide, but rapid seed germination, emergence, and development are more effective in reducing the likelihood of damping-off. Sow seeds thinly to avoid overcrowding and to ensure good air movement and adequate light. Some growers top seed flats with a thin layer of milled sphagnum moss. The low pH of the sphagnum is naturally fungicidal. Prior to germination, provide temperatures of 70 to 75°F. When the seedlings emerge, remove any plastic covering or lid that was used to increase humidity levels, and keep the seedlings ten degrees cooler.
Proper watering is also critical. Allow the surface of the potting medium to dry before watering again. When watering seeds by hand, use a watering can or clean hose nozzle that has not touched the ground. Also, make sure that seedling trays are not standing in water.
Finally, avoid overfertilization. A water-soluble fertilizer, applied at half-strength once the second true leaves have appeared, is sufficient.