Preventing damping-off disease on your seedlings

I have met many first-time gardeners who attempted to grow their own vegetables and flowers from seed. Lured by the luscious photographs and teasing text of mail-order catalogs, they wanted to grow new and exciting varieties that were not available as transplants at local garden centers and farmer's markets. Unfortunately, their attempt at starting plants from seed was foiled by damping-off disease.

Seeds attacked by damping-off fail to germinate and begin to decay. They become soft and mushy and turn brown to black. Seedlings that have emerged can be attacked just above or below the soil line. Damping-off produces slightly darkened water-soaked lesions 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 and fall over.

Besides killing the crop, damping-off kills a gardener's self-confidence. After just one attempt, many wannabe gardeners retreat and purchase ready-to-plant tomatoes or marigolds, forsaking their dream to try new, unique, and heirloom varieties.

Only a few will try to understand damping-off disease, and learn how to avoid it. Damping-off is caused by several different fungi, which collectively attack and kill seeds and young seedlings. 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 crowded seedbeds all favor the growth of damping-off fungi.

Damping-off is easier to prevent than cure. Use clean containers and tools. If necessary, disinfect them with a 10 percent bleach solution (1 part bleach to 9 parts water) or soak them in hot soapy water (160 degrees) for at least a half hour.

Also, use a soil-less growing mix containing peat, perlite and vermiculite. It's typically free of fungus spores provided that the soil-less mix has not been in contact with open ground.

The real key to thwarting damping-off is to encourage rapid seed germination, emergence and development. Sow the 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 milled sphagnum is naturally fungicidal. Prior to germination provide a temperature of 70 to 75 degrees. When the seedlings emerge, remove any plastic covering or lid that was used to increase humidity levels, and give the seedlings 10-degree cooler temperatures.

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 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.

Don't let damping-off dampen your gardening spirits to try something new and exciting from seed.

Editor's note: This is Bob Polomski's last column for the Independent Mail (for now). Garden Works will go on indefinite hiatus so Bob can spend more time on teaching and research responsibilities at Clemson Extension.

Bob Polomski is an Extension Horticulturist at Clemson University and an ISA Certified Arborist. He is the author of "Month-by-month Gardening in the Carolinas."