Avoid Damping Off of Seedlings

M. Grabowski, University of Minnesota Extension

Damping off is a fungal disease caused most commonly by the fungi Rhizoctonia spp., Pythium spp., and Fusarium spp. All three of these fungi survive quite well in soil and plant debris. Since the tissue of young seedlings is soft and easy to infect, these pathogens can attack a wide variety of flowers and vegetables when they are seedlings. Damping off fungi can kill the seed before it emerges from the soil or it can attack the young stem and new leaves, resulting in tan mushy spots, pinched, rotted stems, and often complete collapse of the seedling. Once an infection has begun, the damping off fungi can move through the potting mix to infect nearby seedlings. Quite often a large section or an entire tray of seedlings is killed by damping off, resulting in few or no surviving seedlings to grow into mature plants.

Damping off is only a disease of seedlings. Once plants have mature leaves and a well developed root system, they are better able to naturally resist the damping off fungi. There is a critical period of growth where special care needs to be taken to protect sensitive seedlings. This period begins before the seeds are ever planted. One of the most import strategies of controlling damping off is preventing it in the first place. Since the damping off fungi typically survive on plant debris, soil, or in contaminated water, all pots, trays, potting mixes and other planting equipment must be sterilized prior to planting. New trays, pots, and potting mix are typically sterile. If you are reusing last year's pots and trays, they should be soaked in a 10% bleach solution for about 30 minutes. It is best to use new unopened potting mix to start seeds.

Once seeds are planted, it is important to maintain a good growing environment for the seed. Potting mix should be moist but not soggy. The damping off fungi thrive in wet cool conditions. Many types of seeds, like tomatoes, need warm soil to germinate and grow well. Warming mats designed specially for heating seedling trays from below are sold through many garden catalogues and nurseries. These may be very helpful in preventing damping off especially if seeds are being grown in cool areas like a basement or garage.

Careful attention should be paid to watering seedling trays. Seeds need moisture to germinate, but too much water will encourage damping off. All pots and trays should have drainage holes to allow excess water to drain away. Seedlings should be watered with lukewarm clean water (like tap or other drinking water). Watering with cold water will slow seedling growth and favor the damping off fungi. Hoses and watering heads should be kept off the floor where they could come into contact with infected soil or plant debris. An oscillating fan can be used to increase air flow around the seedlings, but care should be taken to avoid over drying of the sensitive new plants.

Cultural practices that result in tall, thin, weak plants like growing plants under low light conditions or over fertilizing with nitrogen will result in increased damping off problems. Provide seedlings with just enough of what they truly need.