Damping-off frequently attacks young seedlings of almost all kinds of vegetables. Just after seedlings have emerged from the soil, they are easily killed by fungus organisms likely to be present in the typical garden soil. Seedlings that die or fall over are said to “damp-off.” Seedlings may also die before emergence, referred to as pre-emergence damping-off. If seedlings do not emerge or have a poor stand, it can be useful to dig up the non-emerged ones in order to determine if the seed loss was due to pre-emergence damping-off or to seed rot. The destructiveness of this disease depends on the amount of fungus in the soil, whether any pathogens are seedborne, and on environmental conditions. Abundant moisture in the soil, high humidity, and cloudy days are especially favorable for the development of damping-off. Many damping-off microorganisms are especially damaging in cool soils.
In the greenhouse or seedbed, damping-off can be managed by pasteurizing the soil with heat, aerated steam, or strong disinfectants. A good job of killing off pathogens in soil/soilless mixes requires specialized equipment which can be quite expensive. Fortunately there are simpler remedies for damping-off when it does appear.
What to Do First
Ensure that seed is free of pathogens that can incite damping-off. Obtain high quality seed that is certified as free from pathogens when possible. Treat seed with a seed treatment fungicide or hot water/steam treatments. For recommendations, see discussions of damping-off under the headings of various crops in this handbook. Avoid planting seeds into cold, wet soils.
Just as soon as you see damping-off, reduce watering. Allow the soil to dry out around the plants. If seedlings are in flats or in cold frames, give them as much air and light as possible. The drier the soil, the greater the light, and the better the air drainage, the less danger that damping-off will continue. Of course, soil around the seedlings cannot be allowed to dry completely or the seedlings will die. Watering with a chemical or biocontrol formulation that mitigates damping-off provides water for plants and offers some disease protection.
Do not re-use soils or soilless mixes if damping-off has occurred in these planting media, unless they are treated with heat, aerated steam, or chemical disinfectants. If soil is stored outside, take steps to protect it from surface water run-off that could collect in the soil and bring damping-off pathogens.
Ensure that pots and flats used in greenhouse seedling production are well cleaned between use since damping-off agents can persist in unclean pots and trays. Wash pots and trays well to remove adhering planting medium and plant debris. Follow up with a chemical rinse or soak, such as a 10% bleach solution or 3% hydrogen peroxide. Benches should be cleaned between transplant production cycles. Avoid placing trays of plants on the ground where they can contact raw soil.
Chemical Treatments
Copper drenches, such as C-O-C-S and tribasic copper sulfates, are often listed first because they can be safely and effectively applied to many kinds of vegetables. Do not apply through any irrigation system. However, care must be taken to avoid copper build-up in soils.
Mix C-O-C-S WDG at 2 oz/3 gal water. Stir thoroughly. The dust is not soluble in water but is merely suspended in it. Stir hard and frequently while applying. Gently spray the plants and soil where the damping-off is occurring. The suspension may be applied with a cup or with a sprinkling can after first removing the spray cap from the spout, but a hand-held sprayer will give more uniform coverage. This treatment can be used on cucumbers, melons, beets, spinach, carrots, peppers, lettuce, and tomatoes. Do not use this treatment on cabbage, cauliflower, broccoli, kale, or other crucifer crops. Can be re-applied on a minimum of 3-day intervals. 24-hr reentry for greenhouse applications.
Plant beds can be treated with fumigants, soil-applied fungicides, and/or biocontrol formulations before seeding. Follow all label directions and precautions.
Do not try to stop damping-off with acids, compounds containing chlorine, or wood ashes. Chemical treatment with metalaxyl as a seed treatment (Apron XL) or as a soil treatment (Ridomil formulations) can be used to decrease damping-off problems caused by Pythium.