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 any 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 and on environmental conditions. Abundant moisture in the soil, high humidity, and cloudy days are especially favorable for the development of damping-off. Some damping-off fungi are especially damaging in cool soils.
In the greenhouse or seedbed, damping-off can be almost completely eliminated by partially sterilizing the soil with heat or strong disinfectants. A good job of soil sterilization requires special equipment and is often impossible. Fortunately there are simpler remedies for damping-off when it does appear.
What to Do First
Treat seed with a seed treatment chemical. For recommendations, see discussions of damping-off under the headings of various crops in Section 4 of this handbook. Just as soon as you see damping-off, stop watering for a while. Allow the soil to dry somewhat 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 that controls damping-off provides water for plants and chemicals for disease protection.
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.
Mix 1 oz copper dust with 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 drench 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. This treatment can be used on cucumbers, melons, beets, spinach, carrots, peppers, lettuce, and tomatoes. Do not use this treatment on cabbage, cauliflower, broccoli, or kale.
Plant beds can be treated before seeding with fumigants. 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) or as a soil treatment (Ridomil 2E) can be used to control Pythium damping-off in the field.