If it hasn’t happened already, vegetable growers will soon drop seeds into transplant trays in preparation for the 2020 season. Or, in a few weeks, vegetable growers may drop seed into the ground. In either case, it is possible that one of several fungi that survive in the soil may attack the seed or seedling as it emerges from the ground. This disease is known as damping-off. The symptoms of damping-off range from a poor stand of seedlings when the fungus kills the seedlings before it emerges from the soil to seedlings that have fallen over due to a lesion of the stem (Figure 1).
Figure 1. Several watermelon seedlings have collapsed from damping-off. Note the necrotic lesion at the base of the stem.
There are several fungi which may cause damping off. These include Pythium, Rhizoctonia, Theleviaopsis, Fusarium spp. and many more. The identity of the culprit may seem unimportant, but if the problem persists, knowledge of the causal fungus may help one to know how to alter the environmental conditions or even find a product for management.
Sanitation is key to managing damping-off, especially with transplants grown in the greenhouse. Soilless mix purchased for transplant use should be opened on a clean, sanitized surface. Avoid using tools contaminated with field soil. Bags of soilless mix that have been opened for a long period are likely to be contaminated with fungi that might cause damping-off.
Transplants grown under cool, wet conditions, seem to have more damping-off problems. In general, cool conditions slow plant development and give the soil fungi a chance to infect. Wet conditions may favor fungal growth and spread. Some soilless mixes do not provide good drainage and therefore may cause more damping off. In addition, seedlings grown in low light conditions tend to be tall and thin, a condition which may lead to the collapse of the seedling due to damping-off.
Raised beds may help to warm soil and help direct seeded plants from damping-off. Sometimes, a later planting date, when the soil is warmer, can help to avoid damping-off.
Seed treated with fungicides with the active ingredient captan or thiram by the seed company may help to slow fungi that may rot the seed. However, these fungi are contact fungi, and thus these products will not help the growing seedling. Some seed is treated with systemic fungicides by the seed company that may move with the seedling and protect new growth. However, transplants that are grown in the absence of pathogens in the soilless mix should not have disease problems. It seldom makes sense to treat the soil or soilless mix with fungicides. But feel free to contact me with questions.