Damping off is a disease of seedlings caused by several different fungi. This disease causes emerging seedlings to collapse, often submerged in a mass of white fungal growth. It is particularly a problem when sowing seed indoors or under glass.
What is damping off?
Damping off is caused by several soil-borne fungi including Pythium, Phytophthora, Rhizoctonia and Fusarium, which infect seedlings and cause them to ‘damp off’ or collapse and decay.
Damping off can affect most seedlings, particularly under levels of high humidity, poor air circulation and if seed is sown to thick. It is mainly a problem when sowing early indoors or under glass, but can affect seedlings sown outdoors in situ.
Damping off is especially damaging in spring when light levels and temperatures are low and seedlings grow slowly, but may occur at any time of year.
You may see the following symptoms:
- Seedlings may fail to emerge (pre-emergence damping off)
- Seedlings collapse, often submerged in a mass of whitish fungal growth
- Raise seedlings in commercial growing compost, which is usually free of these fungi. If home-produced compost must be used, consider steam sterilisation to destroy pathogens
- Ideally, use new pots and trays whenever raising seedlings. If they must be re-used, wash them thoroughly and preferably also treat them with a disinfectant such as Jeyes Fluid. Never reuse pots and trays in which damping off has been a problem
- Sow seedlings thinly to avoid crowding
- Use mains water if possible when irrigating seedlings grown in pots and trays. If using rainwater, ensure that the water butt is covered to prevent the entry of leaves and other organic debris that could harbour some of the damping off fungi. Do not overwater
- Keep seedlings well ventilated to reduce humidity
The copper fungicide Fruit and Vegetable Disease Control is very effective if applied as a preventative drench to seed trays.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Species of the fungus-like organisms Phytophthora and Pythium are widespread in soil. Species of the true fungi Rhizoctonia and Fusarium are also common in soil.
All of these organisms, and some others, can infect the delicate tissues of young seedlings and kill them. Infection of the seed can occur before emergence (pre-emergence damping off), leading to patchy emergence. Infection can also occur after emergence, particularly where the seedlings are crowded and where humidity is high, leading to a rapid collapse. The fungi are often visible as a whitish growth of mould on the rotting plants.