Damping off is caused by several soil-borne fungi and fungus-like organisms including Pythium, Phytophthora, Rhizoctonia and Fusarium, which infect

A seedling is a young plant grown from seed.

Damping off can affect most seedlings, particularly under conditions of high humidity, poor air circulation and if seed is sown too thickly. It is mainly a problem when sowing early indoors or under glass, but can also 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 (post-emergence damping off)

Non-chemical control

These measures are very important, particularly given the current lack of fungicides for the control of damping off.

• Raise seedlings in commercial growing compost, which is usually free of damping off organisms. 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, brush off soil, wash and preferably also treat them with a disinfectant such as Jeyes Fluid. Dry before storage. 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 pathogens
• Do not overwater
• Keep seedlings well ventilated to reduce humidity
• Ensure that both greenhouses and water butts are cleaned regularly. After thoroughly cleaning and refilling rainwater tanks or water butts, add a proprietary water butt cleaner to help prevent problems from both damping off and algae

Chemical control

No fungicides are available to gardeners for the control of damping off.

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 causal organism may be visible as a whitish growth of mould on the rotting plants.