The three main fungi that cause the disease are called Phytophthora, Pythium and Rhizoctonia, for those of you that are interested in such things.
The first sign that something is wrong is when patches of seedlings keel over and have visible damage at the bases of their stems. You may also see white fluffy fungal growth on the collapsed seedlings, or the stems of older seedlings may become distorted and wiry. The fungal spores are transmitted either by contaminated soil or by water splash. High temperatures, dense sowing, poor hygiene and waterlogged soils provide perfect conditions for this disease, but any combination of these factors can encourage it.
What can be done to prevent damping off .
Always use fresh seed compost and clean pots and trays. Make sure that any used pots are washed out thoroughly before use. Don’t use collected rainwater on the seedlings, as this may harbour harmful fungal spores. Rainwater will be the best water to use when the plants are established but at the start use tap water instead.
Always sow seed thinly and evenly in the seed tray, and make sure the compost is not kept waterlogged. Water initially using a copper based fungicide, (the Co-op definitely stocks this as I bought some from there last year) which can be applied now and again as the seedlings mature.
There is a saying that I quite like about seed sowing; it really refers to growing plants outdoors but the quote tells me not to be too disappointed if I loose a few seedlings, as there is always something trying to get them!
“Sow seed generously; one for the rook, one for the crow, one to die and one to grow”