Damping off

Pythium and Phytophthora spp., with Rhizoctonia solani and Fusarium

also called Seedling blight

Seedlings collapse at the soil line from fungal attack in cold, wet, stagnant conditions. Sow thinly in fresh compost with clean pots, water from below, and ventilate.

Water mold No cure Spread in waterSoil-borneSpread by contact Spreads to neighbors

What you'll see

Seedlings topple over with a thin, dark, water-soaked pinch right at compost level while the leaves are still green.
The collapse spreads outward as a circular bald patch through the tray over a day or two.
A fine white cobwebby growth may appear over the compost surface and up the fallen stems.
Some seeds fail to emerge at all, rotting before they break the surface.
Pinched-off seedlings pull away leaving the roots behind, or come up with brown, sparse roots.

Why it happens

Several water molds and soil fungi that live in old compost, dirty pots and water butts and attack the soft, unlignified stem of a young seedling. Nothing is wrong with the seed; the trouble is the conditions in the tray. Rainwater from a butt is a common source, as is compost reused from last year.

Cold, wet, airless propagation — sowing too early, watering too heavily, a lid left on, a windowsill with no air movement, or a tray sown so thickly the seedlings never dry.

Treating it

Take the lid off, move the tray somewhere brighter and airier, and stop watering until the surface dries.
Remove collapsed seedlings and the compost immediately around them straight away.
Prick out the healthy survivors into fresh compost and clean pots rather than trying to save the tray.
Water only from below, with mains water, and only when the tray feels light.
If the patch is spreading fast, resow — a saved tray rarely produces good plants.

Keeping it away

Use fresh, bagged seed compost each year and wash pots and trays in hot soapy water before use.
Sow thinly, and thin again as soon as the seedlings can be handled.
Water with mains water rather than from a butt for seedlings, and water from below.
Sow at the right time and temperature rather than as early as possible; heat from below helps enormously.
Ventilate propagators daily and remove covers as soon as germination is through.

Who gets it

Any seedling, but lettuce, brassicas, tomatoes, basil, peas, antirrhinums and hardy annuals sown early are most at risk.