Damping off
Soil-borne fungi that rot seeds and young seedlings at soil level, causing them to collapse suddenly.
Also known as: Seedling rot, Collar rot of seedlings
Symptoms and identification
Seeds fail to come up, or seedlings that looked healthy suddenly fall over. The stem at soil level becomes thin, dark and water-soaked, as if pinched. A white, cottony growth may appear on the soil surface in humid conditions, and patches of seedlings die together.
Causes
Fungi and water moulds such as Pythium, Rhizoctonia and Fusarium live in soil and on dirty trays. They attack seedlings in cold, wet, poorly drained soil, especially when seeds are sown thickly and kept in humid, still air.
Organic treatment
Remove affected seedlings and the soil around them at once. Improve airflow, reduce watering and sprinkle a thin layer of dry sand or ground cinnamon on the soil surface. Mixing Trichoderma into the seed-starting mix helps protect future sowings.
Prevention
Sow in fresh seed-starting mix in clean trays, not in garden soil. Sow thinly, water from below, and let the surface dry slightly between waterings. Give seedlings good light and airflow, and remove covers once seeds have germinated.
Chemical treatment (use with care)
Seeds can be treated with a fungicide labelled for seed treatment, such as captan or thiram, before sowing, following the label. Affected trays can be drenched with a copper-based fungicide.