Damping-off of seedlings (black leg)

Symptoms

Seedlings and young transplants suddenly wilt, topple over and die, often in patches. At the base of the stem, just above the soil surface, the root collar turns black, softens and constricts, so the plant loses its support and falls over. Seeds may also rot before emergence, which shows as sparse, uneven emergence of vegetable transplants, annual flowers and herbs raised from seed. Two phases are distinguished: pre-emergence damping-off (rotting of the germinating seeds, absence or gaps in emergence) and post-emergence damping-off (toppling of seedlings that have already emerged). The exact picture depends on the pathogen. With infection by oomycetes (Pythium) the rot is watery, translucent and soft — the constricted stem looks as if 'cut with a knife', and the seedling falls flat, especially in cold and excess water. Rhizoctonia solani produces a brown to rusty-red constriction, the tissue is drier and more pinched than watery, the seedling may remain standing for some time, and a delicate, brown, cobweb-like mycelium is sometimes visible on the surface of the medium and on the collar. Fusarium causes wilting with yellowing and discoloration at the base and of the roots. The focus spreads in a circular, patchy way from the first infected seedling, because the pathogen travels through the moist medium and water; the roots of diseased seedlings are brown, poorly developed, with sloughing bark.

Treatment

Removing the infected seedlings together with the surrounding soil and cutting back on watering to let the top layer of the medium dry out. If needed, the transplants are watered with fungicides or biological preparations, and heavily infected batches are better discarded and re-sown in clean, sterilised medium. The disease progresses rapidly and a single, already-toppled seedling usually cannot be saved — the goal is to stop the spread within the focus and protect the healthy transplants. Humidity must be reduced immediately: stop watering until the top layer dries out, remove any cover (film, glass), increase air movement and lower air humidity, provide good light and slightly raise the medium temperature to speed up the development of the surviving plants. The choice of product depends on the pathogen, which matters here: oomycetes (Pythium, and among related genera e.g. Phytophthora) are insensitive to some fungicides that act on true fungi, and vice versa. Against oomycetes, products targeting Oomycota are used (e.g. from the phenylamide or phosphonate group), while against Rhizoctonia and Fusarium a soil-drench fungicide with a different mode of action. As support, the medium is drenched with biological preparations containing antagonists. Heavily infected trays are best discarded entirely and re-sown in fresh, sterile medium and clean containers.

Prevention

Sowing not too densely in a permeable and sterilised medium, moderate watering, and ensuring ventilation and good light. It helps to avoid excess nitrogen and too low temperatures, and to use healthy, dressed seed and clean containers. Prevention is more effective here than any treatment, because the disease develops faster than an intervention can act. Hygiene is key: fresh, sterile medium (not garden soil, nor medium reused after diseased transplants), disinfected containers and tools, and watering with clean water — not from a tank or pond, where oomycetes can live. Keep the medium moist but not waterlogged; water preferably in the morning and from below, avoiding condensation and standing water. A warm medium and good light speed up emergence, so the seedling stays for a shorter time in the most vulnerable phase. A thin layer of dry, sterile material on the surface (sand, perlite, vermiculite) can help by keeping the root collar dry. In addition, the medium can be inoculated preventively with biological preparations containing antagonists (e.g. Trichoderma).

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.