Why Are My Cabbage Seedlings Collapsing at Soil Level?

Brassica seedlings collapsing at the base of the stem — the stem pinching in and going dark and wire-like at soil level, causing the seedling to keel over — have either wire stem or damping off. Both cause very similar above-ground symptoms (the seedling collapses at the stem base) but have different causes and occur in slightly different conditions. Understanding the difference helps in preventing the same loss the following season.

Wire stem

Wire stem is caused by the soil fungus Rhizoctonia solani, which attacks the stem base of brassica seedlings at or just below soil level. The affected stem section turns dark brown and constricts — becoming noticeably thinner than the healthy stem above and below — giving the characteristic "wire" appearance. Affected seedlings fall over at this constriction point. Wire stem is most common in cold, wet conditions where the fungus is favoured and the seedling's growth is slow. Using fresh, sterile compost in clean trays prevents the problem effectively.

Damping off

Damping off (caused by Pythium and Phytophthora species) also attacks seedlings at stem level, causing collapse, but the affected stem tends to be water-soaked and slimy rather than dry and wire-like. Damping off spreads rapidly through a seed tray under warm, moist conditions with poor air circulation — once a few seedlings are affected, the infection spreads quickly to neighbours. The pathogen thrives in contaminated old compost and in trays that have not been cleaned between uses. Prevention: always use fresh, unused compost for seedlings; sterilise trays between uses; avoid overwatering; provide adequate ventilation.

What to do

Remove and dispose of affected seedlings immediately — do not leave them in the tray where they can spread infection. Move the remaining seedlings to better-ventilated conditions and allow the compost to dry slightly between waterings. Do not try to save the compost from an infected tray — discard it and start again in fresh compost and clean trays. Resow in good conditions.