Why Are My Beetroot Seedlings Collapsing at Soil Level?

Young beetroot seedlings that appear healthy one day but are found collapsed, pinched, and withered at the stem base the next day are suffering from damping off — a collective term for seedling death caused by several soil-borne and water-borne fungal and oomycete pathogens, primarily Pythium, Rhizoctonia, and Fusarium species. The pathogens attack the thin stem at or just below soil level, causing the tissue to rot through and the seedling to topple as if it has been cut. The collapse usually spreads outwards from the first affected seedlings in a circular pattern as the pathogens spread through wet compost or soil.

Why damping off occurs

Damping off pathogens are present in most unsterilised compost and garden soil and become active under specific conditions: persistently wet, poorly drained compost or soil; low air circulation around seedlings (dense planting, still air, enclosed trays covered with lids); cool temperatures that slow seedling growth but allow fungal activity; and reusing unsterilised seed trays or compost from previous seasons. Watering from above with cold water in cool, overcast conditions is a classic set-up for damping off. The pathogens spread through waterlogged compost, infecting neighbouring seedlings rapidly once established.

Saving remaining seedlings

Once damping off appears in a tray or bed, it cannot be cured in affected seedlings — remove collapsed plants immediately to reduce the spread of spores. Move remaining seedlings to a better-ventilated position and reduce watering significantly, allowing the surface to dry between waterings. Thin surviving seedlings to improve air circulation. Watering the tray with diluted chamomile tea (a mild antifungal) or a copper-based fungicide registered for use on food crops may slow spread but is not reliable once damping off is established.

Prevention for future sowings

Use fresh, sterile seed compost every season. Clean seed trays with diluted bleach (1:10 in water) and rinse before use. Sow thinly — crowded seedlings increase humidity and reduce air movement. Water from below by placing trays in a shallow tray of water rather than watering over the top, and allow the compost to approach dryness between waterings once seedlings are through. Provide adequate light to encourage rapid, strong seedling growth. Outdoor direct sowings into warm, well-drained soil rarely suffer damping off compared to indoor sowings in cool, damp conditions.