A common fungal disease of seedlings where the stem collapses at soil level. Caused by several soil-borne pathogens (Pythium, Rhizoctonia) favoured by overwatering, poor drainage, and high humidity.
Damping off is caused by soil-dwelling fungi, primarily Pythium and Rhizoctonia species, which thrive in warm, moist conditions. The disease attacks seedlings just below the soil surface, rotting the stem and causing the young plant to topple over. In Australia's subtropical and tropical zones (zones 10-12), this is a year-round concern, particularly during the wet season. In temperate zones (zones 8-9), it's most problematic in spring (September-November) when seedlings are germinating and humidity levels rise.
To prevent damping off, ensure excellent drainage in seed-raising mix—purchase quality seed-raising mix from Bunnings rather than using garden soil, which harbours fungi. Maintain good air circulation around seedlings with a small fan, and avoid overwatering; water from below or early morning only. Sterilise pots and trays with a weak bleach solution before use. In humid climates, consider using fungicide-treated seed-raising mixes or applying a light dusting of sulphur powder around seedlings. If damping off appears, remove affected seedlings immediately and improve ventilation. Reduce watering and allow the soil surface to dry slightly between waterings. Consider bottom heat (around 20-24°C) to encourage faster germination and reduce disease risk.