White mould is a disease caused by the fungus Sclerotinia sclerotiorum, which attacks hundreds of crops including hemp, sunflower, beans and canola. On cannabis it causes stem cankers: water-soaked, bleached lesions on the main stem or branches that become covered in cottony white mycelium. Hard, black, seed-like resting bodies called sclerotia form on and inside the stem.
In practice
Branches above a canker wilt and die because the stem is girdled. Splitting the stem often shows sclerotia inside the hollow pith. The fungus survives in soil as sclerotia for several years and releases spores in cool, wet conditions, so it is an outdoor and greenhouse disease, worst in dense plantings after rain. Management relies on spacing for airflow, avoiding fields with a history of sclerotinia in susceptible crops, removing infected plants with their sclerotia, and long rotations. Check what is registered for cannabis locally.
Don't confuse with
- Botrytis: grey fuzzy spores, usually in buds rather than stem cankers.
- Powdery mildew: thin white powder on leaf surfaces.
- Fusarium wilt: brown streaks in the vascular tissue, no cottony growth or sclerotia.
