Rhizoctonia rot
A soil-borne fungus that rots seedling stems, roots and cuttings, and causes dark scabby patches on potatoes and roots.
Also known as: Rhizoctonia root rot, Wirestem, Black scurf
Symptoms and identification
Seedlings and cuttings develop sunken, reddish-brown lesions at the soil line, become thin and wiry, and fall over. On older plants, roots rot and leaves yellow and wilt. Potatoes carry black, crusty patches that scrape off like soil.
Causes
The fungus survives in soil and on plant debris and is active in cool to warm, wet soil. Overwatering, cold wet seed beds, deep planting and crowded seedlings all help it.
Organic treatment
Remove collapsing seedlings and the soil around them. Let the growing mix dry slightly between waterings, and use a beneficial Trichoderma product in the mix as a preventive.
Prevention
Use clean pots and a fresh, well-drained mix, plant when soil is warm, do not sow too deep, and water in the morning. Rotate crops and plant certified disease-free seed potatoes.
Chemical treatment (use with care)
Seed treatments and fungicide drenches labelled for Rhizoctonia are available. Follow the label and local advice.