Southern blight
A soil-borne fungal disease of hot, humid climates that rots stems at the soil line under a white fan of mould dotted with tiny round seed-like bodies.
Also known as: Southern wilt, Sclerotium rot
Symptoms and identification
Plants wilt suddenly, often in the heat of the day, and do not recover. The stem at ground level turns brown and soft and is wrapped in white, thread-like mould. Mustard-seed-sized round bodies, white at first and then brown, form on the mould and the soil surface.
Causes
The fungus lives in the soil for years as hard resting bodies and attacks over 500 plant types. It thrives in warm soil above about 27 degrees Celsius, with frequent rain or irrigation and a thick layer of plant litter.
Organic treatment
Dig out infected plants with the surrounding soil, and bin them. Solarise bare soil under clear plastic for 4 to 6 weeks in summer to reduce resting bodies.
Prevention
Rotate to grasses or cereals for several years, bury crop residue deeply or remove it, and avoid piling mulch against stems. Water the soil evenly and improve drainage in beds.
Chemical treatment (use with care)
A soil drench with a fungicide labelled for Sclerotium rolfsii can protect nearby plants, but results are mixed. Follow local agricultural advice.