Blackleg
A disease that blackens and rots the base of potato stems from soil-borne bacteria, and causes sunken cankers and leaf spots on cabbage-family plants.
Also known as: Potato blackleg, Brassica blackleg
Symptoms and identification
In potatoes, the stem base turns black, soft and slimy, leaves yellow and roll upward, and plants wilt and die. Tubers may rot with a foul smell. In brassicas, grey leaf spots with tiny black dots appear, and sunken, dark cankers form at the stem base.
Causes
Potato blackleg is caused by soft-rot bacteria that spread from infected seed tubers and in wet, warm soil. Brassica blackleg is a seed-borne fungus that survives on crop debris and splashes up in rain.
Organic treatment
Remove and bin affected plants straight away, and do not compost them. Do not save tubers from plants with blackleg for seed.
Prevention
Plant certified disease-free seed potatoes or seed, in well-drained soil, and avoid cutting seed potatoes. Rotate crops, clear debris and do not overwater.
Chemical treatment (use with care)
There is no cure for blackleg in potatoes. Seed treatment with fungicide on brassica seed may be used in commercial growing, and prevention is best in home gardens.