Symptoms
Black scurf: Hard, irregular raised black patches on the surface of tubers.
Stem canker: Sunken brown lesions at the base of the stem and on the stolons. If the lesions girdle the stem the plant may wilt, and small, green aerial tubers sometimes form in the leaf axils.
If the shoots of newly-planted tubers are slow to emerge they can be attacked and killed. Although further shoots develop, this results in delayed emergence and weak plants.
Cause
Caused by a widespread soil fungus (Rhizoctonia solani), and is most prevalent in cool, dry, light soils. The black scurfs are the resting bodies (sclerotia) of the fungus.
Control
Black scurf on harvested tubers for consumption is a cosmetic problem only, and can be easily scrubbed off potatoes before cooking.
There is no chemical control available to home gardeners. Many infections arise from contaminated seed tubers, so check these carefully and do not plant any with high levels of black scurf. Pre-sprout tubers to speed up emergence and reduce the risk of shoots being attacked before they emerge.
The fungus is also quite capable of surviving in the soil in the absence of a potato crop. Growing potatoes on long rotations should reduce the risk if clean seed is used, and will also help prevent problems with other soil-borne pests and diseases such as common and powdery scab, sclerotinia, and potato cyst eelworm.