Black scurf of potato is caused by

The black scurf of potatoes is caused by Rhizoctonia solani.
Black scurf is caused by the fungus Rhizoctonia solani, which can exist in the soil in the absence of potatoes. It does not feed on the tubers, and stored tubers with black scurf on them will not decay or deteriorate.
Rhizoctonia causes a variety of symptoms including damping off, stem lesions, stem rot, root rot, crown rot, and aerial web blighting. The infection causes wilting, stunting, and possibly plant death.