The symptom is characterised by small, grey or black-brown scablike formations (sclerotae) on the surface of the tuber. This is the dormant stage of the Rhizoctonia fungus. The sclerotae are fairly firmly attacked but can be removed with the nail without injuring the skin.
Black scurf (Rhizoctonia solani) is of most significance when it attacks seed potatoes, as it develops on the plants, and by withering the shoots cam lead to a reduced yield. The attack can be aggravated by late development in account of cold and damp soil.
