Rhizoctonia (black scurf) – Rhizoctonia solani, fungus
"This fungus is a common soil inhabitant and tuber colonizer, most easily identified by the small black sclerotia (black scurf stage) on the tuber surface. Sclerotia range in size from as small as a pinhead to as large as half a pea and are best described as dirt that won't wash off, but can be removed with a fingernail. Rhizoctonia also causes a russeting or checking of the tuber surface, which is common in heavier soils. These symptoms can be confused with those of common scab. The sclerotia remain inactive in storage, but contribute to poor appearance and reduced marketability. Tubers used for seed should be treated with a fungicide, for sclerotia can be a source of inoculum for sprout "burn" and stem cankers on early emerging plants. Delayed harvest increases sclerotia size and number"
Information and photo source: Zitter, T.A., Loria, R. "Vegetable Crops", Cornell University Vegetable MD Online