Rhizoctonia solani causes stem canker on vines resulting in wilt and black scurf on tubers resulting in poor appearance and potential spread into soil.

Rhizoctonia stem canker  Rhizoctonia stem canker  Black scurf -- Atlantic  Black scurf -- Yukon Gold

I. On vines, the disease is called stem canker. It is an early season wilt.

a) Early symptoms are reddish-brownish, usually rectangular, blotches on stems near ground level — slightly above, at and/or slightly below. These blotches can also be seen in severe cases on the stolons which will result in pinching off the stolon and loss of the tuber (stolon pruning).

b) Later symptoms are wilting of the vine and the blotches become darker to black, enter stems and decaying them. Root pruning also may occur. As the disease progresses, a cross-cut of the lower stem will reveal that the vascular ring is brown to black and with further disease progress the whole inside decays. In the last stages, it's impossible to discern this wilt from other wilts. Microbiological plating and growth of organism in culture is needed for identification.

II. On tubers, the disease is called black scurf.

This disease does not rot tubers. It's appearance is on the surface as black hard spots that do not wash off. These fungal colonies give rise to the vine symptoms noted above when the tuber is planted. The colonies are formed through contamination by the disease in the soil. Tuber malformation is often associated with this disease, but the direct cause is not known.