Symptoms

Light silvery lesions (photos 1 and 2) are clearly visible on the tuber surface of red-skinned cultivars (photo 3).

These patches spread and eventually are covered with very fine black dots; these are the fructifications of the fungus (conidiophores that carry spores) (photo 4).

At harvest, these symptoms are barely visible and appear mainly during storage when the temperature and humidity are favourable.

The symptoms of silver scurf and black dot are often confused with each other, even though some criteria make it possible to distinguish them. For H. solani, the surface lesions are better defined and more silvery with finer black punctuations than for black dot (photo 5: silver scurf, on the left - black dot, on the right). In favourable moist conditions, sporulation of H. solani occurs at the edge of the silver scurf lesions and this differentiates it from black dot.

When the fungus has covered a large portion of the tuber, the superficial layer of the skin becomes detached; the tuber shrivels and shrinks due to dehydration (photo 6).