Black dot (Colletotrichum coccodes) is a common superficial fungal blemish on potatoes. The black dots are microsclerotia that are often just visible to the naked eye. They can be found on tubers, stolons, roots and stems. On tubers, affected areas may show a light brown background colour, which can darken during storage.

Black dot is a weak pathogen that is more prevalent from soils low in nitrogen or in warm, wet growing conditions. Warm, moist storage also promotes the blemish.

Black dot may develop a silvery sheen during storage, which can be confused with silver scurf. However, black dot tends to show much more irregularly shaped patches with less well-defined margins than silver scurf. Inspection with a hand lens (10x) will quickly differentiate the regularly spaced black dots from the bunched threads of silver scurf.

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This page was last amended on 25th June 2003