Black dot

Black dot (Colletotrichum coccodes) is a fungal blemish of potatoes that are often only just visible to the naked eye. Black dot can be found on stolons, roots and stems, on tubers they can produce a light brown background, which can darken when in 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.