Black dot

Black dot (Colletotrichum coccodes) is a fungal blemish of potatoes and tomato roots that are often only just visible to the naked eye. It is also known as brown rot of tomato, aubergine and pepper fruit. 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.