Why Are My Sweet Potatoes Rotting Black Inside?

Cutting open a sweet potato to find dark black or brown-black areas of rotted flesh — sometimes with a bitter smell — is a sign of black rot, caused by the fungus Ceratocystis fimbriata. Black rot is one of the most economically damaging sweet potato diseases worldwide and can destroy tubers in the field, during curing and in storage. Unlike surface mould that can be cut away, black rot penetrates deeply into the tuber flesh and the affected tissue is bitter and inedible.

Identifying black rot correctly

On the outside, affected tubers show dark brown to black, circular, slightly sunken lesions. These may be small at harvest but expand rapidly in storage. Cut the tuber open: the interior around the lesion is black, dry and firm — not soft and slimy like bacterial rot. The fungus may produce faint green or dark fruiting bodies in the lesion over time. Affected tissue has a bitter taste — a single black rot lesion can make the entire cooked tuber taste bitter even if the affected area appears small.

How it spreads

The black rot fungus survives in soil for several years and in infected plant debris. The primary entry routes are: infected planting material (slips from diseased tubers carry the fungus internally even if they look healthy), wounds at harvest (abrasions and cuts in the skin at digging time allow the fungus immediate entry into the tuber), and soil splashing onto wounded surfaces during wet harvesting conditions.

Prevention in the field

Source certified disease-free slips from a reputable supplier each year — do not propagate from your own tubers if black rot has been present. Rotate sweet potato to a new bed every three to four years. Improve drainage in heavy soils — the fungus thrives in wet, poorly drained conditions. Harvest carefully to minimise skin damage; handle tubers gently during lifting and transport.

Curing and storage

Curing sweet potatoes correctly after harvest dramatically reduces post-harvest black rot losses. Cure by holding tubers at 29–32°C and 90–95% relative humidity for four to seven days immediately after harvest — this allows the skin to harden and minor wounds to heal over (suberise) before the fungus can enter. After curing, store at 13–15°C in a dry, well-ventilated location. Do not store tubers from a black-rot-affected harvest alongside healthy tubers — discard all suspect ones promptly.