Why Do My Sweet Potato Skins Have Brown Crusty Patches?

Sweet potatoes harvested with brown, dark, scab-like or crusty patches on the skin — as if the skin surface has been roughened or scorched in irregular patches — are showing symptoms of soil rot scurf, caused by the fungus Monilochaetes infuscans. Scurf is one of the most common cosmetic problems in home-grown sweet potatoes and is often seen when growing conditions are wet or when infected planting material was used. The good news: scurf is a surface-only problem and does not affect the edibility or eating quality of the tuber inside.

Identifying scurf

Soil rot scurf appears as flat, dark brown to black crusty patches on the skin surface. The patches may cover small or large areas of the tuber surface. Unlike surface mould, the lesions are part of the skin itself rather than a soft growth on top of it. Cut the tuber open — the flesh inside is completely normal and unaffected. The patches do not indicate internal rot. In storage, scurf does not spread from affected tubers to neighbouring ones.

Causes and conditions

The scurf fungus overwinters in soil and in infected plant debris. Infection is most severe when: infected planting material (slips from scurf-infected parent tubers) is used; the soil is consistently wet or poorly drained during the growing season; and the tubers are left in cool, wet soil too long after vine senescence. The disease is primarily carried on planting material — sourcing certified clean slips is the single most effective prevention.

Prevention

Use certified disease-free slips from reputable suppliers each year. Rotate sweet potato to a different bed every three to four years. Improve drainage in the growing bed — scurf is worse in wet soils. Harvest promptly in autumn; do not leave tubers in wet, cooling soil after the vines die back. After harvest, cure immediately to harden the skin — cured tubers with light scurf develop no further disease during storage.

Using affected tubers

Scrub the skin under running water to remove loose scurf material before cooking. Affected tubers can be peeled before cooking if the patches are extensive and you prefer a clean appearance. The flesh inside is unaffected by the disease and tastes identical to clean tubers. Store affected tubers normally after curing and use within the normal storage window.