Scurf

Monilochaetes infuscans

Scurf is caused by a fungus that grows on the surface of the sweet potato tuber. It shows itself as purple/brown/black patches on the skin (fortunately it is mainly cosmetic and doesn’t affect edibility). These lesions enlarge in storage and in extreme cases can cause the skin to crack, allowing the flesh of the tuber to dry out.

This disease is usually spread by planting slips taken from contaminated roots, though it can also come from the soil (it can survive in the soil for up to 2 years), Like many other fungus diseases it is most problematic in wet weather. The most important control measure is to use certified disease free plants. If this isn’t possible then cut the slips above soil level and they should be free of the fungus spores. You should also check roots for signs of the disease before storing (use any badly infected tubers first). Wait three years before planting sweet potatoes in the same soil.

Image: Robert Wick, University of Massachusetts, Bugwood.org