Q: I have been raising sweet potatoes for several years. Now, about five percent of my potatoes have black skin. The potato underneath the skin is good but the black skin is unsightly. What is wrong?
A: I have sour news for your sweet potato garden. Your potatoes likely have scurf, a common soil borne disease of sweet potatoes. The fungus destroys to skin of the potato root, causing it to quickly decline when stored.
You make a good observation, though: scurf does not affect the eating quality of a potato. You can scrape it off and cook your fresh potatoes as you normally do.
There is no fungicide treatment for scurf. It is avoided by planting only certified disease free potato slips in clean fields. Additionally, don’t plant potatoes in an infested field for at least three years.