Answer: There are a number of fungal diseases that cause black skin on sweet potatoes. Black rot causes circular, dark depressions on the tubers. Scurf creates small, round, dark spots on the surface, but doesn't affect the eating quality. Stem rot results when plant stems are injured. To minimize problems with your future crops, plant in a different garden spot, plant only healthy slips, choose disease-resistant varieties, and provide the best growing conditions possible. Problems with scurf can be minimized by growing in an acid soil. Don't overfertilize the vines, especially with a nitrogen-rich fertilizer, as this can cause tough, stringy roots.
The wrinkling skin may have been due to improper curing. After harvesting, dry the sweet potatoes for 2 or 3 hours, then spread them out in baskets lined with newspaper. Set them in a dry area where the temperature will remain between 80 and 85F for about 2 weeks. Once they're cured, store them at 55 to 60F with a humidity of about 85 percent. Check the stored sweet potatoes occasionally and remove any that show signs of decay.
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