Sweet potato scurf or soil stain is caused by Monilochaetes infuscans, a soil-borne fungus. This disease causes superficial damage to the sweet potato and will reduce its storage time due to moisture loss through the diseased areas. Copper-skinned sweet potatoes usually have brown lesions, and red-skinned sweet potatoes have black lesions. Symptoms are restricted to the skin of storage roots and do not directly affect the underlying tissue. Affected tissue can be easily scraped off. The fungus is prevalent in wet, heavy soils high in organic matter with high soil pH.
Use only disease-free potato roots for propagation; plant cuttings into new soil. If a soil pH test comes back higher than 6.5, consider adding sulfur to increase soil acidity.
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