Sweet Potato Scurf

Sweet potatoes can be grown in large containers and look lovely on a patio.

And, sometimes, they get sick with a fungal disease called sweet potato scurf.

Purple sweet potatoes growing in a large container (Kate Russell)

Lush aboveground growth of your sweet potato plant may give no sign of diseases at first. But if you dug up a portion of the root system, you would see discolored areas of various sizes and shapes. These superficial lesions are gray on yellow-fleshed sweet potatoes and purple, brown, or black on orange-fleshed varieties. These discolorations look similar to black scurf and silver scurf of regular potatoes. Unlike silver scurf, sweet potato scurf does not penetrate beyond the skin. Eventually, these lesions can be seen on the stems and leaves of heavily infected plants.

While sweet potato scurf (or soil stain) doesn’t look too terrible and can be scraped off, these fungal infections make it more likely that other diseases and pests will get in. In storage, infected sweet potatoes dry out faster.

Conducive conditions

Humidity is necessary for sweet potato scurf to develop. It often appears during the rainy season. And it is more likely to occur in areas with heavy soil, after top dressing with a lot of manure, or in soil with high levels of organic matter. Organic matter usually helps plants grow better. Apparently, sweet potatoes stay healthier when grown in light, sandy soils.

Sweet potato infected with scurf (Scot Nelson) Public Domain.jpg

Sweet potato scurf management

Sweet potato scurf is caused by the Monilochaetes infuscans fungi. This isn’t a resilient soil-borne fungus. Without a host, it generally only survives for 1–3 years in the soil, depending on the soil type. This makes crop rotation a very effective control. In areas where sweet potato scurf has been a problem, growers generally rotate crops every three or four years.

Spores can be spread via tools, clothing, baskets, and shoes, but infected plant material is the primary culprit. This means you can often avoid it by only planting certified disease-free slips. Slips are baby sweet potato vine cuttings used to make new plants. Commercial growers treat slips with fungicide before planting as a preventative measure. There are no fungicides that control the disease once it takes hold.