My Sweet Potato Plant Is Rotting

Causes of Rot

• Black rot fungus causes sweet potato roots to develop firm, black patches. It also produces black lumps or cankers on the underground sections of stems and can cause whole plants to yellow and wither. Soak seed sweet potatoes for 10 minutes in a solution made from two-thirds cup of borax powder in a gallon of water prior to planting. Fusarium wilt has similar above-ground symptoms to black rot but is caused by a different fungus and does not cause black root lesions. It persists in the soil for many years.

Other Diseases

• Sweet potatoes are vulnerable to scurf fungus, which causes shallow, black lesions on the skin of the roots. It is most common in gardens with fine-grained soil and in gardens fertilized with manure. Discard the remains of plants after harvest and do not replant sweet potatoes in the infected soil for at least two years. Root knot nematodes are tiny worms that infest sweet potato roots and cause weak growth and sickly foliage. They can be identified only by sending a soil sample to a laboratory.

Cultivation Practices

• Avoid rot by limiting sweet potatoes to an inch of water per week. If this amount of water comes naturally due to rainfall, no supplementary water in required. Remove all debris such as old leaves and stems, as they act as a reservoir for fungus. Harvest your potatoes before hard frosts and before the soil temperature drops to 50 F to prevent damage. Treat sweet potatoes with care as any bruising may develop into rot during storage. Leave harvested roots to cure for up to three weeks in a warm, humid spot. Store between 50 and 60 degrees F.

Prevention

• Avoid disease by buying commercial roots or transplants that are certified disease free, rather than replanting your own roots year after year. Select cultivars that are resistant to any diseases you know are present in your area. Take transplant cuttings from well above the soil line to avoid soil-based fungus. Never plant sweet potatoes in soil where they have previously had diseases or rot and always rotate sweet potato crops on a three- or four-year cycle.