Black scab is a widespread fungal disease that commonly affects potatoes, appearing as dark, scaly patches on the tuber’s surface. While the marks impact visual appeal, the disease is primarily a cosmetic issue. The presence of black scab does not mean the potato is spoiled or inedible, but it does affect the quality and marketability of the crop.
Identifying Black Scab
The most recognizable sign of black scab is hard, black, slightly raised patches on the potato’s skin. These formations, known as sclerotia, are often called “black scurf” and resemble soil that cannot be washed away. While they cling tightly to the skin, these black spots can be scraped off with a fingernail or will come off when the potato is peeled. The fungus does not cause the tuber to decay or deteriorate in storage.
The fungus can also impact the growing plant. It may cause reddish-brown, sunken lesions, or cankers, to form on the stems near the soil line. In cases of a severe infection, these cankers can hinder plant development.
Causes and Favorable Conditions
Black scab is caused by a soil-borne fungus named Rhizoctonia solani. This fungus is very common in soil and can survive there for several years, even without a host plant. It can be introduced into a garden on infected seed potatoes or may already be present from previous plantings.
The development of black scab is most severe in cool, wet, and poorly drained soils, particularly during the early stages of tuber formation. These conditions create an ideal environment for the fungus to infect the developing potatoes. The disease also tends to worsen if potatoes are left in the soil for an extended period after the plant tops have died back, giving the fungus more time to form the black sclerotia on the tuber’s surface.
Prevention and Management Strategies
Starting with certified disease-free seed potatoes greatly reduces the chance of introducing the Rhizoctonia solani fungus into your soil from the start. Heavily affected seed tubers should not be planted, as the fungus can attack the emerging sprouts and lead to poor plant establishment.
Crop rotation is another important strategy for managing this soil-borne disease. Avoid planting potatoes or related crops, such as tomatoes and eggplants, in the same location for at least two to three years. This practice helps to reduce the buildup of the fungal pathogen in the soil over time. Rotating with non-host crops like corn or beans can help break the disease cycle.
Adjusting your planting and harvesting schedule can also limit the impact of black scab. Planting seed potatoes in warmer soil promotes faster sprouting and growth, which can help the plant outpace the fungus. Harvesting tubers promptly once they have reached their mature size is also beneficial. Limiting the time mature potatoes spend in cool, damp soil minimizes the window for the fungus to form the unsightly black scurf on the skins.
Improving soil conditions can make the environment less hospitable for the fungus. Rhizoctonia solani favors overly wet conditions, so ensuring your garden has good soil drainage is a significant preventive measure. Amending heavy clay soils with organic matter can improve their structure and prevent waterlogging, creating a healthier environment for potato growth and discouraging the development of black scab.