Black scurf of potato

Black scurf, caused by the soil-borne fungus Rhizoctonia solani, is a major disease affecting potato crops worldwide. This pathogen is known for its ability to attack various parts of the potato plant, including tubers, stolons, and stems, leading to both qualitative damage and quantitative yield losses through plant stand reduction.

The fungus is highly versatile and possesses an extensive host range, affecting numerous vegetables and ornamentals beyond potatoes, such as tomatoes, beets, and many grains. Because of this wide host range and its ability to persist in soil as sclerotia, the disease management requires an integrated strategy to reduce inoculum levels effectively.

Diagnostic symptoms on tubers include dark brown or black, hard, flat fungal structures called sclerotia, which are firmly attached to the tuber surface. During the growing season, the pathogen causes rhizoctonia canker, which manifests as brown necrotic lesions on underground stems. This damage often results in stunting, leaf curling, and the formation of aerial tubers due to the blockage of vascular tissue.

The development of the disease is heavily influenced by environmental conditions during planting. Cool and moist soil temperatures (12–18°C) are highly conducive to fungal infection, as they delay potato sprouting. During this lag phase, the pathogen easily infects the susceptible succulent sprouts, potentially killing them before they even emerge, which is the primary cause of uneven stands.

Effective control measures focus on preventive strategies and chemical intervention. Farmers should utilize disease-free certified seeds and practice long crop rotations to diminish the soil-borne inoculum. Seed treatment with appropriate fungicides is essential to protect the emerging sprouts during the critical early growth stage, ensuring robust development and better yields.

• Planting high-quality, certified seed potatoes free of sclerotia.
• Implementing long-term crop rotation cycles to lower soil pathogen levels.
• Applying systemic fungicide seed treatments before planting.
• Ensuring optimal planting conditions in warm, well-drained soil to speed up emergence.