Potato rhizoctoniosis

Pathogen and disease type. Potato rhizoctoniosis, commonly referred to as black scurf, is caused by the soil-borne fungus Rhizoctonia solani. This pathogen is a major concern in potato production worldwide, as it can attack the plant at various growth stages, causing necrosis and decay in roots, stolons, and tubers.

Susceptible crops and pathogen nature. While Rhizoctonia solani is a broad-spectrum pathogen affecting numerous vegetable and cereal crops, potatoes are particularly vulnerable. The fungus persists in the soil as sclerotia, which are dormant masses of hyphae that act as the primary inoculum for infection during the next planting season.

Symptoms and signs. The most visible sign is the presence of black, dirt-like sclerotia on the surface of harvested tubers. However, the most damaging symptoms occur underground: stem and stolon cankers, which appear as reddish-brown lesions. In severe cases, the fungus causes the base of the stem to rot, often accompanied by a white mycelial mat near the soil surface.

Conditions for development and impact. Development is favored by cool, moist soil conditions during the early growth period. The economic impact is substantial, as the disease can lead to poor stand establishment, reduced tuber size, and skin blemishes that significantly lower the market quality of the harvested produce.

Protection and prevention measures. Managing rhizoctoniosis requires a combination of cultural and chemical strategies to reduce inoculum levels. Essential practices include:

• Utilizing clean, certified disease-free seed tubers.
• Applying appropriate seed treatments or in-furrow fungicides.
• Maintaining a healthy crop rotation and avoiding planting in cold, wet soil.
• Ensuring rapid emergence by using sprouted seed or optimal planting depths.