The Q&A Archives: Potato Disease

Question: I have grown potatoes in the Helena valley for a number of years. Each year I have some potato tubers that have black spots on them. The spots look like dirt particles and they do not penetrate beyond the skin of the potato. I believe this is a resultof a disease called rhizoctonia. If this is a correct diagnosis, what can be done to control this disease.

Answer: The symptoms sound very much like "Black Scurf", a fungal disease caused by Rhizoctonia. This disease causes hard, black lesions on the surface of the tuber. It doesn't affect the quality of the tubers, but they may not store as well as tubers uninfected by the disease. Rotate your potato crop, avoid overwatering, especially early in the season, and be sure to purchase certified disease-free seed potatoes.

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