Irregular dark brown to black spots, 1/8 to 1/2 inch in diameter, appear on the lower leaves. Concentric rings develop in the spots, which enlarge, causing the leaf to die and fall off. Tubers may develop dark, sunken spots, often with a purplish raised border.
Clean up and destroy plant debris after harvest. Do not store infected tubers. Avoid overhead watering by using drip or furrow irrigation. Plant certified, disease-free seed pieces. Apply a fungicide labeled for this disease, following label directions.