Potato brown spot and black pit are fungal diseases of potato caused by the fungus Alternaria alternata. On leaves, it causes dark brown spots of necrotic tissue with a dark brown margin. Starting as small lesions, the spots can coalesce to cover a large percentage of leaf or petiole surface. On tubers, the disease causes black, deep sunken pits with definite margins that often develop during in storage.
The pathogen causing these diseases survives on infected soil-bound plant debris and susceptible weeds, and spreads to leaves via windborne spores and tubers through soil-bound spores, often after mechanical damage. Favorable conditions include long dew periods, standing water on foliage, and warm or hot temperatures. Reduced airflow, plant maturity, and low nitrogen status of a potato crop can also favor these diseases.
Cultural management involves properly managing soil moisture, post-harvest plant debris, crop fertilization, rotating away from Solanaceous crops, and avoiding bruising tubers during harvest. Properly-timed foliar fungicide applications can also aid in managing the disease.