Do not grow potatoes in the same area year after year, but rotate with crops other than tomatoes. Common diseases of Irish potatoes in home gardens are described below. This is a potentially serious disease of potato and tomato, caused by the fungus Phytophthora infestans . Late blight is especially damaging during cool, wet weather. The fungus can affect all plant parts.
Young lesions are small and appear as dark, water-soaked spots.
These leaf spots will quickly enlarge, and a white mold will appear at the margins of the affected area on the lower surface of the leaves. Infected potato tubers have a dry, corky rot that may be brown or reddish. Tubers are symptomless at the initial stages of infection but often develop symptoms in storage. The fungus produces a foul odor when infection is severe. Fungal spores are spread between plants and gardens by rain and wind.