Early blight may also start to show up on potatoes. Both of these foliar diseases do best during periods of humid weather, however, early blight develops most rapidly under conditions that are slightly warmer than those most favorable for Septoria leaf spot. Early blight usually starts to appear on the older leaves at the base of the plant. Small brown to black spots develop that may be surrounded by a yellow margin. The spots enlarge rapidly and concentric rings, or a target spot pattern begins to develop in the brown tissue.
Spots can also form on petioles, stems, and fruit.
Fruit is usually infected at the calyx end, and the lesions can get quite large spreading down the shoulders of the fruit. Lesions on the fruit are usually dark brown to black in color and usually contain the concentric ring pattern. High levels of disease can lead to significant defoliation, which exposes the fruit and results in sunscald. Septoria leaf spot also first appears as small spots on the older leaves. Spots are usually tan to gray in color with a dark brown margin.