Early blight of potatoes can reduce yields significantly. Infections can be very severe when plants are under stress from something else like fertility, another disease, heat etc. Alternating wet and dry conditions will increase severity of infection. Temperatures of around 20°C favour fungal growth, while night time temperatures of around 15°C favour sporulation.
Lesions may not be visible until weeks after infection first takes place. They begin to form on leaves, usually shortly after flowering. Lesions will not grow past main leaf veins, producing angular shapes. They start as small spots and grow out in rings, forming a “bullseye” shape. Small, fleck like lesions may form on stems.