Early blight and late blight, two serious diseases of potato, are widely distributed. Both are found everywhere potatoes are grown. Early blight of potato is caused by the fungus, Alternaria solani , which can cause disease in potato, tomato, other members of the potato family, and some mustards. This disease, also known as target spot, rarely affects young, vigorously growing plants. It is found on older leaves first.
Early blight is favored by warm temperatures and high humidity.
The spots are often bordered by veins which make them angular. The spots usually have a target appearance, caused by concentric rings of raised and depressed dead tissue. A yellowish or greenish-yellow ring is often seen bordering the growing spots. As the spots become very large, they often cause the entire leaf to become yellow and die. This is especially true on the lower leaves, where spots usually occur first and can be very abundant.