The spots often have pale green to yellow rings surrounding them. Leaf spots begin as small, pale to dark green, irregularly shaped spots. It is favored by cool, moist weather and can kill plants within two weeks if conditions are right. In recent years, it has reemerged as a problem. Late blight continued to be a devastating problem until the 1880s when the first fungicide was discovered. It is famous for being the cause of the 1840s Irish Potato Famine, when a million people starved and a million and a half people emigrated. It was first reported in the 1830s in Europe and in the US. It affects potato, tomato and, occasionally, eggplant and other members of the potato family. Late blight of potato is a serious disease caused by Phytophthora infestans. See current recommendations for chemical control measures. Use adequate nitrogen levels and low phosphorus levels to reduce disease severity. Surround the field with wheat to keep wind-blown spores from entering. Keep this year’s field at least 225 to 450 yards away from last year’s field. Do not use a field for potatoes that was used for potatoes or tomatoes the previous year. Do not dig tubers until they are fully mature in order to prevent damage. Keep plants healthy stressed plants are more predisposed to early blight. In general, late maturing varieties are more resistant than the earlier maturing varieties. Varieties resistant to this disease are available. Early blight spots are less likely to become rotted by secondary organisms than the other tuber rots. As the disease advances, the potato flesh often becomes water soaked and yellow to greenish yellow. When the tuber is sliced open, the flesh under the spots is usually brown, dry, and leathery or corky in texture. The edges of the spots are often raised and purple to dark metallic gray in color. Tubers are affected, as well, with dark, circular to irregular spots. Dark brown to black spots can occur on stems. This is especially true on the lower leaves, where spots usually occur first and can be very abundant. As the spots become very large, they often cause the entire leaf to become yellow and die. A yellowish or greenish-yellow ring is often seen bordering the growing spots. The spots usually have a target appearance, caused by concentric rings of raised and depressed dead tissue. The spots are often bordered by veins that make them angular. Spots begin as small, dark, dry, papery flecks, which grow to become brown-black, circular-to-oval areas. Early blight is favored by warm temperatures and high humidity. This disease, also known as target spot, rarely affects young, vigorously growing plants. The terms “early” and “late” refer to the relative time of their appearance in the field, although both diseases can occur at the same time.Įarly 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. Both are found everywhere potatoes are grown. Early blight and late blight, two serious diseases of potato, are widely distributed.

Early blight disease