For home gardeners who have enough space to grow them, potatoes are a popular crop. Potato varieties that are hard or impossible to find in stores are delicious treats, even if they are not “keepers” but have to be enjoyed while they are fresh. There are many potato diseases, but with a few precautions most of them can be avoided.
The most famous potato disease is late blight, which caused the tragic Irish Potato Famine. Only one variety of potato was grown in Ireland at the time, so when the blight hit it wiped out virtually the entire crop. Since potatoes were a staple food for the Irish poor, the blight resulted in widespread famine from 1845 to 1852. It is believed that the late blight was carried to Europe from America, where it had decimated crops from 1842 to 1844.
Foliage diseases include early blight and late blight, which are fungal diseases. Two wilt diseases are also caused by fungi—fusarium wilt and verticillium wilt. Soil fungi and several types of bacteria attack young plants and the tubers themselves. Virus diseases include mosaic and leafroll.
Early blight begins as dark spots on the lower leaves. Late blight begins as light brownish spots on leaves, that turn black; a faint white fuzz can be seen on the underside of the leaf. Wilt diseases begin with the yellowing of leaves that then droop and curl. The bacterial disease called blackleg may prevent germination of seed potatoes, and/or result in emerging plants that have bronzed leaves that rapidly wilt and die; stems above and below the soil line are black, slimy and rotted. The virus disease mosaic causes yellowing of the leaf veins or a mottled, crinkly leaf surface, while leafroll causes the lower leaves to discolor yellow or a reddish-bronze, then curl upwards.
It is most important to plant certified disease-free seed tubers. Infected seed not only produces sick plants, if it produces any at all, it introduces disease organisms into your garden soil, where they may remain for years. If you do find any diseased plants, remove them completely from your garden—bag them, roots and all, and take them to the dump. Do NOT add them to the compost pile. At the end of the growing season, remove all plant materials—do not leave them in the soil.
Since most potato diseases are caused or encouraged by excess moisture in the soil or on the leaves, try to keep the soil barely moist and never wet; and avoid overhead watering by using drip irrigation. Do not grow potatoes, tomatoes, peppers or eggplants in the same location in your garden the following year. These plants are all related and share many of the same pests and diseases.
The safest rule of thumb is to deal with all potato diseases as incurable, which most of them are. Promptly remove diseased plants and dispose of them, and never add them to the compost pile. Make sure you remove all tubers left in the soil, too. If diseased plants are quickly, totally and carefully removed from the garden, the remaining plants may survive and produce a crop.