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Potato Diseases and Their Control

Source: NDSU Extension Service Horticulturist

A number of diseases attack home garden potatoes, and many of these diseases can be prevented. For example, there is a disease of the potato tuber called scab. Potato scab causes the tubers to have a brown, rough, bumpy or corky surface. There is no cure for scab, but frequently it can be prevented by planting scab-resistant varieties such as Agassiz, Bison, Crystal, Norgold Russet or Redsen. Do not use animal manure or straw on potatoes as this will promote scab. Also, do not rotate potatoes with beets on the same land if scab has been a problem.

When planting potatoes, here are a few hints that will help prevent disease problems. It is very important to plant good quality seed pieces, as poor quality seed or seed saved from last year's crop can be a source of many diseases. If possible, buy certified potato seed since this will reduce the chances of destructive bacterial diseases such as black leg and ring rot. It will also reduce the chances of getting virus diseases, some which do not even show up but still reduce yield. Other virus diseases, such as rugose mosaic, produce a blotchy or mosaic pattern of light and dark green on the leaves. Whenever this appears, even on just a few plants, you have fairly serious virus problems, because rugose mosaic is a combination of two viruses, both which reduce yields but give no visible signs of their presence, unless they are both infecting the same plant.

Later in the season, when the plants are growing well, watch for leaf spot diseases. There are two that can seriously damage the leaves, causing them to drop prematurely. The most common leaf spot problem is early blight. These spots become large, up to inch or even 1 inch in diameter. They are circular to irregular in shape and dark brown to black in color. Early blight spots have concentric rings arranged in a target pattern. The spots have a definite margin. Leaves with early blight spots turn yellow and drop off. As soon as early blight appears, the potato plants should be sprayed with a fungicide such as Captan (Orthocide) or Daconil 2787 (Ortho Multi-Purpose Fungicide). These fungicides are available at garden care centers.

Another leaf spot disease develops in years with cool, wet weather. This is late blight. When late blight attacks, the leaves develop large dark-colored wet spots. The whole leaf rots quickly. Late blight spots do not have definite margins. As soon as late blight is found, spray the plants with a fungicide. The same fungicides can be used as for early blight control.

For future reference, you may want a copy of Extension bulletin PP-756, "Potato Disease Control in the Home Garden," which is available at your county office of the NDSU Extension Service.

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