Document Type


Publication Date



Plant Pathology Department

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potato, potato blight, fungicides


South Dakota has an important potato- growing area in Clark, Codington, Hamlin and Deuel counties. Production of certified seed has become one of the important phases of the potato industry in South Dakota. In addition to the main commercial seed and table stock production areas indicated above, smaller producing areas are located in various sections of the state, not to mention the familiar potato patch in most home gardens. Likewise, with the coming of irrigation in the James River Valley of South Dakota, potato production will probably increase further. Losses in yield and quality of potatoes are severe in seasons favorable to the development of foliage diseases. While these diseases do not occur in serious amounts every season in South Dakota, the profits may be wiped out in those years when they are prevalent. When these diseases strike in epidemic proportions, one of the important effects which they have on the crop is the prevention of normal "sizing" of the tubers, thereby resulting in a reduction of U. S. No. 1 potatoes. For this reason, it is important to know what control measures are effective and can be easily and quickly applied when these diseases strike the crop. An effective fungicide, properly applied, can control these foliage diseases, and by removing another crop hazard can thereby add to the stability of commercial and certified seed potato production in South Dakota. Chemical fungicides, applied as sprays or dusts, differ in their effectiveness in controlling crop diseases. A particular fungicide might be highly effective in combating one type of disease, but may not be very effective against another. Consequently, it becomes necessary to test many chemicals over a period of several years to determine their effectiveness as well as the dosage rate and the proper time of the season to apply them. The most prevalent fungus diseases affecting potato foliage in the state are early blight, caused by Alternaria solcmi, and late blight, caused by Phytophthora infestans. Early blight is present to some extent every season in South Dakota, and causes losses through reduced grade and yield. The disease first appears as small oval or angular brown spots on the leaves with characteristic target-like markings. As the spots increase in size and number the affected leaves die. As a result of this defoliation the yield, especially of U. S. No. 1 grade potatoes, is reduced. During those seasons when conditions are favorable for the development of late blight, losses through reduced yields and tuber rot become costly to potato growers. Late blight is favored by cool temperatures and relatively good moisture conditions. This disease first appears on the leaves as pale green, water-soaked, irregular-shaped spots which may enlarge rapidly, turn brown or black, and show a white mildew-like appearance on the lower surface of diseased leaves. The stems can become infected and the entire plant may be killed in a few days. Under conditions favorable for the disease (moderate temperatures, with high humidity, heavy clews or frequent rains), it can spread rapidly throughout a field. Spores from diseased foliage washed down to the tubers by rain or brought in contact with them during digging operations can expose tubers to infection with late blight. Late blight-infected tubers develop what is known as late blight tuber rot, either in the field or in storage. To protect potato plants from these fungus diseases, it is necessary to keep the foliage coated with an effective fungicide. It has been shown experimentally that fungicides applied as sprays provide better coverage and disease control than those applied as dusts. A series of field experiments were initiated in 1945 at the Experiment Station at Brookings with the object of developing a potato spray program adapted to South Dakota conditions. In 1946, these tests formed a part of a regional potato fungicide trial in which six states in the upper Mississippi Valley participated. From 1947 to 1949 the tests were included in the national cooperative fungicide experiments sponsored by a Special Committee on the Coordination of Field Tests with New Fungicidal Sprays and Dusts, appointed by the American Phytopathological Society.










South Dakota State College of Agriculture and Mechanic Arts, Agricultural Experiment Station