Answer: Sounds as though your spuds have scab. Common scab is caused by a naturally-occuring soil microbe called Streptomyces scabies. The problem is most prevalent and severe in neutral (pH 7) or slightly alkaline soils. The optimum pH for scab development is from pH 6.0 to 7.5. In general, scab is not a serious problem in soils with a pH of 5.5 or lower. The scab organism is very sensitive to soil moisture, pH, and temperature. The incidence of the disease is generally greater in sandy or gravelly soils than in heavier soils. The amount of soil moisture at the time of tuber development is very important.
Several practices can limit the severity of scab.
1. Planting resistant varieties is the most feasible and practical control. Varieties showing some level of scab tolerance include: Norchip, Norgold Russet, Reliance, Russet Burbank, Shurchip, Norland, Hi-Plains, Pungo, Redskin, Russet Sebago, Shoshoni, and Superior.
2. Avoid planting seed exhibiting scab lesions.
3. Crop rotation in which potatoes will be planted in the same area only once every 3 or 4 years. Avoid rotating with root crops; instead plant grains, grasses, or legumes.
4. Maintain good moisture levels in the soil while tubers are developing.
5. Avoid spreading fresh manure just preceding the potato crop.
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