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Cause of Potato Scab. The cause of common potato scab is a bacterium, Streptomyces Scabies, found in the soil. It is most common on light, sandy soils, low in organic matter. Soils that were previously grassland are particularly liable to having the disease. Soils with a high pH (recently limed) provide a better environment for the bacteria.
Examine the sweet potato for signs of decay. Decay in sweet potatoes can be seen as dark brown spots or small craters on the surface. Decayed spots can be cut away from the rest of the potato, but the bitter taste may linger throughout. Run your fingers over the skin of the potato.