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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.
1 Do not work manure or compost into the potato bed, as the excess organic matter can lead to potato scab and other diseases. Apply all fertilizer before the end of July, as fertilizing later can cause the plant to produce smaller, undeveloped potatoes.  Spade.