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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.
See 3 dimensional views of Black Dot, Black Scurf and Silver Scurf. Black dot disease of potato, caused by the fungus Colletotrichum coccodes, is generally considered to be a weak root pathogen of potato. Recent studies in New York and elsewhere have revealed, however, that this disease must be considered as part of the total disease complex affecting potato.