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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.
Phytophthora infestans is an oomycete that causes the serious potato disease known as late blight or potato blight. (Early blight, caused by Alternaria solani, is also often called potato blight.) Late blight was a major culprit in the 1840s European, the 1845 Irish and 1846 Highland potato famines.