On tubers, scabs are somewhat circular and consist of raised, scablike lesions. Damage generally does not extend much below the skin. There are no above-ground symptoms. Excessively wet soils can cause lenticels on the tubers to enlarge and become whitish and corky. Lenticels are pores in the skin that allow gas exchange.
Black scurf is another common disease that extends no deeper than the skin.
This pathogen generally is introduced into the soil via infected seed pieces. Once the soil is contaminated, the pathogen can survive indefinitely on decaying plant material and manure so that ongoing control measures become necessary. High soil moisture from one week before emergence until eight weeks after emergence can really reduce scab severity. The logic behind this is that the scab bacteria are inhibited in moist soils. Keep the soil moist, but do not drown the plants since this can encourage other diseases such as blackleg and bacterial soft rot.