Black spot, nonpathogenic
Black spot occurs in the tuber flesh just beneath the tuber periderm and appears as gray to black circular areas with diffuse borders. Because this injury is always the result of bruising, including pressure bruising, internal symptoms may be associated with flattening or other signs of mechanical injury on the tuber surface. However, black spot may occur in the absence of any obvious external damage, and mechanical injury to the tuber does not always result in internal symptoms. Many factors have been reported to affect the susceptibility of tubers to black spot. Tubers with low turgor pressure are most likely to be affected by black spot. The stem end is more susceptible than the apical end, and mature tubers are more susceptible than immature tubers. Seed quality is not appreciably reduced by black spot. Avoid deep storage piles, especially with more susceptible varieties.