There are two major types of potato bruises caused at harvest. While both are caused by the potato hitting or being hit by another object, they occur under different conditions and for different reasons. Blackspot bruise appears as small, dark, discoloured areas beneath the potato skin which usually are not visible unless the tubers are peeled. Injured tissue first turns pink, then red, then darkens. Stem ends of tubers and tubers with lower water content are more susceptible to blackspot bruise.
Varieties vary greatly in their susceptibility.
Shatter Bruise - Impact of sufficient strength to rupture the tuber skin and several layers of cells beneath it causes a shatter bruise. The resulting small splits in the skin may not be visible at first but are easy to see after the injured tuber tissue dries out. The margins of shatter bruises show the same discolouration as blackspot bruises, but the injuries are visible on the tuber surface and usually penetrate deeper than blackspot bruises. Diseases such as fusarium dry rot or bacterial soft rot can infect tubers and cause tissue breakdown because the flesh is open. Other general forms of bruises or mechanical damage include skinning, cutting and scraping.