Pressure bruises develop in storage on tubers usually late in the season. Tubers are most susceptible to pressure bruising when the humidity in storage drops below 90% causing the tubers to lose moisture, particularly in the initial months of storage. This defect is most often the result of pressure at points of contact with adjacent potatoes, or the floor and appears as flattened or depressed areas on the tuber (80). In most instances, there will be no discolouration of the underlying flesh at time of packing; however, grayish to black discolouration may develop in the affected tissue with time (81). It is impossible to predict whether discolouration will occur, or how extensive it will be, from the appearance of the flattened or depressed areas. Over time pressure bruises may become sunken (82, 83) and develop into a rot (84)Footnote 9.
(85) Pressure bruises are scored under a special tolerance for pressure bruises when the sunken and discoloured areas are in excess of 10% of the surface area of the tuber. To approximate percentage coverage, tubers can be divided into quadrants as illustrated below. This is a two dimensional image, one side is shown as 50 %; this represents one half of a tuber sliced down the middle.