Other pages about bruise defects:
In addition to the external bruises discussed in the previous issue, during storage another external bruise called pressure bruise can occur. Pressure bruising appears as a depressed, dark, softened, and circular area on the tuber surface (Picture; Chase and Silva, 1987). It may be small, quarter inch diameter, to as large as a couple of inches. The depression may be as much as a half inch deep. [At home, you may have noticed apples with pressure bruising since the symptoms are very evident on them.] The bruise enlarges over time once it forms. When the center of the bruise dries, it may collapse and form a cavity. Obviously, this bruise markedly reduces marketability and must be sorted out when tubers leave storage. However, in storage, as cells at the surface break down, an entry portal for pathogens develops and tuber rots in the pile develop.
Pressure bruise occurs on tubers at the bottom of a pile and results from the weight of tubers above. Do not pile tubers higher than 18 feet. It is aggravated under conditions of low relative humidity. Storage humidity should be greater than 90% RH. Dehydrated tubers are especially susceptible. Tuber water loss is not uncommon with immature tubers, wounded tubers, and tubers harvested from soil with low moisture. Potato cultivars having tubers with low solids are most at risk. The condition is a phenolic reaction in the periderm (tuber skin cells) that also produces a gas, ethylene, a growth hormone, which also promotes further cell wall breakdown in an auto-catalytic reaction thereby making the condition increasingly worst.
References:
Chase, R.W. and Silva, G.H. 1987. Potato Bruising. Mich. St. Univ. Coop. Ext. Bull. E-2074.