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diggin’ in BRUISE MANAGEMENT primarily tyrosine (an amino acid), and eventually results in the formation of a pigment called melanin that gives the black/blue discoloration. It typically takes about 24 to 48 hours for the pigment to develop. In some instances this biochemical reaction does not occur even if the tissue is damaged. For example, Figure 1 shows a Ranger Russet potato that was impacted with the same amount of force on both the stem and bud end. The stem end developed lots of melanin, resulting in formation of a dark bruise. The bud end damage resulted in a white spot of damaged tissue, but little melanin formation. Why the difference between the bud and stem end, even though the same amount of force was applied? It may be due to the physical differences between the ends, maturity and/or availability or levels of enzymes and amino acids to react and oxidize. This complex interaction between impact forces, chemical reactions and tuber physical properties provides insight as to why potato bruising may be more prevalent in certain fields, varieties or seasons. Monitoring bruise incidence throughout your harvest system is a useful way to determine whether bruise susceptibility is higher for a particular lot or field. Taking tuber samples at different points throughout the harvest and handling process (eg. hand dug, harvester, conveyors, any drop points or turns, etc.) and holding them at warm temperatures will allow you to assess the tubers for blackspot damage. The higher the temperature, the quicker the pigment formation will occur and the sooner the results will be available. PG Black and Blue Tuber bruise management (emphasizing Blackspot Bruise) by William H. Bohl, UI extension educator I CAN’T SAY FOR SURE, BUT I SUSPECT potato producers were less concerned about potato bruising some 60 or so years ago before mechanical potato harvesters. is more complicated because it is more than a physical damage. The blackening just beneath the tuber skin results from an impact that damages several layers melanin, which causes the black, gray or brown discoloration seen only if the tuber skin is removed. The reaction takes 24 to 48 hours to complete. Shatter bruise, in contrast, is less complicated because it’s simply a physical damage resulting from an impact that breaks the tuber skin and possibly several layers of cells underneath. The cracks may become more visible upon drying. BLACKSPOT SUSCEPTIBILITY There is a greater potential for large BRUISED RANGER. An artificially bruised Ranger Russet. Photo courtesy of Nora Olsen. Yet, no one wants to go back to those days just to minimize bruising. Although a considerable amount of tuber damage can occur on a harvester, management practices to minimize bruising cannot be confined to the harvester or the harvest period. Of the two main types of tuber bruising, blackspot and shatter bruise, blackspot 24 Potato Grower | JULY 2010 of cells, causing a chemical reaction. Tyrosine (a substrate) mixes with a copper-containing protein, polyphenol oxidase (an enzyme), commonly called PPO, forming dihydroxyphenyl-alanine (DOPA). DOPA is further oxidized (oxygen is added), producing several intermediate compounds with the end product being tubers to be damaged compared with smaller ones. Likewise, larger cells have a greater potential to be damaged from an impact than smaller cells. Also, a small radius (pointed tuber end) is more likely to be damaged than a large radius because the force of the impact is confined to a smaller area. Hence, oftentimes damage is referred to as shoulder bruise. Growing conditions and practices during the season play a large role in tuber size. Because blackspot bruise is a chemical reaction, the internal quality of the tuber plays a role in susceptibility. Nitrogen and phosphorus play an indirect role in potential blackspot bruising, in that potato plants lacking these two nutrients will likely die earlier than adequately fertilized plants, thus, resulting in more mature tubers, and more mature tubers have a