This article appears in the December 2020 issue of Potato Grower.
Blackspot and shatter bruises are some of the greatest contributors to potato quality degradation. Ensuring proper tuber hydration and adequate soil moisture at harvest, avoiding cold pulp temperatures that promote both blackspot and shatter bruise, and minimizing impacts by lowering drop heights, reducing rollbacks, and adding cushioning are all common practices used to reduce the risk of bruise.
However, when bruises still show up under good handling conditions, it can be abundantly frustrating. Variability in susceptibility of tubers to bruise damage can range from year to year, field to field, and plant to plant, which only adds to the frustration. In addition, the location on an individual tuber where an impact occurred tends to have a large influence on the type of bruises (blackspot or shatter bruise) that will appear. That is why blackspot bruise is commonly called shoulder bruise and predominantly found on the stem end.
To further tease out where a bruise is more likely to form on an individual tuber, a recent study by the University of Idaho compared two varieties, Russet Burbank and Russet Norkotah, using an impact force from a 100-gram weight dropped 7 inches onto several locations on each tuber. This was done at harvest and repeated twice more in storage. Each tuber was impacted in 20 different spots, and each spot was evaluated for blackspot and shatter bruise susceptibility. These spots were carefully chosen to include placement on the shoulder and the flat surface (face) of the potato, as well as the bud end, the middle of the potato, and the stem end.
This study confirmed the bud end was more susceptible to shatter bruise and the stem end more susceptible to blackspot bruise for both varieties. Interestingly, blackspot bruise had a gradient in both discoloration and incidence from the bud end of the tuber to the stem end. The greater risk of blackspot bruise is not solely at the stem end of the tuber; rather, it extends about halfway toward the bud end. The shoulders had higher blackspot bruise incidence than the face of the tuber, although shatter bruise incidence did not differ between these locations. Russet Burbank and Russet Norkotah had similar gradients within the tuber from bud to stem end, although the incidence and severity of both blackspot and shatter bruise was lower in Russet Norkotah.
A blackspot bruise forms when there is internal tissue damage due to an impact. The impact can cause a loss of compartmentalization within a cell, and the enzyme polyphenol oxidase (PPO) reacts with tyrosine, eventually forming melanin (black discoloration). Not all varieties show an extreme difference between the bud and the stem of the tuber, but in general the stem end is more sensitive to a physical impact and more likely to discolor. Several reasons are given as to why the stem end is at greater risk for blackspot discoloration:
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Older/more mature part of the tuber,
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Greater potential area for stress,
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Higher potential for weakened cells,
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Different cell size and number, and
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Greater potential for higher levels of tyrosine and PPO
An individual tuber’s response to a physical impact can be determined by a tuber’s physical, biochemical and physiological attributes, which are in turn influenced by growing and harvest conditions. The tuber’s response makes fine-tuning a bruise management program that much more difficult, but exceptionally important to mitigate overall bruise damage.
Nora Olsen is a professor and extension potato specialist based at the University of Idaho’s Kimberly Research & Extension Center. Rabecka Hendricks is a graduate research assistant at the Kimberly R&E Center. Mike Thornton is a professor, extension potato specialist and chair of UI’s Parma R&E Center.