Potato Progress

Volume 22 Number 3

Practices associated with blackspot bruise susceptibility

Mike Thornton¹, Nora Olsen¹, Jeff Miller², Aymeric Goyer³, Ruijun Qin³, and Kenneth Frost³

¹University of Idaho; ²Miller Research; ³Oregon State University

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Bruise incidence can have a big influence on the value of potatoes in both fresh and process channels due to incentives for bruise free and the losses associated with rejection for not meeting grade standards.  It can be very frustrating to growers to see bruise levels fluctuate from year to year, field to field, and sometimes even day to day during the harvest season. Much variation in bruise incidence can be traced to two factors – impact severity and pulp temperature.  Research has shown that the greater the force of an impact, the more likely it is that bruise will occur, and that shatter bruise will develop over blackspot bruise. Pulp temperature influences the physical properties of tuber cells by changing how much they deform during impacts and the amount of impact energy that goes into rupturing the cell contents or tearing the cells apart. That is why bruise incidence greatly increases at temperatures below 50^oF, and shatter bruise tends to predominate in colder conditions. In addition, cell chemistry determines how extensively the injured cells form the visible pigment associated with blackspot bruise. Together, the physical and chemical properties of cells determine the susceptibility of a potato to bruise injury.

‍Why study vine maturity and nitrogen management?

It is impossible to avoid all impacts given the amount of equipment potatoes are exposed to as they move from field to storage and from storage to end use. Likewise, the limited number of days to get the crop harvested in the fall doesn’t always allow for operating when potato pulp temperatures are within the recommended range of 45 - 65^oF. Therefore, we need to understand which management practices influence how susceptible potatoes are to bruising.  The extent of vine maturity at the time of vine kill or harvest is one of the factors that has previously been shown to be related to bruise susceptibility. Corsini et al. (1999) reported that potatoes from commercial fields in Idaho that exhibited greater vine senescence in late August were more susceptible to blackspot bruise compared to fields that were still relatively green (Figure 1). They also reported vine maturity influenced blackspot susceptibility of Ranger Russet more than Russet Burbank. Prior to that study, most of the research on bruise susceptibility conducted in the Northwest had focused on Russet Burbank, which raises questions about how some of the newer varieties like Umatilla Russet and Clearwater Russet bruise as a function of vine maturity. Also, fields rarely senesce at a uniform rate. Could some of the extreme incidence and severity of blackspot bruise we occasionally see be associated with areas of fields where vines senesce early compared to the rest of the field?

‍The research outlined above suggests that delaying vine maturity might be a useful strategy for reducing a crop’s susceptibility to blackspot bruise. One of the most effective ways to delay vine maturity is to increase seasonal nitrogen (N) applications. However, past research has shown a complicated interaction between N management and bruise susceptibility. For example, Corsini et al. (1999) reported a direct relationship between high late season vine N content and increased bruise susceptibility of Russet Burbank in Idaho. They pointed out that many fields with high vine N concentration (above 15,000 ppm petiole nitrate in early August) were being managed with extra in-season N applications to combat early dying but were still senescing fast at the end of the growing season. This is the opposite of what would be expected if delayed vine maturity is the key to increasing bruise resistance. Nitrogen applications have also been reported to increase the total amino acid content of potatoes including tyrosine, the substrate oxidized to the black pigment after bruising (Hoff et al., 1971). Therefore, it is possible any reduction in cells’ physical response to an impact associated with delayed maturity is being offset by increased chemical potential for blackening. We felt it was worthwhile to revisit in-season N management effects on bruise susceptibility to try to resolve these conflicting reports.

‍Is there a relationship between within-field variation in plant maturity and blackspot bruise susceptibility?  

Tuber samples were collected from 20 locations in commercial fields in Idaho and Oregon during both the 2020 and 2021 growing seasons. Fields were selected to represent a range of commonly grown varieties and included one field of Russet Burbank, Ranger Russet, Umatilla Russet, and Clearwater Russet each year. Vine maturity was visually rated at each in-field location prior to hand-harvesting 20 tubers for controlled impact tests using the method of Olsen et al. (2019).  The data set over the two years consisted of bruise evaluations on 3200 individual tubers (8 fields x 20 locations per field x 20 tubers per location). Samples were transported to Kimberly, then held at a constant temperature of 55^oF for a few days to standardize pulp temperature during impact tests.  Three additional tubers from each field location were assessed for mineral (focusing specifically on N) and tyrosine content and biochemical blackening potential using established methods (Goyer and Pelle, 2017). Each factor was evaluated for correlation with bruise susceptibility.

‍Most of the fields were exhibiting large variation in visual vine senescence when the tuber samples were collected in early September. The controlled impact procedure resulted in moderate-to-high blackspot bruise incidence and lower levels of shatter bruise. Therefore, we focused on the relationships among vine senescence, tuber N content, tuber blackening potential, and tuber tyrosine content with blackspot incidence and severity, and bruise depth.

‍Several measured parameters were significantly correlated with blackspot bruise susceptibility, but the relationships were not consistent among the eight fields. For example, blackspot severity was negatively correlated with vine senescence in Ranger Russet, but this relationship was positive in Clearwater Russet and no relationship was observed in Umatilla Russet or Russet Burbank in 2021 (Figure 2). These results seem to conflict with what was reported for commercial fields of Russet Burbank and Ranger Russet in Idaho by Corsini et al. in their 1999 paper.  However, that study relied on a visual evaluation of whole-field maturity, whereas the present study focused on the maturity of individual spots within a field. Soil moisture may also be important here -- Corsini et al. (1999) found that the most mature fields tended to have the lowest soil moisture content, indicating that tubers were potentially dehydrated to the point that blackspot susceptibility increased.

‍Another potential difference between this study and Corsini et al. (1999) is the time of the season that tubers were sampled.  We collected tubers for bruising on the same day the visual senescence ratings were taken, which was early to mid-September in both years.  It is possible that leaving the tubers for longer periods under dead vines would result in more dramatic differences in bruise susceptibility than what was measured in this study.

‍The most consistent relationships were found between tuber N content and tyrosine content. We found in all eight fields across four different varieties tyrosine increased as tuber N content increased (significant correlation in 5 locations). Tyrosine also increased in seven of the eight locations as plant senescence increased (significant correlation in 3 locations). Our results indicate that either tuber N content or tyrosine might be useful as an indirect indicator of bruise susceptibility. However, neither tyrosine content nor N content was consistently related to any of the gauges of bruise susceptibility we measured in our controlled impact tests. In fact, the relationship between tyrosine content and bruise incidence, severity, and depth were weakly negative in both years. This is opposite to expectations if tyrosine was limiting the extent of darkening after a damaging impact. The role of tyrosine is further questioned when looking at the relatively low levels of tyrosine in tubers of Clearwater Russet and Umatilla Russet compared to Russet Burbank and Ranger Russet (Table 1). Both Clearwater Russet and Umatilla Russet expressed high bruise incidence and moderately severe symptoms in both years of the study, despite having low levels of tuber tyrosine. This raises the possibility that Clearwater and Umatilla are more susceptible to cellular damage which makes them susceptible to blackspot bruise despite having low tuber tyrosine content.

‍Does in-season N fertilization affect blackspot bruise susceptibility?  

Field trials were established in 2020 and 2021 with Russet Burbank grown at Miller Research in Acequia, Idaho and Russet Norkotah grown at the Hermiston Agricultural Research and Extension Center in Oregon to evaluate the impact of in-season N fertilizer rates on plant maturity at harvest. Plots were 4 rows wide by 25 to 30 feet long replicated four times. In-season nitrogen was applied weekly starting in mid-to-late June and ending in early August. Petiole samples were collected the first week of August and analyzed for nitrate-N. Tubers from the center two rows of each plot were collected at crop maturity to determine N and tyrosine content, biochemical blackening potential, and blackspot bruise susceptibility using the same methods outlined for the vine maturity study. Based on previous research reports, we expected the highest N application programs to result in higher petiole nitrate levels in early August. Research is less clear about whether some of the excess nitrogen in the vines is transferred to tubers where it could be stored in the form of tyrosine. High tyrosine levels have previously been reported to be associated with differences in susceptibility to blackspot bruise among potato varieties (Corsini et al., 1992).

‍The Russet Burbank trial in Idaho exhibited very high petiole nitrate levels in early August (> 15,000 ppm) under the 150 and 200% in-season nitrogen programs (Table 2). There was relatively little vine senescence in any treatment by late August in 2021 (1 week before vine kill), however, in 2020 the vines were more senesced at the end of the season. At high fertilizer rates there was a trend for potato plants to produce tubers with high tuber N contents in both years (Table 2). In contrast, there was no evidence that higher tuber N content reflected an increase in tyrosine content as there were no significant differences among the four N rates. The lack of influence of in-season N applications on tyrosine content was consistent across both 2020 and 2021. Likewise, no bruise susceptibility parameters were significantly affected by in-season N rates (Table 2).

‍In the Russet Norkotah trial in Oregon the in-season N treatments exhibited relatively small differences in petiole nitrate levels in the first week of August, and all nitrogen programs exhibited levels above 15,000 ppm (Table 3). In contrast to the Idaho trial, the higher plant N content was not associated with reduced vine senescence in late August. In-season fertilizer rate had no effect on tuber N content in Russet Norkotah in 2021 but resulted in a small increase in 2020 (Table 3). Tyrosine content of Russet Norkotah tubers was relatively high (455 to 1452 ug/g DW), but there was no effect of in-season N rate on tyrosine content or bruise susceptibility. This is further evidence of the lack of relationship between tuber tyrosine content and bruise susceptibility as Russet Norkotah had lower blackspot severity, depth, and incidence despite having double the amount of tyrosine in tubers compared to Russet Burbank.

‍Does this mean that vine maturity and nitrogen management are not important?

As with many aspects of potato production, the relationship between plant maturity and bruise susceptibility is very complicated. Both the physical and chemical properties of tubers change as the plant matures. Skin set, dry matter content, and tyrosine content are changing relatively quickly at the end of the season. Additionally, pulp temperatures tend to decline with air temperatures as we get into September and October.  Together these changes indicate that there are probably no easy answers to questions about the level of plant senescence and bruise susceptibility. However, there are good reasons to pay attention to the relative health of fields at the end of the season. Previous research by Rick Knowles at WSU has shown there is a decline in processing quality due to sugar problems in over-mature potatoes. The decline in quality occurs as tubers sit under dead vines where they are exposed to fluctuations in soil temperature. This problem would be most apparent during seasons with warm fall temperatures.  We have also shown in past research in Idaho that potatoes sitting under dead vines for long periods tend to have a shorter dormancy period and sprout quicker in storage (Thornton and Olsen, 2016).

‍Even though we did not see significant effects of in-season N management on bruise susceptibility, there is no doubt that over-fertilizing can impact quality. Specific gravity, skin development (i.e., level of russeting), and skin set can all be negatively impacted by high rates of N fertilizer. High N applications not only affect visual appearance and processing quality, but also have implications for storage losses as the skin is an important barrier against diseases such as Fusarium dry rot, Pythium leak, and pink rot.

‍Summary

• Tuber tyrosine content tended to increase with visual vine senescence ratings (significant correlation in 3 of 8 commercial fields).  This indicates tubers from the earliest senescing portions of fields might have higher potential to produce more pronounced blackspots following an impact.
• However, vine senescence level within a field was not a good indicator of bruise susceptibility. Likewise, potato varieties such as Clearwater Russet and Umatilla Russet that have relatively low tuber tyrosine content tended to be equally susceptible to blackspot bruise as varieties with higher tyrosine content.  This indicates that tyrosine is not always the limiting factor that determines blackspot susceptibility.
• Nitrogen management did not significantly affect tuber tyrosine content and tubers harvested from crops managed with very high rates of in-season nitrogen (up to double the recommended rate) showed no evidence of being more susceptible to blackspot bruise compared to the recommended nitrogen program.

‍Literature Cited

Corsini, D., J.J. Pavek and B. Dean. 1992. Differences in free and protein-bound tyrosine among potato genotypes and the relationship to internal blackspot resistance.  American Potato Journal 69:423-435.

Corsini, D., J. Stark and M. Thornton. 1999. Factors contributing to the blackspot bruise potential of Idaho potato fields.  American Journal of Potato Research 76:221-226.

Goyer, A. and J. Pelle. 2017. Relationships between tyrosine, phenylalanine, chlorogenic acid, and ascorbic acid concentrations and blackspot biochemical potential and blackspot susceptibility in stored russet potatoes. Journal of the Science of Food and Agriculture 98:3735-3740.

Hoff, J.E., C.M. Jones, G.E. Wilcox, and M. D. Castro. 1971. The effect of nitrogen fertilization on the composition of the free amino acid pool of potato tubers. American Potato Journal 48:390-394 

Olsen, N., M. Thornton and J. Miller. 2019.  Quality evaluation and maintenance of fresh Idaho potatoes. Progress report to the Idaho Potato Commission, 36pp.

Thornton, M. and N. Olsen. 2016. Bracing for another hot summer. Potato Progress. Volume XVI, number 5.

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