Growers, does your sense of urgency rise as harvest time approaches? Sense of urgency can be thought of as stress driving performance timing and quality. Some of that stress is relieved by setting potato vine kill date related to desirable harvest date. It’s not necessarily a case of “set it and forget it,” but, in general, potato harvest occurs when tuber skin is set, which usually occurs two to three weeks after vine kill product application. Of course, the date for vine kill is also scheduled depending on the crop’s end use, speed of desiccation needed, and specific variety characteristics. Other needs sometimes taken into account include late-season weed control and a desire to kill in order to prevent disease translocation from foliage and stems to tubers.
University of Idaho research has shown that currently available products can provide close to 100 percent overall vine kill, measured as leaf and stem kill, as soon as three days (sulfuric acid), one week (diquat—Reglone and others), or two or more weeks after application (glufosinate-ammonium—Rely and others; carfentrazone—Aim; or pyraflufen-ethyl—Vida).
What about blackspot bruising and stem end discoloration (SED)? Shouldn’t choice of vine-kill date also depend on the impact of vine-kill speed on tuber susceptibility to blackspot bruising and SED, and whether vine-kill speed should be different for immature vs. mature tubers?
Tissue of more mature tubers is lower in sucrose and higher in starch and tyrosine than tissue of immature tubers. In layman’s terms, sugars in the tuber convert to starch over time as the tuber matures. University of Idaho research conducted by Mike Thornton and Nora Olsen revealed higher black pigmentation and dark bruising when tuber starch/tyrosine is relatively high compared to sucrose level. In contrast, when tuber sucrose is high and starch is low, SED is higher than when the opposite occurs. In other words, mature tubers are more susceptible to bruising, and immature tubers are more susceptible to SED. Talk about being caught between a digger and a cellar floor!
But wait…There’s that harvest = skin set = two to three weeks after vine kill. Fields are harvested according to a schedule, but tuber maturity can differ from field to field and as the harvest season progresses. Perhaps vine-kill speed on immature tubers impacts bruise and SED differently than vine-kill speed on more mature. It is likely, however, that the same vine-kill product is being used in all of a given grower’s fields.
Answers
An interaction between vine-kill speed and vine/tuber maturity at time of application occurred in a preliminary University of Idaho trial conducted on Russet Burbanks. Fast (sulfuric acid vs. medium Reglone vs. slow Vida) vine-kill products were applied to vines with less than 5 percent natural senescence or more mature vines with 40 percent or greater natural senescence. Application at either timing was made mid-day. Harvest for each treatment occurred two weeks after overall desiccation reached 95 percent. Treatment harvest was not always on the same date. A non-treated control was harvested at each treatment harvest date. Blackspot bruising was assessed.
Vine-kill Speed
The number of days it took to reach 95 percent overall vine-kill after application of sulfuric acid, Reglone and Vida at the early-timing was 17, 22 and 36, respectively. The number of days to 95 percent vine kill of 40 percent senesced vines was more similar: 21, 21 and 27, respectively.
Vine-kill speed by Reglone was similar with the two timings, while sulfuric acid slowed slightly, and Vida was quicker when vines were mature than when vines were immature at application time.
Other studies have shown that desiccation of mature vines with vine-kill products is usually faster than when the same are applied to immature vines.
Blackspot Bruise
As expected, since mature tubers have more starch than immature tubers, harvest after vine-kill products were applied to mature plants resulted in more tubers with bruising than when harvest occurred after these products were applied to immature plants.
Proposed future research will be conducted with the varied vine-kill speed products and tuber maturity timing, and will also include such factors as sugar levels in tubers at application time and harvest. SED occurrence will be assessed.
Meanwhile, impact of tuber maturity in a given field at vine-kill time on blackspot bruising should be kept in mind when choosing the most appropriate vine kill product and timing for that field.
Study results will be eventually combined with those from other trials looking at the impact of factors such as soil moisture level and other environmental stresses at vine kill and harvest time on tuber quality. The outcome will be a more complete, overarching set of vine kill recommendations.
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