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Control bitter pit in Honeycrisp apples

By DOROTHY NOBLE
AFP Correspondent

SMITHSBURG, Md. (July 11, 2017) — At the twilight meeting in early June at Barr Orchards, Penn State and University of Maryland researchers and Extension educators supplemented their presentations with materials on how to prevent bitter pit in Honeycrisp apples.
The variety ‘Honeycrisp’ was the focus because of its demand in the marketplace plus its production challenges.
Bitter pit can show up several weeks before harvest and increase two- to four-fold after a month in storage. While starting internally, the disorder causes unsightly external blemishes. Fortunately, multiple-year orchard studies are providing clues for management.
Crop load and nutrient management have resulted in improvements.
Low calcium levels in fruit peel; high nitrogen, potassium, and/or magnesium to calcium ratios; excessive terminal shoot length; and low crop load have all been associated with the incidence of bitter pit.
In 2016, heat and water stress predisposed the problem. Also excessive fruit size was highly correlated to incidence.
However, total actual calcium applied per season was inversely related to bitter pit, with the best suppression applying at least eight pounds per acre, with calcium chloride the source. Growers should begin with the first cover spray of calcium chloride and continue until just before harvest.
Most growers spray approximately every two weeks. However, with Honeycrisp, the growers who have achieved some of the best results apply a special calcium chloride spray weekly. Research revealed that fruit calcium levels of 0.04 percent to 0.05 percent had the lowest amount of bitter pit in Honeycrisp.
Those growers had developed a season-long foliar calcium program that totaled at least eight to 13 pounds per acre of actual calcium.
Also, the Mg+K+N to Ca in fruit was strongly correlated to the incidence of bitter pit.
That explained 70 percent of the occurrences.
As a result to this finding, growers should avoid or minimize sprays of magnesium, potassium and nitrogen on Honeycrisp.
This agrees with recent Cornell recommendations.
The research showed that calcium fruit levels were higher and bitter pit was reduced when average terminal shoot length was 10 to 15 inches. High levels of bitter bit historically have been from overly vigorous trees — shoot length has been over 20 inches.
Although growers have found that Honeycrisp trees often need more severe pruning the first several years to encourage growth, a more balanced approach for mature tree pruning should mitigate bitter bit occurrences.
The studies also revealed that bitter pit was reduced with crop load levels of four or give fruit per cm² trunk cross-sectional area.
Hence, that finding indicates that thinning Honeycrisp to an optimum crop load often involves chemical thinning followed by careful follow-up hand thinning.
In addition, the equilifruit apple thinning gauge has shown more accuracy in adjusting the crop load than spacing fruit. Plus, it can leave more fruit per tree.
In using the equilifruit disc, Penn State pomology professor James Schupp instructs growers to start at the lowest limb and work to the top. Place the disc about 3 cm from the trunk.
Find the notch that fits snugly, and refer to the F-value that corresponds. That is the number of fruit that should remain after hand thinning.
Fruit to be removed should first be the injured or damaged fruit, then small fruit, and finally those fruit necessary to break up clusters.
Penn State’s “Fruit Times” presents the latest research with links to thinning and bitter pit reduction. It is accessible at http://extension.psu.edu/plants/tree-fruit/news.