Jun 14, 2017
Crop load, better nutrient management curb bitter pit

Multiple years of studies in Pennsylvania grower orchards are providing additional clues for managing Honeycrisp in the orchard to prevent bitter pit following storage.

Research led by Penn State University Extension Educator Tara Baugher, Professor of Horticulture Rich Marini and Professor of Pomology James Schupp should help growers attack bitter pit. It begins showing up on Honeycrisp several weeks before harvest and the incidence can increase two- to fourfold after a month in storage.

During multiple years of research on bitter pit in commercial Honeycrisp orchards, incidence was associated with low calcium levels in fruit peel; high ratios of nitrogen, potassium, and/or magnesium to calcium in fruit peel; excessive terminal shoot length; and low crop load.

Heat and water stress predisposed Honeycrisp to bitter pit in 2016, and incidence was also highly correlated to excessive fruit size, the research indicated. Total actual calcium applied per season was inversely related to bitter pit, with the best suppression of bitter pit being with at least 8 pounds and the source being calcium chloride.

Key findings

  • Fruit calcium levels of 0.04 percent to 0.05 percent were associated with the lowest bitter pit levels in Honeycrisp. Whereas most apple varieties have fruit calcium levels of 0.05 percent to 0.06 percent, Honeycrisp often only had a fruit calcium level of 0.03 percent. Growers who achieved the desired fruit calcium level had developed a season- long foliar calcium program that totaled at least 8 to 13 pounds of actual calcium. Growers will find a useful tool for making decisions regarding calcium materials and rates at the Penn State Extension Tree Fruit Production website, www.psu.edu.
  • The ratio of Mg+K+N to Ca in fruit was most strongly correlated to bitter pit incidence, and explained 70 percent of the occurrence (probability level of 99 percent), researchers stated. This finding indicates that growers should avoid or minimize sprays of magnesium, potassium and nitrogen on Honeycrisp, which is in agreement with recent Cornell University recommendations for the variety.
  • Calcium levels in the fruit were higher, and bitter pit was reduced, when average terminal shoot length was 10 to 15 inches. The commercial blocks of Honeycrisp with historically high levels of bitter pit were overly vigorous, with shoot length over 20 inches. Whereas growers find that Honeycrisp trees often need to be pruned more severely the first several years following planting in order to encourage growth, it is important to develop a more balanced approach to pruning mature trees, the researchers determined.
  • Crop load levels of four to five fruit per square centimeter trunk cross-sectional area were associated with reduced bitter pit levels. Thinning Honeycrisp to an optimum crop load often involves chemical thinning followed by careful follow-up hand thinning. Tom Kon and Jim Schupp’s research with an equilifruit apple thinning gauge has shown that it is more accurate in adjusting crop load than spacing fruit a certain distance apart and often results in leaving more fruit per tree.

Investigated were the orchard experimental design, fruit sampling and determining fruit nutrient levels.

Three trees each with high, medium, and low crop loads were tagged in each orchard plot for measurements of crop load, fruit size, shoot length, fruit nutrient levels and bitter pit incidence. Peel slices for nutrient analysis were taken from around the apple circumference, three centimeters from the calyx. The peels were air dried for two weeks and then ground with a hand- held coffee grinder. Nutrient analyses were conducted by the Penn State Agricultural Services Analytical Lab.

Bitter pit was reduced when nitrogen, potassium and magnesium were maintained at low levels relative to calcium. Source: Penn State University

Multiple years’ findings

Regression analyses indicated bitter pit was very highly correlated to the ratios of N/Ca, K/Ca, Mg/Ca, (K+Mg)/Ca, (N+K+Mg)/Ca and ((N+K+Mg)/Ca)-38 and inversely correlated to the level of Ca.

Bitter pit incidence increased with increasing shoot length and decreased with decreasing crop load. During 2015, bitter pit incidence was significantly different at each harvest timing, with apples harvested too green having 57 percent more bitter pit than fruit harvested at the proper stage of maturity, researchers found.

Growers applied their preferred calcium products and reported the source of calcium and number of sprays. Total elemental calcium applied per season was inversely related to bitter pit, with the best suppression of bitter pit being with 8 to 13 pounds (source – calcium chloride).

In all years of the research, bitter pit showed up in the first month of storage and was not progressive. One of the original reasons for setting up the research was to test the accumulated ratio ((N+K+Mg)/Ca)-38) used by Washington growers to predict bitter pit.

“Now that we have three years of data with uniform plot designs, we have an opportunity to better adapt this tool for Pennsylvania growers,” Baugher said. Fruit from grower cooperators was also sent to Chris Watkins’ postharvest laboratory in Geneva, New York, and the results of this study are reported at “Postharvest Practices to Manage Storage Disorders in Honeycrisp” on the PSU Extension website.

“In several associated studies, we evaluated the effect of harvest maturity on bitter pit development, and bitter pit was increased by 55 percent to 60 percent when fruit were harvested with background color that was too green,” Baugher said.

Based on data from this Honeycrisp research and fruit quality statistics from Pennsylvania packinghouses, researchers developed calculators to assist growers in making economic decisions regarding number of harvests and whether or not to stem-clip, and these are available at the Tree Fruit Production website, http:// extension.psu.edu/plants/tree-fruit.

— Gary Pullano, managing editor


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