Malus ×domestica Borkh. cv. Honeycrisp has been widely planted in North America during the past two decades. However, it is susceptible to many disorders that result in high postharvest losses. Excessive vegetative vigor in apple trees can reduce fruit calcium (Ca) concentrations and increase bitter pit incidence in apple fruit. Plant growth regulators are used routinely in tree fruit orchards to control vegetative growth to increase light penetration into the canopy. The objective of this study was to determine whether shoot growth inhibition using the application of prohexadione-calcium (P-Ca; Apogee®) or stimulation via application of gibberellic acid (GA3; ProGibb®) affected bitter pit incidence in ‘Honeycrisp’ apple. In 2016 and 2017, the experiment was conducted in a commercial ‘Honeycrisp’ orchard with five treatments [untreated control, 62.5 mg·L–1 P-Ca (low P-Ca); 125 mg·L–1 P-Ca (high P-Ca); 16 mg·L–1 GA3 (low GA3); and 32 mg·L–1 GA3 (high GA3)]. Treatments were applied twice during the growing season. Shoot length and the number of internodes for new growth were measured 4 weeks apart after treatment. Overall yield and fruit quality were assessed at harvest, and bitter pit incidence was assessed after 4 months of storage. Low and high P-Ca rates limited shoot growth extension; high GA3 increased shoot extension compared with the untreated control. However, the number of internodes did not change substantially for each shoot. The number of internodes is one of the primary factors affecting leaf area and, consequently, the transpiration balance between fruit and leaves. In both years, treatments with either GA3 or P-Ca did not affect fruit elemental concentration or bitter pit incidence. These results indicate that growth-inhibiting plant growth regulators that reduce shoot extension may not be useful for managing bitter pit incidence in ‘Honeycrisp’ apple.
Corina Serban and Lee Kalcsits
Corina Serban, Lee Kalcsits, Jennifer DeEll and James P. Mattheis
‘Honeycrisp’ apples are susceptible to bitter pit, a physiological disorder that impacts peel and adjacent cortex tissue. ‘Honeycrisp’ is also susceptible to chilling injury (CI) that can be prevented by holding fruit at 10 to 20 °C after harvest for up to 7 days. This temperature conditioning period reduces CI risk but can enhance bitter pit development. Previous research demonstrated a controlled atmosphere (CA) established during conditioning can reduce ‘Honeycrisp’ bitter pit development without inducing other physiological disorders. The objective of this research was to evaluate the duration of CA needed to reduce bitter pit development. Experiments were conducted in 2014, 2016, and 2017 with fruit obtained from commercial orchards in Washington State and, in 2017 only, Ontario, Canada. Half the fruit were treated with 42 µmol·L−1 1-methycyclopropene (1-MCP) for 24 hours at 10 °C immediately following harvest. The untreated fruit were held at the same temperature (10 °C) in a different cold room. Following 1-MCP treatment, all fruit were conditioned at 10 °C for an additional 6 days, then fruit was cooled to 2.8 °C. During conditioning, fruit were held in air or CA (2.5 kPa O2, 0.5 kPa CO2) established 1 day after harvest, for 1 to 8 weeks, then in air. All fruit were removed from cold storage after 4 months and then held 7 days at 20 °C. Fruit from most orchards/years stored in CA developed less bitter pit compared with fruit stored continuously in air. CA during conditioning also reduced poststorage peel greasiness but CA for 2 weeks or longer enhanced cortex cavity development in some orchard lots. Treatment with 1-MCP did not reduce bitter pit but enhanced development of peel leather blotch and core browning for some orchards/years. 1-MCP–treated fruit slowed the loss of soluble solids content, titratable acidity, and reduced internal ethylene concentration. Results suggest the potential for postharvest management of bitter pit development in ‘Honeycrisp’ apples by CA established during conditioning with minimal development of other postharvest disorders.