Altering Shoot Extension Did Not Affect Bitter Pit Incidence in 'Honeycrisp' Apple

Malus xdomestica 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 (R)) or stimulation via application of gibberellic acid (GA(3); ProGibb (R)) 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 GA(3) (low GA(3)); and 32 mg.L-1 GA(3) (high GA(3))]. 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 GA(3) 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 GA(3) 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.