Gibberellic Acid₄₊₇ and Benzyladenine, Cambium Disconnection, Nitrogen, and Tip Removal Influence on Branch Induction in Newly Planted Poorly Feathered “Fuji” Apple Trees

Abstract

The use of well-feathered nursery trees on dwarfing rootstocks is crucial for establishing high-density apple orchards to ensure early production. Training trees to develop a tall spindle canopy architecture has gained popularity in high-density orchards during the past decade. However, commercially produced nursery trees often lack feathers or are unevenly branched, making initial training decisions difficult and delaying precocious fruiting. In this project, we studied the vegetative meristem activation response to partial scoring or partial girdling of the bark cambium above the bud, with and without application of 500 ppm Promalin (gibberellin 4+7 + benzyladenine), tip removal of the tree leader at the silver tip stage on April 23, 2019, Promalin spray at 500 ppm at post-bloom stage on May 7, 2019, and nitrogen (N) as UAN-32 (32% N) application at five levels (60, 80, 100, 120, or 140 g actual N/tree) on June 20, 2019, in newly planted un-feathered or unevenly branched “ Aztec Fuji ” trees on Budagovsky 9 (Bud 9) in southwest Idaho, USA. Grower’s Short Cut trees received a heading cut to the leader at 60 cm above the ground and resulted in the emergence of 5 - 6 strong branches near the heading cut. These trees had larger trunk cross-sectional areas than scored or girdled trees. Scoring or girdling, with or without Promalin, significantly improved branching by increasing the number of branches and bourse shoots. In many cases, the effects of partial scoring, with or without Promalin, were similar to those of bark girdling which is a more risky and time-consuming process. Branches induced by scoring and girdling had sharper crotch angles relative to the vertical leader. Spraying Promalin during the post-bloom stage at 500 ppm was not as effective as scoring or girdling to initiate branches. Additional N rates or removal of the leader tip did not significantly impact branch induction.