Effects of Growth Regulator Sprays and Notching on Growth and Branching of ‘Starkrimson Delicious’ Apple Trees

Abstract A block of 2-year-old ‘Starkrimson Delicious’ apple ( Malus domestica Borkh.) trees growing in Somerville, Victoria, Australia were treated with 0 to 500 ppm BA when terminal growth was 2 cm. BA application initially increased the rate of lateral shoot growth. However, the growth rate rapidly slowed and extension growth on the lateral shoots stopped earlier than on untreated trees. There was a linear increase in the number of lateral shoots and a linear reduction in the length of these lateral shoots as the concentration of BA was increased. BA forced more lateral shoots to grow at the tip of the branch than at the base, although shoots at the base of the limbs grew longer. A reduced concentration of BA was required to stimulate spurs to grow out into lateral shoots on the vigorous, more upright central leader. Lateral shoots were longer on the central leader than on the scaffold branches, indicating tree and/or branch vigor may be a major factor in producing good scaffold limb development following BA application. Notching forced more spurs to form lateral shoots on the central leader, and these shoots grew longer than those on the scaffold limbs. Notching increased the number of buds developing into lateral shoots following BA application. May and Baker 25-105 increased the number of lateral shoots on a tree. This result occurred both on untreated trees and on those that previously received 400 ppm BA. Reducing the number of lateral shoots by pinching apices of shoots soon after BA application was the most effective method to increase the length of BA-induced lateral shoots. Chemical names used: N -(phenylmethyl)-1 H -purine-6-amine (BA) (2-chloroethyl)phosphonic acid (ethephon), butanedioic acid mono(2,2-diemthylhydrazide) (daminozide), propyl 3- t -butylphenoxyacetate (May and Baker 25-105).