Factors influencing branch production in young apple trees

In commercial apple (Malus domestica Borkh.) orchards, early cropping is a very important economic consideration. Orchard turnover is now occurring more rapidly through the release of new varieties and this demands the minimisation of plant establishment costs. It has been reported that young trees with branches produce crops much earlier than those with no branches. A series of experiments have been conducted to explore the factors involved in branch production, and to examine some methods of manipulating or inducing branches in nursery and one-year-old apple trees, particularly spur type 'Red Delicious'. The results presented confirm that spur type Red Delicious apple nursery trees produce very few branches when compared with Golden Delicious and Red Fuji, on MM106 rootstocks. Chemical treatments, of Cytolin (BA+GA₄₊₇) at 800 ppm (single application) and four sequential applications of 200 ppm, induced lateral shoot formation on nursery trees; but the sequential applications produced narrow branch crotch angles. A more detailed study showed that BA is the main effective ingredient of the mixture, when applied as a single spray. GA₄₊₇ alone induced branching when applied in 4 sequential applications at 200 ppm, but the lateral shoots had narrow crotch angles. The GA₄₊₇- induced branch crotch angles became wider, while the branches were growing. NAA at 10 ppm did not influence the Cytolin- or GA₄₊₇- induced branch crotch angle, when applied at different periods of growth. Cytolin was also applied to one-year-old apple orchard trees to induce lateral shoots, at lower concentrations than applied to the nursery trees. An optimum concentration range of Cytolin concentration of between 80 and 100 ppm is suggested. Concentrations of Cytolin were used up to 200 ppm without phytotoxicity symptoms. The higher concentrations resulted in excessive lateral shoot production but shoot length was reduced. Other growth regulators i.e. M&B 25,105, thidiazuron, or paclobutrazol were also tested on nursery trees, but they produced unwanted side effects without any significantly increase in the number of lateral shoots. Mechanical treatments i.e. the removal of young or mature leaves, lower lateral shoots or buds did not induce any increased lateral growth. The removal of leading shoot tips induced a cluster of lateral shoots, immediately below the apical node but these had narrow crotch angles. The results are discussed in terms of the possible relationships between the plant hormones and their influence on lateral shoot induction. The practical considerations required to fully utilise the spur type apple varieties' natural advantages in early cropping are also discussed