Evaluate methods to produce and handle containerised nursery trees

The aim of the project was to investigate the feasibility of growing apple nursery trees in bags in a sterile medium and potentially planting trees already in autumn rather than the normal spring planting in order to attenuate transplant shock and preclude the need to cold store trees.

‘Golden Delicious’ and ‘Big Bucks Gala’ on M9 rootstock were grown in bags in a shaded tunnel in a commercial nursery. Different treatments were evaluated to try and induce growth cessation and enhance hardening-off during autumn. Treatments to chemically defoliate trees in autumn, reduce reproductive bud formation and increase nitrogen reserve status were also evaluated. In addition, trials were done to compare autumn to spring planting of containerised trees in the orchard, evaluate cold storage versus chemical restbreaking and different planting methods in establishing containerised trees in the orchard.

Prohexadione calcium and abscisic acid showed no significant effect on growth cessation during the nursery phase, and no significant effect in spring bud break and new growth in the orchard. Trees planted in autumn showed earlier bud break whereas trees planted in spring had a higher bud break percentage. Foliar nitrogen (urea) during the nursery phase did not significantly affect spring bud break and new growth. At the rates applied, foliar copper, in comparison to 1-aminocyclopropane-1-carboxylic acid proved to be a more successful chemical defoliant in the nursery with no significant negative effect on the subsequent performance in the orchard. Dormancy management practises improved bud break in spring as well as the architecture of young trees planted in a commercial orchard. It is recommended that trees receive a six-week 6 °C cold storage period and a chemical rest breaking treatment to improve establishment. Different planting methods of containerised nursery trees did not significantly influence bud break in spring, but planting containerised trees with an undisturbed growing medium or only slightly loosening the growing medium before planting improved new lateral shoot growth and apical extension growth.

No clear conclusion could be made on the use of PGRs to harden-off trees or chemical defoliants in our trials. Trees planted during autumn, in a warm winter region, did not accumulate sufficient chilling, which resulted in reduced bud break during spring. This time of planting resulted in a basal dominant tree architecture. As trees were probably not managed ideally in the orchard, the treatment effects could have been masked by a lack of tree vigour during the first growing season in the orchard. A period of a six-week cold storage at 6 °C as well as a chemical rest breaking treatment are important dormancy management practises, regarding spring bud break and tree architecture, when trees are planted in warm regions. Because containerised nursery tree propagation is a new concept in South Africa, further research is needed on propagation and subsequent management of these trees