CHANGES IN FRUIT SET OF 'GALA' APPLE IN RESPONSE TO ENVIRONMENT AND ARTIFICIAL SPUR EXTINCTION

Abstract

In commercial ‘Gala’ apple orchards, natural bloom typically results in many more flowers and fruit than are required for high quality commercial crops. During flowering and early fruit development, strong competition for limited resources between florets or fruit within and among spurs induces their natural abscission, which affects fruit set. Removal of excess flowers and fruit routinely requires chemical and hand thinning. However, hand thinning is expensive, and chemical thinning can result in unpredictable outcomes because of climatic variation and differences in floral bud load among seasons, orchards, and trees within an orchard. Artificial spur extinction (ASE), a thinning method that manipulates the density and distribution of floral buds in trees, has been shown to improve the predictability of fruit set. A model to predict fruit set reliably would be a useful technology for commercial orchards. We investigated fruit set responses of ‘Gala’ over five regions and two growing seasons in New Zealand and Australia. The pattern of natural fruit set on spur and terminal bud types was affected by region more than by season. Among regions, the proportion of floral buds failing to set fruit varied from 10% to 60%, whereas within a region between seasons, this proportion varied by less than 10%. ASE applied shortly before budbreak removed all axillary buds and set ‘spur plus terminal’ bud densities to 3 or 5 buds.cm-2 basal branch cross-sectional area. As floral bud density was reduced, the proportion of buds that failed to set fruit declined and the proportion of buds setting multiple fruit increased. These responses contribute to the development of a predictive fruit set model for ‘Gala’, aimed at enabling accurate setting of potential crop loads before budbreak, using ASE as an alternative to conventional thinning practices.