Managing Bourse Shoot Growth to Maximize Flowering in Apple

Consistent apple fruit production depends on regular flowering. Irregular flowering (biennial bearing) leads to challenges for apple industries around the world. Factors such as fruit weight, seed number per fruit and bourse length are thought to influence flowering and could contribute to biennial bearing. Bourse shoots are vegetative shoots that arise just below flowers on flowering spurs. Over the last two decades the literature suggests that longer bourse shoots have a higher probability of forming flowers, however in a preliminary study on 'Honeycrisp' we found that bourses longer than 25 cm rarely formed flowers. In this study we hypothesized that bourse length is negatively correlated with flower formation and the time of bourse shoot growth termination plays an important role in flower formation. In two cultivars, 'Honeycrisp' (biennial cultivar) and 'Gala' (annual cultivar) we measured bourse length throughout the season on fruiting and de-fruited spurs, and determined floral status of buds at the end of the growing season. We found that, in 'Honeycrisp', bourse length correlated negatively with flower formation but in 'Gala' there was no effect of bourse length on flower formation. The time of terminal bud set was not affected by fruiting, although 'Honeycrisp' growth terminated at 42 days after full bloom whereas 'Gala' bourse shoots grew slowly throughout the growing season. Therefore bourse length plays a role in determining flower formation in 'Honeycrisp' but not 'Gala' and future research should develop methods to manage bourse length to optimize flowering.