Localised and tree total crop loads influence trunk growth, return fruit set, yield, and fruit quality in apples

Abstract: Localised fruit thinning strategies must be investigated to improve precision crop load management in narrow-canopy, multileader apple trees. This study aimed to determine the effects of within-leader and tree total crop load on leaders’ and trunk’s growth, fruit set, yield, and fruit quality in ‘Ruby Matilda’ apples (marketed as Pink Lady®) over three years. Different crop loads were imposed on two leaders (primary and secondary) of bi-axis trees. Leader and trunk relative growth rate, return fruit set, yield, and fruit quality parameters at harvest were measured. High within-leader crop loads led to a significant increase in yield and reductions in trunk growth, return fruit set, and deterioration of fruit quality parameters except for flesh firmness and starch index. Similar trends were observed in whole-tree relationships. High crop load in secondary leaders had moderate negative effects on trunk growth, yield, and fruit mass of primary leaders; it only marginally affected their return fruit set and had no significant effect (p > 0.05) on their fruit quality. A crop load of 6.8 fruit no. cm−2 of leader cross-sectional area was estimated to achieve a relatively consistent return fruit set within the same leader. At a whole-tree level, a similar crop load (6.9 fruit no. cm−2 of trunk cross-sectional area) produced a consistent return fruit set despite its higher variability. These crop loads produced high yields (120 and 111 t ha−1, respectively) and good quality fruit. Using individual leaders as management units is recommended to simplify operations and reduce variability.