Improving fruit set of 'Kordia' and 'Regina' sweet cherry with AVG

Sweet cherry cultivars require different management strategies depending on whether they are prone to oversetting or have a tendency towards poor fruit set. The aim of this study was to investigate the efficacy of Retain® (15% a.i. (S)-trans-2- amino-4-[2-aminoethoxy]-3-butenoic acid hydrochloride, AVG) for improving fruit set on the difficult to set cultivars ‘Kordia’ and ‘Regina’ under Tasmanian conditions. Trials were established in commercial orchards of ‘Kordia’ in 2011/12 and ‘Regina’ in 2012/13. To evaluate the impact of application time, whole-tree applications of AVG were made separately by hydraulic hand lance at 30 and 80 percent bloom, or as a double application at 30 and 80 percent bloom. On each application date, AVG was applied at 75 or 112.5 g/ha to ‘Kordia’, and 75 or 150 g/ha to ‘Regina’. The surfactant Maxx™ was added to the spray solution at the label rate of 0.5 ml/L in ‘Kordia’, no surfactant was used in the ‘Regina’ orchard. AVG applications increased fruit set by 57 percent in ‘Kordia’ (from 9.7 to 15.3 %) and 33 percent in ‘Regina’ (from 23.1 to 30.7%), compared to untreated control trees. We found no significant differences between the two application rates, nor was there any effect of application timing. In the ‘Kordia’ orchard we recorded a significant (4.4%) reduction in fruit diameter in the AVG-treated trees. The double application also reduced size by 3.7% compared with a single application (P < 0.01). However fruit size was increased slightly (2%) by the lower, but not the higher, rate of AVG in the ‘Regina’ trial (P < 0.001). AVG reduced the incidence of rain-cracked fruit from 25% to 14% in the ‘Kordia’ orchard. Fruit firmness in ‘Kordia’ was significantly higher in AVG treated fruit than the control fruit (7%), but stem retention force was reduced by 16%. AVG had no effect on fruit color. AVG treated trees had significantly less starch in trunk tissue, from the time of pit hardening onwards, than control trees, implying that greater investment of tree carbohydrate reserves was associated with the relatively high fruit set. This work has demonstrated that AVG may be useful for improving fruit set (yield) in sweet cherry cultivars that are chronically low-yielding. Further work is needed to substantiate these results in different production regions and clarify the effect of AVG on fruit firmness.