Screening of eco-friendly thinning agents and adjusting mechanical thinning on 'Gala', 'Golden Delicious' and 'Fuji' apple trees

Fruit thinning is the most important yet difficult practice that drives orchard profitability. High labor costs and difficulty to improve return bloom by hand thinning have left chemical thinning as the main method used by growers. However, unpredictability and safety/environment concerns regarding chemical thinning have set mechanical thinning as a sound alternative. Thirteen field experiments were performed during 2004-2016 in order to evaluate several agents for their use as new thinners, and adjust mechanical thinning on 'Gala', 'Golden Delicious' and 'Fuji'. Olive oil applied at bloom reduced crop load, but russetting was also increased. Therefore, while their use is not advisable for russetting prone cultivars such as 'Golden Delicious', it could be a good thinner for cultivars like 'Red Delicious'. Lime sulfur did not have a consistent thinning effect in our study when applied at bloom. Overall, no differences regarding economic value between hand, chemical, and mechanical blossom thinning were observed, suggesting mechanical thinning as a valid alternative approach. For 'Gala' strains, 6 km h−1 and 250 rpm with 270 strings was the best configuration to provide an ideal crop load of ∼6 fruit/cm2 of TCSA and an average fruit size of 170 g. For 'Fuji', 5 km h−1 and 320 rpm with 270 strings provided a crop load in accordance to the optimum range for this cultivar in our conditions. However, combination of mechanical thinning plus chemical treatments might be the ideal strategy for 'Fuji' strains when the initial number of flower clusters per tree is above 500. For 'Golden Delicious' strains, 6 km h−1 and 230 rpm with 270 strings was the best configuration to provide an ideal crop load within the optimum range. Mechanical thinning timing was also examined at different phenological stages (E2, F1, F2, and G), with no significant differences regarding yield, fruit size or crop load between them. Two prediction models ('Gala' & 'Golden Delicious') were developed to adjust the right tractor and rotational speeds depending on the initial number of flower clusters. The method begins with first calculating the final fruit number needed per tree (crop load for each particular cultivar) in order to achieve the desired yield. Then, tractor and rotational speeds can be determined by the model once knowing the initial number of flower clusters per tree.