Alternative Approaches to Chemical Thinning for Regulating Crop Load and Alternate Bearing in Apple

In the past, chemical thinning dominated in fruit orchards. This paper for the special issue outlines alternatives to chemical thinning for crop load management (CLM) and its effect on fruit size, firmness, sugar, starch, and weight, indicating ripeness and fruit quality, yield, and alternate bearing. A total of 450 apple trees (Malus domestica Borkh., cv. ‘Roter Boskoop’; six years old) on M9 rootstock were used at the Klein-Altendorf experimental station (50° N) of the University of Bonn, Germany. As the first alternatives, trees were mechanically blossom-thinned at the balloon stage (BBCH 59) with a rotor speed of 320 rpm or 380 rpm at 5 km/h tractor speed or were chemically thinned at the full bloom stage (BBCH 65) with ammonium thiosulfate (ATS), ethephon (ETH), and/or 6-benzyladenine (BA) at 10–12 mm fruit size (BBCH 71) after applying ATS/ETH. Flower clusters and/or cluster leaves (source) were manually removed to determine the optimum sink-source ratio to achieve different ratios of fruitlets (sink) relative to the leaves (source) at fruit set (BBCH 67–69). Un-thinned, adjacent trees served as the control. The majority of CLM methods improved fruit size and weight. Removing cluster leaves at fruit set increased fruit size and weight of the remaining fruit, which has not been observed before. The most effective treatment for fruit size and weight and return bloom was the 75% flower cluster and complete cluster leaf removal. Removal of more than 50% of flower clusters successfully improved return bloom, indicative of alternate bearing. The mechanical blossom thinning had a positive effect on fruit size and weight with a return bloom similar to that of removal of 50% flower clusters.