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Effect of Crop Load Management by Blossom Thinning or cluster Leaf Removal on Apple Quality and Alternate Bearing

Prud Netsawang, Peter Schulze Lammers, Achim Kunz, Michael Blanke

The influence of Crop Load Management (CLM) on fruit quality, yield, and alternate bearing was assessed by comparing three methods of modifying the source: sink relationship: a) Mechanical, b) Chemical and c) Manual thinning. A total of 450 apple trees (Malus domestica Borkh., cv. ‘Roter Boskoop’; six years old) on M9 rootstock at the Klein-Altendorf field laboratory (50°N) of the university of Bonn, Germany were used. 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 full bloom stage (BBCH 65) with Ammonium Thiosulfate (ATS), Ethephon (ETH), and/or 6-Benzyl Adenine (BA) at 10-12 mm fruit size (BBCH 71) after applying ATS/ETH. Flower clusters and/or cluster leaves 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 quality in terms of fruit weight and size. 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 quality and return bloom improvement was the 75% flower cluster and complete cluster leaf removal. Removal of more than 50% of flower clusters successfully improved return bloom. The mechanical blossom thinning had a positive effect on fruit quality with a return bloom similar to that of removal of 50% flower clusters.