‘Elstar’, the main apple cultivar grown in the Netherlands, requires adequate thinning to reach marketable fruit sizes and to achieve regular yields by preventing alternate bearing. At the moment, chemical thinning of ‘Elstar’ is the only economically feasible way of thinning. Thinning by hand is labour intensive and thus too expensive. Furthermore, hand thinning is usually carried out after June-drop, by which time only increases in fruit size are achieved and no reduction in alternate bearing. Up to 2004, carbaryl was the only effective fruit-thinning chemical Dutch growers had at their disposal. However, carbaryl is no longer officially registered for thinning apples. Thus, another agent for thinning ‘Elstar’ and other apple cultivars is urgently needed. This paper reports on two trials made with the thinning agent MaxCel containing 6-benzyladenine (BA) as the active ingredient. Generally, MaxCel applied at 10-12 mm fruit diameter thinned well, yielded bigger fruits and improved return bloom. However, thinning efficacy of MaxCel was negligible for weak growing trees or branches and increased with the vigour of the tree or branch. Adequate thinning and target fruit sizes were obtained with BA-concentrations in between 100 and 300 mg L-1 active ingredient. Results of thinning on pack-out and fruit quality are also presented.
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