Mechanical selective removal of flowers in a fruit tree canopy

Abstract

Thinning is a pre-requisite in fruit crops in order to overcome or prevent alternate bearing and to provide regular yields of high quality in terms of size, colour and fruit constituents. A novel device was developed to achieve a wide variety of three-dimensional thinning motions, which consisted of three horizontal rotors with rotating tines. The number and position of tines was varied, while the rotor number, speed, angle and vehicle velocity were kept constant (380 rpm at 5 km h(-1)). The objective was to detach or reduce flowers not evenly within a tree, but selectively, i.e., remove more inner canopy flowers towards the tree trunk, which would otherwise develop into small, firm, green and unripe fruit. To achieve this selective thinning, i.e., to remove particularly those inner flowers, tines for the outer canopy were stepwise detached. Six-year-old apple cultivar 'Pinova' trees on M9 rootstock at 3.5×1 m spacing at Klein-Altendorf near Bonn, Germany were mechanically blossom-thinned in April. Overall, the success of the selective thinning depended on the branch angle; branches facing the thinning device were more strongly thinned. Detachment of tines along the tree branches led to selective thinning in that flowers were progressively eliminated towards the tree trunk. The portion of desirable singlets increased with a reduction of the tines for the outer canopy by 33-66% relative to the standard setting with all tines in motion/attached. This removal of tines destined for the outer peripheral canopy improved the quality of apple fruit in terms of fruit size and fruit colouration (class I) from 70 to 86%. Apple served as a model crop, but the results are suitable for the majority of fruit crops such as pears, plums, apricots and for both cultivation systems, integrated fruit production (IFP) and bio-organic.