Regulation of fruit set by mechanical flower thinning

The objective of the present study was to thin apple flowers without use of chemicals, in order to improve fruit quality, reduce labour for hand thinning and overcome alternate bearing. The newly developed device comprises three rotors with adjustable angles and vertically rotating ropes, which remove excess apple flowers. Eleven year-old cv. 'Braeburn' apple trees were thinned at flower opening (BBCH growth stage 61) with the new device in Klein-Altendorf near Bonn, Germany. Adjacent untreated, hand-thinned or chemically (benzyladenine-)thinned apple trees of the same rows served as controls. Tree branches remained un-damaged by the vertically rotating ropes. Slight leaf damages of less than 8% were observed at the fastest rotor speed of 320 rpm, which also gave the best thinning results. The portion of class one fruits > 70 mm was increased by 10% without yield loss and by up to 20% with yield losses of ca. 5-10%, depending on the settings, relative to the untreated control. This was equivalent to fruit mass gains of 10 g without yield loss and of 20 g with 10%-20% yield loss with economic gain in both cases. The single or double-sided mechanical thinning required 1.2 h ha(-1) or 2.4 h ha(-1) at a tractor speed of 2.5 km h(-1) and reduced the subsequent hand thinning by respectively 20% or 45% (by 7 h or by 15 h/ha or its cost by 50-135 (sic)/ha). The new device gently removed up to one third of both peripheral and central flowers at a cost of less than 100 (sic)/ha and with a negligible risk of over-thinning and without effect on return bloom.