Influence on apple trees of selective mechanical thinning on stress-induced ethylene synthesis, yield, fruit quality, (fruit firmness, sugar, acidity, colour) and taste

The overall objective of this work was to improve fruit quality, break alternate bearing and reduce hand thinning using fewer chemicals in fruit crops. A device was constructed for mechanical thinning, which consisted of three independent horizontal rotors with ropes and freely adjustable angles on a frame, mounted on a front three point hitch and powered by the tractor hydraulics. This can be adapted to any fruit tree trained as spindle, Solaxe, (tall) vertical axis or fruit wall (le mur fruitier) irrespective of rootstock employed. Rotor speed varied from 300 to 460 rpm at either 5 or 7.5 km/h tractor speed. Eight-year-old or twelve-old apple trees cvs. 'Gala' and 'Golden Delicious' were mechanically thinned in 2007 between pink bud and full bloom (flower bud stages 6-8 or F1-F2) near Bonn, Germany; non-thinned and hand-thinned apple trees of the same block and variety served as control. Mechanically thinned flowering branches showed a similar amount of ethylene efflux (0.4-0.6 ppm C₂H₄/branch) as non-thinned flower branches, preventing potentially unexpected subsequent fruit drop, except for those removed by the rotors. The impact of the horizontal rotors on the branches was from the upper side and removed excessive flowers right to the tree trunk viz. the centre of the tree canopy, where fruits of lesser quality are expected leaving 2-3 flowers per cluster. Leaf damage was less than 10%, even at the fast rotor speed of 420 rpm, which was associated with negligible wood injury. Mechanical thinning induced firmer and sweeter fruit, i.e. tastier apples with longer shelf life, relative to control fruit from non-thinned apple trees. The greatest efficacy in terms of final fruit quality in the grading/sorting was achieved by a rotor speed of 360 rpm at a tractor speed of 5 km/h: Fruit mass increased by up to 20 g and the proportion of fruit larger than 70-75 mm by 10-30% compared with the fruit from non-thinned trees. Mechanical thinning with this newly constructed device led to a 10-20% reduction in yield, but increased returns due to better fruit size and colouration in apple with the potential to overcome alternate bearing.