Mechanical flower thinning improves the fruit quality of apples

Abstract

Apple 'Golden Delicious Reinders' and 'Gala Mondial' trees were mechanically blossom-thinned with 30-77 x g (300-480 rpm rotation) and 5 or 7.5 km h(-1) vehicle speed to improve fruit quality, minimise leaf damage, reduce hand and chemical thinning and to prevent or overcome alternate bearing; adjacent untreated or manually thinned apple trees served as controls. Mechanical thinning (43 x g, 360 rpm, 5-7.5 km h(-1)) had a positive effect on fruit size (15% larger), firmness (8.4 in Gala vs. 7.6 kg cm(-2) in the unthinned control), sweetness (124 vs. 117 g kg(-1) sugar in the control), contained the largest malic acid content (4 g kg(-1) vs. 3.4 g kg(-1) in the control) and 17% more anthocyanin (normalised anthocyanin index = 0.8 in Gala vs. 0.7 in the control); fruit of Golden and Gala showed additionally advanced starch breakdown and ripened earlier. Since increases in rotor speed, viz. centrifugal force, versus increases in the vehicle speed resulted in opposing effects, an integrated coefficient of thinning (ICT) was devised with optimum values of 10-40 (at 43 x g, 5-7.5 km h(-1)), where an ICT > 50 led to tree damage and ICT < 8 led to sub-optimum thinning efficacy.