Mechanical flower thinning improves fruit quality of apples and promotes consistent bearing

Abstract

A novel device is presented for mechanical blossom thinning with first test trials in the US on seven-year-old apple [ Malus domestica (Borkh.)] ‘Buckeye Gala’ near Yakima, WA, USA. Trees were mechanically blossom-thinned with the novel string thinning device, developed by Damerow et al. (2007), at 260 or 360 rpm rotor speeds and a 2.5 km h −1 vehicle speed to improve fruit quality, reduce hand and chemical thinning and to prevent or overcome alternate bearing; manually thinned trees served as controls in a replicated, randomized block trial. The strongest mechanical thinning treatment (360 rpm, 2.5 km h −1) had a positive effect on apple fruit size (75 mm versus 72 mm in the hand-thinned control), firmness 9.0 kg cm −2 versus 8.9 kg cm −2, advanced ripening i.e. starch breakdown, sweetness 12.3-12.8 °Brix versus 11.9 °Brix in the control, contained the largest malic acid content 0.43% versus 0.37% in the control and more red blush, i.e. fruit coloration. The sugar:acid ratio was maintained particularly after the strongest 360 rpm mechanical thinning (29:1) and resembled that of the chemical thinning and the hand-thinned control (32:1), since fruit with higher sugar content also contained more malic acid. The best return bloom was achieved by the conventional chemical standard (105%), followed by the strongest mechanical thinning (92% with 360 rpm) then the combination of mechanical and chemical thinning (85%) compared to much lower values in the control (69%), showing the efficacy of blossom thinning to overcome alternate bearing.