Chemical thinning programs for 'Fuji Mishima' apple trees under black anti-hail net

Abstract

The objective of this work was to evaluate chemical thinning programs containing mixtures of different plant growth regulators, at low concentrations, applied to 'Fuji Mishima' apple trees at post-blossom, as well as to identify the most effective in reducing fruit set, decreasing the need of manual thinning, and improving fruit quality. Under a black anti-hail net, the apple trees were sprayed with the six following chemical thinning protocols (treatments): benzyladenine (BA) + gibberellin₄₊₇ (GA) at full bloom (FB) and BA + carbaryl (CB) on fruitlets with a 15 mm diameter; BA + GA at FB and naphthaleneacetic acid (NAA) + CB on fruitlets with a 7 mm diameter; BA + GA at FB and BA + CB on fruitlets with a 15 mm diameter; ethephon (ETH) + CB on fruitlets with a 15 mm diameter and metamitron (MM) + ETH on fruitlets with a 20 mm diameter; and MM on fruitlets with a 7 mm diameter and MM on fruitlets with a 20 mm diameter. The treatments were compared with an untreated and a manual thinning control. The black net reduced photosynthetically active radiation in 22%. For 'Fuji Mishima' apples under a black anti-hail net, the chemical thinning program consisting of BA + GA (47 + 47 g a.i. ha^-1) at FB, followed by BA + CB (80 + 288 g a.i. ha^-1) on fruitlets with a 15 mm diameter, consistently reduces fruit set, requires less manual thinning, and improves fruit weight and size.