Tree-adapted mechanical flower thinning prevents yield loss caused by over-thinning of trees with low flower set in apple

Abstract

Flower thinning compromises the continuous yield and fruit quality in the production of apple fruit. The aim of the present study was to investigate the potential of managing spatial heterogeneity in flower set by adapted tree-individual mechanical flower thinning. In the years 2011, 2014-2016, commercial orchards of 'Elstar'/M26 and 'Gala'/M9 trained as slender spindle, with an abundance of varying flower set were mechanically thinned at balloon stage (BBCH 59) with the Darwin 250 device at constant vehicle speed of 8 km h-1 with varying rotational frequency ranging from 200 revolutions per minute (rpm) to 380 rpm. Rotational frequency of the thinning device was translated to average kinetic energy (Ekin [J]) that one string transfers into the canopy. Thinning treatments ranged between 0.15 J and 0.66 J. For low and medium flower sets, majority of trees underrun production target of 119 fruit and, therefore, no thinning was necessary. Treatments >0.23 J caused yield loss by over-thinning without any positive effect on fruit mass. At high flower set, thinning treatments of 0.23 J and 0.33 J were adequate settings to reduce crop load in 'Elstar' and 0.33 J in 'Gala' without yield loss. The concept of tree adapted flower thinning in comparison to field uniform thinning would result in the avoidance of yield loss by over-thinning of trees with low and medium flower set by 1.4-4.2 t ha-1 in 'Elstar' and 2.6-7.6 t ha-1 in 'Gala'. Results indicate the importance of quantifying flower set for precisely defining the intensity of thinning treatment according to the tree's yield capacity.