Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti Short communication Effect of non-chemical crop load regulation on apple fruit quality, assessed by the DA-meter Lena Peifer, Samuel Ottnad, Achim Kunz, Lutz Damerow, Michael Blanke ⁎ INRES-Horticultural Science, University of Bonn, Germany ARTICLE INFO Keywords: Apple (Malus domestica Borkh.) Crop load management (CLM) DA-meter Fruit quality June drop Mechanical thinning Return bloom Shading ABSTRACT The objective of the present work was to investigate alternatives to chemical crop load management (CLM) for cv. ‘Gala Mondial’ to improve fruit quality, particularly fruit size and portion of class I and its effects on June drop and return bloom as well as evaluate fruit maturation with the new, non-invasive DA-meter. Moderately flowering apple trees cv.’Gala Mondial’ at Klein Altendorf under white hail net in 2016 were subjected to four crop load motions: a) shading (90% for 77 daylight hours) at the end of May, b) mechanical blossom thinning with the Bonner device with three horizontal rotors at the balloon stage, and c) hand thinning at the end of June, with d) a combination trial of b) and c) adjacent un-thinned apple trees served as control. The weaker (320 rpm) and the stronger (360 rpm) mechanical thinning both at 5.5 km h -1 tractor speed caused a 5% or 22% reduction in fruit set, followed by shading with 27% reduction in comparison with the control trees. After June drop, the stronger mechanical thinning reduced fruit set by 23% and the combined mechanical and hand-thinning by 67%, while shading caused a reduction of 99% at 50°N and without spring frosts. The best pack-out viz. greatest percentage (86.8%) of fruits > 70 mm diameter was achieved in the combined mechanical and hand thinning followed by 53.8% in the hand thinning, the strong mechanical thinning with 21.4% and control fruit with only 19.4% resulting in a ca. 68% increase in both class I and II fruit with an additional economic net gain of 3500 €/ ha. The relationship between the DA-meter and Streif index is delineated to determine optimum harvest date (OHD). Although apple cv. ‘Gala’ appears insensitive to alternate bearing, return bloom in the control was 4–6, in the stronger mechanical thinning (360 rpm) 5–7, in the hand thinning 6–8 and in the combined mechanical and hand thinning as strong as 8–9 on the 0 (no flowers) - 9 (white blossom) scaleand reflects the physical response of the tree to the previous CLM.© Elsevier B.V. 1. Introduction With 10–12 million tons, apple is the largest fruit crop in Europe. In apple cultivation, crop load management (CLM) is one of the most important technologies to ensure good fruit quality, especially fruit size, taste and colour. Therefore, orchardists have to remove excessive flowers and fruitlets from the apple viz fruit trees. CLM is also im- portant to break or overcome alternate bearing, i.e. change in years of high and low yields (Westwood, 1998). Of particular interest are CLM methods, which reduce the use of chemical agents or act as a supple- ment in years (Damerow et al., 2007), when the ambient temperature is too cold e.g. for chemical (BA) fruitlet thinning (Kong et al., 2009). These CLM methods include mechanical thinning, which has played an increasingly significant role in the last few years, hand-thinning as well as shading (Blanke et al., 2012; Costa et al., 2012; Seehuber et al., 2014; Veal et al., 2011). The apple cultivar ‘Gala Mondial’ is precocious with a tendency to produce many small fruits; therefore, thinning may improve not only inner fruit quality but also fruit size. Fruit trees produce more flowers and fruitlets than they can provide with photo-assimilates to fruit ma- turation; therefore, trees drop a proportion of their own fruitlets during three abscission phases. However, the remaining fruits do not meet the quality for marketing as class I. In apple cultivation, it is satisfactory for a good harvest, if seven percent of the flowers develop to fruitlets (Untiedt and Blanke, 2001). Fruitlets compete with each other for a limited range of assimilates, minerals and nitrogen compounds, pro- vided by the trees. The removal of redundant flowers or fruitlets im- proves the leaf: fruit ratio. This will allow a better supply of photo- assimilates to the remaining fruitlets. To ensure a good fruit quality in apple, 20–30 leaves (’source’) per fruit (‘sink’) to the corresponding fruit branch are necessary (Westwood, 1998; Jackson, 2003; Seehuber et al., 2014). Fruit drop takes place in three periods (Luckwill et al., 1953), after bloom, as June drop and before harvest. One to four weeks after flowering, the fruit tree discards unfertilized flowers. At June https://doi.org/10.1016/j.scienta.2017.11.011 Received 29 August 2017; Received in revised form 27 October 2017; Accepted 3 November 2017 ⁎ Corresponding author. E-mail address: mmblanke@uni-bonn.de (M. Blanke). Scientia Horticulturae 233 (2018) 526–531 0304-4238/ © 2017 Elsevier B.V. All rights reserved. T