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Effect of non-chemical crop load regulation on apple fruit quality, assessed by the DA-meter

Peifer, Lena, Ottnad, Samuel, Kunz, Achim, Damerow, Lutz, Blanke, Michael
Scientia horticulturae 2018 v.233 pp. 526-531
Malus domestica, alternate bearing, apples, deblossoming, flowering, fruit quality, fruit set, fruits, hail nets, harvest date, rotors, shade, solar radiation, spring, tractors, trees
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 (320rpm) and the stronger (360rpm) mechanical thinning both at 5.5kmh−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 >70mm 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 (360rpm) 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.