Mechanical thinning of apples reduces fruit drop

The effect of the intensity of mechanical thinning on fruit drop and final fruit set was investigated in the years 2011 and 2014 on apple trees with varying flower set considering the cultivars ‘Elstar’, ‘Gala’ and ‘Pinova’. Thinning was carried out with the device Darwin 250 at balloon stage (BBCH 59) at constant vehicle speed of 8 km h^‑1 with rotational frequencies of 200, 240, 280, and 320 rpm. By calculating kinetical energy at the end of the string, thinning treatments equal 0.68, 1.01, 1.42 and 1.89 J. Absolute numbers of removed flowers increased with increasing flower set. Additionally, removal of flowers increased with enhanced rotational frequency. Thinning showed no effect on fruit drop on ‘Elstar’ and ‘Pinova’ in 2011. In 2014, thinning treatment of 1.42 J and higher reduced fruit drop on ‘Elstar’ and ‘Gala’ due to a reduced number of sinks competing for available carbohydrates. Consistently, fruit drop was enhanced on trees with high flower set in comparison to trees with low flower set in every trial. Final fruit set underrun yield capacity of the trees when trees had flower set of 1-200, and on trees with higher flower set when thinned above 1.01 J. Therefore the concept of precise thinning shows an encouraging potential to balance heterogeneity of flower set within an orchard by means of adaptive thinning intensity for each tree with varying rotational frequency based on flower set.