Mechanical thinning of apples reduces fruit drop

Martin Penzel is a PhD candidate in the Working Group of Precision Horticulture of the Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB), Potsdam, Germany. For his doctoral study he is focusing on precision thinning of apples and the gas exchange patterns of pre-harvest fruit and leaves. His goal is to determine the optimum crop load according to the carbon balance of the trees. The effect of mechanical thinning on fruit drop and final fruit number of ‘Elstar’, ‘Gala’ and ‘Pinova’ apples was investigated in 2011 and 2014. Mechanical thinning was carried out at the balloon stage, at a constant vehicle speed of 8 km h-1 with rotational frequencies of 200, 240, 280, and 320 rpm of the rotating arms. By calculating the kinetical energy at the end of a string, such thinning treatments equaled 0.68, 1.01, 1.42 and 1.89 J, respectively. The absolute number of removed flowers per tree was proportional to the initial number of flowers per tree (flower set) and increased with enhanced rotational frequency. In 2014, natural fruit drop was reduced on ‘Elstar’ and ‘Gala’ when thinning treatments of 1.42 J or more were applied. We suggest this was because of a reduced number of sinks competing for available carbohydrates. Fruit drop was more consistently enhanced on trees with high flower set than on trees with low flower set in every trial. When trees had 200 flowers or less, the final fruit set was below the target crop load of 125 fruit per tree on the control. The trees with higher initial flower set, >200, were thinned above frequencies of 1.01 J. Therefore, the concept of precision thinning shows an encouraging potential to balance the heterogeneity of flower set within an orchard by means of adaptive thinning intensity for each tree by varying the rotational frequency based on the initial flower set.

Martin Penzel won an ISHS Young Minds Award for the best poster presentation at the International Symposium on Understanding Fruit Tree Behaviour in Dynamic Environments at IHC2018 in Turkey in August 2018.

Martin Penzel, Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB), Potsdam, Germany, e-mail: mpenzel@atb-potsdam.de

The article is available in Chronica Horticulturae