Influence of tree-adapted flower thinning on apple yield and fruit quality considering cultivars with different predisposition in fructification

Production of fruit providing high quality, good storability and long term consistent yields requires regulation of the crop load on fruit trees by reducing the number of flowers. Due to several disadvantages of chemical thinning, mechanical approaches have become preferable. Despite the spatial variation of flower set between the trees, current thinning systems work with uniform intensities throughout the orchard. Uniform thinning intensity causes excessive thinning at some of the tress, while trees with exhaustive flower set might be under-thinned. A new mechanical system for flower thinning combines (i) camera based sensor for identifying the flower density in situ; (ii) a mobile geographic information system, which includes (iii) a decision support system for determining the optimum thinning intensity based on the actual flower density and ancillary data (yield, soil); and (iv) the mechanical thinning unit. Field trials have been conducted on Malus x domestica Borkh. 'Elstar' and 'Pinova' to quantify the relation between the string thinner's rotational speed per minute (RPM) and the number of flowers, the yield and fruit quality parameters from single trees. A slight improvement of fruit quality in terms of yield of medium fruit sizes of >65 mm and a significant increase of soluble solids content were obtained at average optimum thinning intensities of 286 RPM for 'Elstar' and 296 RPM for 'Pinova'. Considering the individual flower density per tree, low flower density (200 tree(-1)) needed 247 RPM for 'Elstar' and 240 RPM for 'Pinova' to maximize yield of fruits larger 65 mm. Excessive flowering (1000 tree(-1)) required intensities of 306 and 310 RPM for 'Elstar' and 'Pinova', respectively. The economic impact of adaptive thinning will depend on the variability within the orchard. Based on data from one season, the yield of marketable fruits could be improved by 5%. Moreover, this evaluation does not account for the benefits from supressing alternate bearing.