17 Principles of Thinning in Fruit Tree Crops – Needs and Novelties G. Costa 1, a , M.M. Blanke 2, b and A. Widmer 3 1 Department of Fruit Trees and Woody Plant Science, Alma Mater Studiorum, University of Bologna, viale G. Fanin 46, 40127 Bologna, Italy 2 INRES – Horticultural Science, University of Bonn, Auf dem Huegel 6, D-53121 Bonn, Germany 3 Research Station Changins-Wädenswil ACW, Research Department Plant Protection and Fruit and Vegetable Extension, Schloss 1, P.O. Box, 8820 Wädenswil, Switzerland Keywords: alternate bearing, biennial bearing, fruit colouration, fruit firmness, fruit quality Abstract Fruit trees bear an abundance of flowers, which they cannot support until fruit maturation; ca. 7% of flowers are necessary in apple for a sufficient harvest while 25% of flowers are necessary in peach. Thinning is hence a prerequisite in fruit crops i) to achieve high quality fruit with sufficient size and colouration for class I marketing including sugar (as a parameter for taste) and sufficient firmness (as a parameter for good storability), ii) to reduce labour-intense hand thinning and iii) to overcome alternate bearing (change of low and high yielding years) by providing regular moderate yields. INTRODUCTION Many fruit species require a reduction of fruit load to reach a vegetative/ reproductive equilibrium; normally this objective is achieved with the fruit thinning operation. This operation is performed yearly mainly in peach and nectarines among the stone-fruit species and in apple among the pome-fruit specie. Fruit thinning is also performed on other fruit specie such as apricot, cherry, plum, kiwifruit, orange, table and grape vine, etc, although in some of these, pruning might represent an effective tool to achieve the desired fruit load. However many fruit trees bear an abundance of flowers, which they cannot support until fruit maturation; ca. 7% of flowers are necessary in apple to guarantee a satisfactory yield (Untiedt and Blanke, 2001) while only 20-25% of flowers are needed in peach (Costa and Vizzotto, 2000). In anticipation of this, the major fruit specie developed an immature fruit physiological drop as self regulatory mechanism. The physiological fruit drop leads to a reduction of the fruit number per tree, but this is still insufficient to guarantee, at harvest, fruits of good marketable size and maximum commercial yield. For this reason, fruit thinning is performed to achieve the optimum fruit load and quality. Fruit thinning is hence a prerequisite in fruit crops for: i) to achieve high quality fruit with sufficient size and colouration for class I marketing (Kong et al., 2009) including sugar (as a parameter for taste) and sufficient flesh firmness (as a parameter for appropriate storage length); ii) to reduce labour-intense hand thinning; and iii) to overcome alternate bearing (change of low and high yielding years) by providing regular moderate yields. OBJECTIVE OF FRUIT LOAD CONTROL The reasons for reduce the crop load are different and are related to the different species. For instance there are beneficial effects of fruit load reduction that can be distinguished between those on long-term whole tree physiology and current year fruit quality (Table 1). Taking apple as an example, the fruit load reduction via fruit thinning is mainly a guglielmo.costa@unibo.it b mmblanke@uni-bonn.de Proc. EUFRIN Thinning Working Group Symposia Eds.: M.M. Blanke and G. Costa Acta Hort. 998, ISHS 2013