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Selective mechanical thinning for crop load management (CLM) of fruit trees

C. Seehuber, L. Damerow, A. Kunz, M. M. Blanke
Acta horticulturae 2014 v. no.1058 pp. 405-410
flowering, European Union, rotors, fruit trees, supermarkets, sulfur, sugars, fruit quality, agrochemicals, cultivars, tractors, farming systems, storage quality, firmness, alternate bearing, temperature, brushes, taste, canopy, fruits, tree age, wages and remuneration, flowers, color, fruit crops, Germany, United Kingdom
Re-registration of all agricultural chemicals under Annex 1 of EU 91/414 reduced the number of compounds available for thinning. Additionally, supermarkets in the UK and Germany require growing certificates, which either limit the amount of chemicals used to a maximum number or exclude particular practises or chemicals. Mechanical thinning is an innovative approach for thinning fruit crops trained to any kind of spindle trees, irrespective of species, cultivar, temperature and tree age and is an alternative to or compliments chemical thinning. Four concepts of its implementation are presented here: a) sole use of mechanical thinning to overcome alternate bearing, b) combination of mechanical thinning with chemical (and hand-thinning) in IFP, c) combination with lime sulphur where approved for organic fruit farming, and d) combination of mechanical thinning with hand–thinning where no chemicals are approved. The device with three horizontal rotors and front mounting on the tractor has been developed at the University of Bonn in 2005-2010. Using a range of combinations of brushes, rotor speeds (320-420 rpm) and tractor speeds (5 to 7.5 km/h), its vertically operating brushes remove ca. 25-33% of flowers. Because of its use at early developmental stages, i.e., during flowering (between pink bud and petal fall), mechanical thinning on its own can break or prevent alternate/ biennial bearing. Mechanical thinning on its own or in combination with either chemical or hand-thinning improved fruit quality, particularly fruit size (by improving the source sink relationship and enlarging the photo-assimilates partitioned to the remaining fruit) and colouration (by allowing more light access to the fruit and removal of shade fruits from the inner canopy), sugar (taste), and sometimes firmness for better storability. The device saves labour costs, otherwise required for hand-thinning, in the order of 15-30 hours per hectare, equivalent to € 100-150/ha, i.e., in the same order as the cost of mechanical thinning of ca. 120 €/ha, based on 20 ha and 10 years depreciation.