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Concepts of selective mechanical thinning in fruit tree crops

Seehuber, C., Damerow, L., Blanke, M.M.
Acta horticulturae 2013 no.998 pp. 77-83
alternate bearing, brushes, canopy, color, cultivars, developmental stages, early development, farming systems, firmness, flowering, flowers, fruit crops, fruit quality, fruit trees, fruits, labor, rotors, source-sink relationships, storage quality, sugars, sulfur, taste, temperature, tractors, tree age
This contribution presents an innovative approach for thinning fruit crops trained to any kind of spindle trees, irrespective of species, cultivar, temperature and tree age. Mechanical thinning is suggested as an alternative or to compliment 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 of the tractor has been developed at the University of Bonn in 2005-2007. By selecting a range of combinations of brushes, rotor speeds 350-450 rpm) and tractor speeds (4-8 km/h), its vertically operating brushes remove ca. 25-33% of flowers. Due to 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 alone 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 mechanical thinning of ca. 120 €/ha, based on 20 ha and 10 years depreciation.