C. Seehuber, L. Damerow, A. Kunz, M.M. Blanke
In the absence of any chemicals approved in Europe for flower thinning pears there is a market demand for large particularly ‘Conference’ fruit. The objective of the present study was to improve fruit quality i.e., achieve large fruit sizes. Therefore, 18-year-old ‘Conference’ and ‘Lucas’ pear trees, trained as super spindles, on quince A rootstock with a spacing of 4×0.4 m were used in Klein-Altendorf near Bonn, Germany; un-thinned adjacent trees served as control. These pear trees with intense flowering were blossom-thinned on 15 April 2009 with rotor speeds of either 300 or 400 rpm both at 5 km h-1 tractor speed. 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 (300-450 rpm) and tractor speeds (4-8 km h-1), 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 on its own or in combination with either chemical or hand-thinning may improve fruit quality, particularly fruit size (by improving the source sink relationship and enlarging the photo-assimilates partitioned to the remaining fruit, sugar (taste), and sometimes firmness for better storability.
June drop was stronger reduced in ‘A. Lucas’ (by -38%) than in ‘Conference’ (by -27% relative to the control). Mechanical blossom thinning reduced the June drop with increasing rotor speed. In ‘A. Lucas’, the natural June drop was as strong as mechanical thinning and resulted in large-sized 80-mm pear fruit in the control; mechanical thinning with 400 rpm further increased fruit mass by 16 g relative to the un-thinned control or 6 g larger than in the thinning with 300 rpm. In ‘Conference’, fruit fell within the next larger fruit size class after the stronger mechanical thinning with 400 rpm at 5 km h-1: the portion of pear fruit in the 75/80 mm size class was 2-fold and in the 80/85 mm size class was 6-fold that in the control. The portion of small fruit such as <60, 60/65 and 65/70 mm was halved relative to the control; the average fruit mass was increased by 40 g without affecting vegetative growth.
The risk of both over-sized fruit and over-thinning appeared small, since mechanical thinning reduced the natural June drop without inducing a subsequent fruit drop after thinning. Mechanical thinning was more efficient in ‘Conference’, as intended, despite more intense flowering in ‘A. Lucas’. Overall, the results indicate the potential of mechanical thinning as a substitute or combination partner for chemical fruit thinning if approved such as BA in pear.
The device saves labour costs, otherwise required for hand-thinning, in the order of 30-60 h ha-1, equivalent to € 150-300 ha-1, i.e., almost twice as much as the cost of mechanical thinning of ca. 120 € ha-1, based on 20 ha and 10 years depreciation.
Seehuber, C., Damerow, L., Kunz, A. and Blanke, M.M. (2015). MECHANICAL THINNING OF 'LUCAS' AND 'CONFERENCE' PEAR IMPROVES FRUIT QUALITY. Acta Hortic. 1094, 289-295
DOI: 10.17660/ActaHortic.2015.1094.37
pear (Pyrus communis L.), fruit firmness, fruit quality, fruit size, thinning

Acta Horticulturae