Mechanical thinning improves source: sink relationships, fruit quality viz fruit size, fruit firmness and fruit colouration and portion of singlets of cv. 'Gala' apple

The objective of the MSc project was to improve fruit quality, i.e. fruit firmness, colour and size, by mechanical blossom thinning. Ten-year-old slender spindle apple cv. 'Gala, Mondial' trees at Campus Klein-Altendorf near Bonn, Germany with an intense flowering of 7-8 on the 0-9 scale were blossom-thinned on 21 April 2009; un-thinned trees served as control. Rotor speeds were 360 rpm or 420 rpm at 5 or 7.5 km/h tractor speed using the new mechanical thinning device developed at the University of Bonn in 2004-2006. Therein, a vertical mast supports three horizontal rotors, whose vertically rotating tines remove excessive flowers; the device was mounted on the front three point hitch of the tractor with the following results: 1. Mechanical thinning reduced the number of apple fruit from 18 per branch section in the un-thinned control, 12-13 in the medium to 8 fruit in the strongest thinning (420 rpm, 5 km/h). 2. Similarly, the number of singlets (one fruitlet/flower cluster) nearly doubled from ca. 2 per branch in the control to 3-4 after mechanical thinning, as intended. 3. The natural June drop after mechanical thinning resembled that in the un-thinned control. 4. Mechanical thinning increased fruit mass by 20 g-48 g relative to the un-thinned control. 5. Mechanical thinning with 420 rpm increased the portion of premium fruit >70 mm (class I) by 43 % (7.5 km/h) and 63 % (5 km/h); combined mechanical and handthinning yielded the largest portion of fruit >70 mm (70 %), but with a dramatic loss in yield (55 %). 6. Mechanical and manual thinning improved the portion of well coloured fruit (75-100 % coloured) (78-98 %) when compared to the un-thinned control fruits with 55 %, probably due to selective removal of shaded fruits from the inner tree canopy. 7. Overall, mechanical thinning alone gave better results than hand thinning alone or its combination with hand-thinning in cv. 'Gala'; mechanical thinning waived the need for hand thinning in this experiment. 8. Mechanical thinning improved fruit firmness from 9.4 kg/cm(2) in the un-thinned control to 10.4 kg/cm(2). 9. Neither vegetative growth nor subsequent fruit drop were stimulated by this type of mechanical thinning. 10. The risk of both alternate bearing and over-sized fruit was waived due to the variety employed. 11. An economic evaluation showed a net profit of (sic) 220/ha in the weak (420 rpm-7.5 km/h) avo (sic) 1,844/ha in the intermediate (360 rpm-5 km/h), but a net loss of (sic) 1,120/ha (420 rpm-5 km/h) for the strongest thinning treatment, assuming a cost of (sic) 120/ha for the mechanical treatment and a saving of (sic) 200 for reduced hand thinning. Overall, the positive results of a better fruit quality (size, colour and firmness) offer the potential of mechanical blossom thinning in cv. 'Gala', which is difficult to blossom-thin chemically.