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Selective mechanical thinning to regulate fruit set, improve quality and overcome alternate bearing in fruit crops

Veal, D., Damerow, L., Blanke, M.M.
Acta horticulturae 2011 no.903 pp. 775-781
alternate bearing, apples, branches, brushes, color, computer software, costs and returns, deblossoming, firmness, flowering, flowers, fruit crops, fruit quality, fruit set, growers, labor, leaves, markets, new technology, rotors, storage quality, sugars, taste, tractors, tree trunk, trees, United States
A novel device was developed at Bonn university to thin fruit crops at the flowering stage in order to improve fruit quality (fruit size, colouration, sugar, taste, firmness and storability) and overcome alternate bearing. The device consists of three horizontal rotors. Their brushes remove up to 1/3 of flowers in slender spindle trees; by selecting a range of combinations of brushes, rotor speeds and tractor speeds, the amount of removed flowers can precisely varied from 0 to 40%. Results from 3 years (2005-2007) on apple varieties including ‘Elstar’, ‘Braeburn’, ‘Gala’ and ‘Golden Delicious’ showed that the novel device can precisely remove flowers not only in the tree periphery, but also close to the tree trunk where fruit of lesser quality otherwise develop. Rotor speeds of between 300-420 rpm showed the best efficacy of flower thinning using tractor speeds of between 5-7.5 km/h. Based on these field data, cycloids were calculated using a commercial software package to simulate the movement of the brushes in a tree region and the percentage of flowers removed at any given tractor and rotor speed. Damage to branches and leaves was less than 8% at the maximum employed rotor speed of 420 rpm used for thinning. The device saves labour cost otherwise required for hand-thinning in the order of 15-30 hours/ha, equivalent to ca. 100 US $/ha. The cost of the device per hectare is estimated at ca. 100 US $/ha/year based on a life-span of 10 years and an area of 20 ha to thin each year. The device may be an alternative for both organic and integrated fruit growers, particularly as chemical thinning agents become less available for environmental reasons. The device with its precise thinning mode may also have a market potential not only for apple, but also certain stone fruit and Mediterranean fruit crops, where thinning and achievement of a high fruit quality is required for successful economic returns.