Mechanical thinning of pome fruit

Thinning is an important practice in pome fruit production which aims to ensure an optimal yield of high quality, large sized fruit as well as an adequate return bloom. In South Africa, pome fruit thinning is generally done by means of chemicals, with follow-up hand thinning. When thinning is effective, set and thus the hand thinning requirement should be reduced. This is important as labour cost associated with hand thinning is high and continually rising. Chemical thinning is weather dependent and can be environmentally harmful, which has led to a shift towards environmentally acceptable methods of thinning such as mechanical thinning.

From 2013 until 2015 the mechanical string thinners, viz. Darwin 300™, BAUM, and Bloom Bandit™, were evaluated. These machines are used to thin trees during full bloom and reduce the number of flowers before fruit set. The aim of the trials was to reduce fruit set and therefore hand thinning requirement, while increasing fruit size and quality, maintaining yield and return bloom. A range of tractor speeds and rotational rates were evaluated with the Darwin 300™ on ‘Forelle’ pears and ‘Cripps’ Pink’ apples, while the BAUM was evaluated only on ‘Cripps’ Pink’ apples. The hand-held Bloom Bandit™ was evaluated on ‘Forelle’, ‘Cripps’ Pink’, ‘Fuji’ and ‘Cripps’ Red’. Thinning intensities of 25%, 50% and 75% of clusters or flowers was applied to mature ‘Forelle’ and ‘Cripps’ Red’ trees during full bloom.

The tractor-driven mechanical thinning devices gave erratic results. The most consistent results on ‘Forelle’ were obtained using the Darwin 300™ at 5.2 km·h-1 and 300 rpm, while the BAUM gave no consistent results. The unreliability of results were due to South African pome fruit orchards currently being unsuitable for tractor-driven mechanical thinning machines. The ‘Forelle’ orchard trained to a Palmette system was the most suited for thinning, which is reflected in the more positive results obtained, but further improvements are possible. The Bloom Bandit™ effectively thinned pear and apple trees and increased fruit size without a decrease in yield or return bloom. More time is spent on thinning with the device compared to tractor-driven machines and this should be taken into account when considering using the Bloom Bandit™. Variable effects were seen on fruit set, yield was reduced to acceptable levels, while fruit size was improved in ‘Forelle’ but not ‘Cripps’ Red’. Results showed that when thinning mechanically, the aim should be to remove between 25% and 50% of flowers clusters in ‘Forelle’ and 50% of flowers clusters in ‘Cripps’ Red’. These levels of thinning gave the best results in terms of the remaining hand thinning requirements and improved return bloom in ‘Forelle’. We, however, only evaluated full cluster thinning and not within cluster thinning, which might also occur during mechanical thinning.

Variable results were obtained with tractor driven thinning machines due to orchards not well adapted to mechanisation. The Bloom Bandit™ gave promising results and can be an alternative to chemical thinning.