Hand thinning is a necessary but costly management practice in peach (Prunus persica) production. Organic apple (Malus xdomestica) production also may require hand thinning to adjust crop load. Mechanical devices to aid in thinning have been developed, but none has proven highly efficient and capable of completely replacing hand thinning. Narrow canopy training systems and novel peach tree growth habits offer new opportunities to examine mechanical methods for thinning peach and apple trees. Our studies evaluated mechanical thinning devices on peach and organically grown apple trees. In 2005 and 2006, a U.S Department of Agriculture-designed spiked-drum shaker was used to thin pillar (columnar) peach trees at 52 to 55 days after full bloom. The drum shaker, driven at two different speeds in the orchard, reduced crop load an average of 58% and follow-up hand thinning time by 50%, and increased fruit size by 9% at harvest compared with conventional hand-thinned or nonthinned control trees in 2005. In 2006, the shaker was driven at one speed but operated at two different frequencies. At 260 cycles/minute, the drum shaker removed more fruit and reduced crop load to a greater extent than when operated at 180 cycles/minute, however, fruit size at harvest did not differ between the two operating frequencies. The drum shaker reduced follow-up hand thinning time between 54% and 81%. Horticultural and economic evaluations of the drum shaker and/or a German-designed blossom string thinner were conducted in 2007 in four commercial peach orchards trained to a perpendicular V or quad V system and an organic apple block trained to a narrow vertical axis system. Mechanical thinners reduced peach crop load by an average of 36%, decreased follow-up hand thinning time by 20% to 42%, and increased fruit in higher market value size categories by 35%. The net economic impact of mechanical thinning versus hand thinning alone ranged from $175/ha to $1966/ha. Mechanical thinning at 20% full bloom resulted in more fruit in the large size categories (2.75 inches in diameter and larger) than thinning at 80% full bloom. Detailed counts of flowers on branches with different orientations indicated that pruning may be adjusted to improve thinner performance. The string thinner effectively thinned dwarf apple trees trained to a vertical axis system in a certified organic orchard, resulting in a reduction in hand thinning time and an increase in fruit size. Based on our tests, mechanical thinning appears to be a promising technique for supplementing hand thinning in apple and peach trees.
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