Submitted to: HortTechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2008
Publication Date: 9/17/2008
Citation: Schupp, J., Baugher, T., Miller, S.S., Harsh, R.M., Lesser, K.M. 2008. Mechanical thinning of peach and apple trees reduces labor inputs and increases fruit size. HortTechnology. 18:660-670. Interpretive Summary: Peaches, like many tree fruits, generally set more fruit than the tree can carry to a marketable size at harvest. Thinning is required to remove some peaches and improve the size and quality of the remaining fruits. Hand thinning is still the primary means available for thinning peaches, but hand thinning is costly and peach growers are looking for more efficient and less costly methods for thinning. This research, conducted over three growing seasons, focused on the application of several new and novel mechanical approaches for removing peach blooms or young fruits. Both mechanical thinners demonstrated effective response in 100 percent of the trials. Additional testing of the mechanical thinners is warranted based on the current results. This information is useful for peach growers, extension fruit specialists, and tree fruit research professionals.
Technical Abstract: Hand thinning is a necessary but costly management practice in peach production. Organic tree fruit production also requires hand thinning to adjust crop load. Mechanical devices to aid in thinning have been developed, but none have 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 USDA designed spiked-drum shaker was used to thin pillar (columnar) peach trees at 52 to 55 days after full bloom (DAFB). The drum shaker, driven at two different speeds in the orchard, reduced crop load an average of 58 percent and follow-up hand thinning time by 50 percent, and increased fruit size by 9 percent at harvest compared to conventional hand thinned or non-thinned control trees in 2005. In 2006 the shaker was driven at one speed but operated at two different frequencies. At 260 cycles per min (cpm) the drum shaker removed more fruit and reduced crop load to a greater extent than when operated at 180 cpm, 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 percent. Horticultural and economic evaluations of the drum shaker and/or a Darwin 300 blossom string thinner (Fruit-Tec, Deggenhausertal, Germany) were conducted in 2007 in four commercial peach orchards trained to either a perpendicular V or quad V system and an organic apple block trained to a narrow vertical axis system. Mechanical thinners reduced crop load by an average of 36 percent, decreased follow-up hand thinning time by 20 to 42 percent, and increased fruit in higher market value size distributions by 35 percent. The net economic impact of mechanical thinning versus hand thinning alone ranged from $175 to $1966 per ha ($71 to $796 per acre). Mechanical thinning at 20 percent full bloom resulted in more fruit in the large size category (7.0 cm or 2.75 in. in diameter and up) than thinning at 80 percent 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 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.