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Home HortScience The Evolution of Mechanical Thinners for Peach Trees
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KEARNEYSVILLE, WV - To produce a crop of marketable size fruit, peach trees must be thinned early in the growing season. Because chemical thinning methods have shown limited success in peach orchards, hand fruit thinning is the generally accepted practice to reduce crop loads in order to increase fruit size. Though the method is effective, hand fruit thinning is one of the most labor-intensive, costly practices for peach growers.

Peach growers are looking to mechanical thinners to provide reliable, economical alternatives to hand and chemical thinning. A study published in HortScience provides important new information for peach and other tree fruit growers. Stephen Miller and Scott Wolford from the U.S. Department of Agriculture, Agricultural Research Service, along with James Schupp, and Tara A. Baugher from Pennsylvania State University, reported on five field trials they conducted over two years in orchards with trees trained to a perpendicular V system. The trials showed that a Darwin string thinner at 60% to 80% full bloom reduced crop load on scaffold limbs by 21% to 50% compared with a hand-thinned control. At the 60% full bloom stage, a USDA-designed double-spiked drum shaker reduced crop load by 27%; in another trial, a USDA prototype single-drum shaker reduced crop load by 9%. The spiked drum shakers (single or double units) removed an average of 37% of the green fruit across all trials.

Fruit size was increased over hand-thinned controls by mechanical thinning in most, but not all, trials. A combined treatment of the Darwin string thinner at bloom followed by a drum shaker (single or double unit) at the green fruit stage produced the greatest net economic impact in a number of the trials.

According to the study's corresponding author Stephen Miller, "All mechanical devices reduced the time required for follow-up hand thinning. Follow-up hand-thinning costs were reduced an average of 27% by mechanical thinning devices over hand-thinned control trees. These results indicate that, despite overthinning in some trials, the mechanical thinning devices provide a potential alternative to hand thinning alone in peach production."

The authors included several caveats regarding the findings. "Although mechanical thinners can clearly reduce crop loads and the time required for follow-up hand thinning in peach, the current state of the art does not replace the need for hand thinning," they noted. The report also pointed out that the mechanical devices described in the study are not capable of selective thinning in the sense they can target a specific bloom or fruit. "Shakers tend to remove larger sized fruits than smaller fruit. This may have a negative impact on the yield of large-sized fruit at harvest. However, the saving in labor and time may offset the small increase in returns gained by a few larger fruit at harvest."

Miller added that additional field studies are recommended, especially with the mechanical drum shakers, to achieve a more economical, reliable, and realistic level of thinning and final crop load.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at


Original Article:

Performance of Mechanical Thinners for Bloom or Green Fruit Thinning in Peaches
Stephen S. Miller, James R. Schupp, Tara A. Baugher, and Scott D. Wolford
HortScience 46:43–51. [Abstract] [Full Text] [PDF]

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