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Peach (Prunus persica L. Batsch) thinning is a costly and time-consuming but necessary practice to produce a crop of marketable size fruit. A number of mechanical devices and methods have been developed and evaluated to reduce the cost and time required for hand thinning peach. This report provides additional evidence that a Darwin string thinner can effectively thin peach at bloom and a spiked drum shaker can thin at bloom or at the green fruit (pit hardening) stage. Five trials were conducted over 2 years in grower orchards with trees trained to a perpendicular V system. A Darwin string thinner at 60% to 80% full bloom (FB) reduced crop load (fruit/cm2 limb cross-sectional area) on scaffold limbs by 21% to 50% compared with a hand-thinned control. At the 60% FB stage, a USDA-designed double-spiked drum shaker reduced crop load by 27% and in another trial, a USDA prototype single-drum shaker reduced crop load by 9%. Across all trials, the spiked drum shakers (single or double units) removed an average of 37% of the green fruit. All mechanical devices reduced the time required for follow-up hand thinning. Follow-up hand-thinning costs (US$/ha) were reduced an average of 27% by mechanical thinning devices over hand-thinned control trees. 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. Despite overthinning in some trials, the mechanical thinning devices described provide a potential alternative to hand thinning alone in peach production.

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