Chemical thinning of peaches was conducted under orchard conditions using CPA, followed by standard hand thinning, on the cultivars ‘Jerseyqueen,’ ‘Sunhigh,’ ‘Redhaven’, ‘Sunqueen’ and ‘Rio Oso Gem.‘ Timing of the 3 sprays at 150 ppm was based on the water volume displacement of 100 fruits selected at random from each plot: a) at 200 ml displacement, b) at 280 ml, and c) at 280 ml plus 4 days. The control was hand thinned only. Ovule length was measured at each spray timing and found to be 7–10 mm at the 280 ml water displacement; both ovule length and water displacement techniques were used to time CPA sprays in over 2000 acres of New Jersey orchards in 1968.
Timing of the CPA thinner for optimum effectiveness in this experiment varied with cultivar. Individual fruit weight was greater on CPA-thinned trees than on the controls for the cultivars ‘Redhaven,’ ‘Rio Oso Gem,’ ‘Sunqueen’ and ‘Sunhigh’ at harvest. Chemically thinned ‘Jerseyqueen’ did not show an increased mean fruit weight at harvest. Yield reductions occurred on ‘Sunhigh’ at the 280 ml timing and on ‘Redhaven’ at the 280 ml + 4 days timing, due apparently to subsequent over-thinning by hand. Shading increased the thinning effectiveness of CPA on all cultivars. ‘Jerseyqueen’ and ‘Redhaven’ were difficult to thin with CPA under the conditions of this experiment. ‘Rio Oso Gem’ was moderately difficult, while ‘Sunhigh’ and ‘Sunqueen’ were thinned readily.