Chemical thinning of peaches. I

Abstract

Effect of N-1-naphthyl phthalamic acid (Peach-thin 322) sprays was tested on Hakuho peaches for three seasons from 1961 to 1963.1. In 1961, four concentrations, 50, 100, 200 and 400ppm, of Peach-thin were applied two days after full bloom to the branch of four Hakuho peach trees, which were twelve years old and moderately vigorous. Two trees of them were properly thinned by 200 and 400ppm sprays, but the two others were not.2. In 1962, the same trees as in the previous year were used. Two concentrations, 200 and 400 ppm, were sprayed two days after full bloom, and on some branches again four days after full bloom. Drop of fruits due to Peach-thin occurred from 10 to 25 days after full bloom. Differences were observed in the response to the sprays among the tested trees as in 1961, but were hardly found among the fruiting branches of various length. Once 400ppm and twice 200ppm sprays thinned fruits to the desirable level.Vigorous young trees of four years old were also treated in this year. Concentration of 200 and 400 ppm were applied to six trees. Fruits were heavily thinned by the sprays, and almost all fruits were dropped by twice 400ppm sprays.3. In 1963, the mature trees used in the previous years were treated with once and twice sprays of 200ppm solution. The effect was not sufficient even of twice sprays, but it was found that growth of young fruits on the sprayed branches was superior to that on the control branches.Effect on the young trees was not so significant as in the previous year.4. The results of experiments for three seasons indicated that thinning effect of Peach-thin was slight on the trees which were likely to set fruits heavily, and it was severe on those likely to have poor fruit setting. A very high correlation was obtained between the fruit setting percentages of control branches and those of branches sprayed with 200ppm Peach-thin at 25th day after full bloom.Peach-thin did not cause any permanent injury on twigs, leaves and fruits.