In order to elucidate the influence of tree age on the thinning effect of Peach-thin, experiments had been conducted on three groups of peach trees (cultivar Hakuho); Group one 12-16 years old (1961-65), Group two 4-7 years old (1962-65), and Group three 4-6 years old (1967-69). In every season, 200ppm solution of Peach-thin was sprayed two or three days after full bloom. In addition, twice spraying of 200ppm solution two and four days after full bloom and 400ppm spray two days after full bloom, were applied in some cases. Thinning effect of Peach-thin was evaluated by comparing percentages of fruit setting of sprayed branches with that of corresponding unsprayed branches about 25 days after full bloom.
On four years old trees, percentages of fruit setting were very low and variable, and thinning effect of Peach-thin was severe, too.
On five or more years old trees, percentages of fruit setting were generally high and stable, and effect of Peach-thin was mild. A high correlation was obtained between the fruit setting percentages of control branches and those of treated branches with 200ppm solution (η=0.519). In the case of mature trees only (12-16 years old), a higher correlation was obtained (r=+0.781).
Influence of tree age on thinning effect of twice 200ppm spray and 400ppm spray were not so clear owing to the lack of data on four or five years old trees.
A high correlation was obtained between the fruit setting percentages of control branches and those of sprayed branches (twice 200ppm spray r=+0.644, 400ppm spray r=+0.515). Correlations in individual years were higher than that throughout the experimental period (twice 200ppm spray 1962 r=+0.996, 1963 r=+0.748, 1969 r=+0.951; 400ppm spray 1962 r=+0.988, 1964 r=+0.868, 1965 r=+0.651).