抄録
The thinning of flowers or fruit is an essential part of the commercial production of quality apples. Although chemical thinners are generally used in apple orchards throughout the world, hand-thinning remains essential for controlling crop load to improve fruit quality and return bloom. The objectives of this study were to investigate factors that determine the time required for hand-thinning and to elucidate cultivar-related efficiency of hand-thinning. In all cultivars, the time taken for hand-thinning per cluster became shorter after petal fall, but the degree of this decrease differed among cultivars. The time taken for hand-thinning depended on both how much and when natural abscission of fruitlets occurred. The times required for hand-thinning were very similar among clusters with four, five, or six flowers/fruitlets, which was twice as long as that required for clusters with two or three flowers/fruitlets. This means that the time required for hand-thinning clusters is critically reduced when the number of flowers/fruitlets within a cluster is three or fewer. The time required for hand-thinning clusters of axillary buds became significantly longer from bloom to 7 days after bloom, and then decreased gradually from 7 to 25 days after bloom. When the proportion of clusters with three or fewer fruitlets is 50% at 15 days after bloom, the planting area of the cultivar that a person is able to hand-thin (six hours per day) from 10 to 30 days after bloom was 24.3 a, which was 40% larger than that of a cultivar in which the proportion of clusters with three or fewer fruitlets is 50% at 30 days after bloom (17.4 a). Therefore, the introduction of cultivars in which fruit abscission occurs at an early stage and on a large scale is a solution for reducing the labor costs of hand-thinning.
