2021 Volume 20 Issue 1 Pages 79-85
NAC (1-naphthyl N-methylcarbamate) has been used as a fruit thinner in apple orchards of Japan. It has long confused apple growers that the thinning efficacy of NAC varies widely from year to year. So, we searched for factors that could influence the thinning efficacy in weather data and data that describe conditions of trees, and tried to estimate the annual variation of the efficacy by developing a regression model using the factors. Whenever trees were treated with NAC, the time at which fruitlets of the trees stopped growing was the same as the time as when the untreated fruitlets destined to drop physiologically stopped growing. The thinning efficacy could be largely estimated by four factors: the number of fruitlets within a cluster when treated with NAC, the number of leaves on a cluster, the maximum temperature for three days after treatment, and the solar radiation for one week after three weeks of treatment. The regression model using the four factors as independent variables indicated that fruitlets on a cluster with many fruitlets and few leaves easily fall, especially under the conditions of a high maximum temperature and low solar radiation. The difference in the thinner efficacy by years was mainly due to the annual fluctuation of solar radiation for one week after three weeks of treatment.