Effect of time of hand-thinning on apple fruit size

The effect of time of hand-thinning on fruit size of Starking, Starkrimson and Starkspur Golden Delicious was studied during two consecutive seasons. In the first season trees were thinned at weekly or fortnightly intervals, commencing at full bloom until eight weeks later, leaving one flower or fruit per cluster. In the second season Starking was thinned to two fruits and Starkrimson and Starkspur Golden Delicious to one fruit per cluster at six weeks from full bloom (WFFB). Thinning during the first two WFFB of the first season significantly increased the cell number in the cortical region of fruit of a similar size. Cell division ceased during the fourth WFFB. Fruit number per tree affected cell number and consequently harvest size significantly. Late thinning compared with early thinning progressively reduced fruit size when similar fruit numbers were present per tree. The differences in fruit size owing to thinning treatments were apparent at six WFFB and persisted until harvest. Harvest size on trees which were thinned at six WFFB during the second season was related to thinning treatments carried out during the first season.