Identifying the optimum thinning time for red ‘Fuji’ apples

Two trials assessed the relationship between time of spray application and the effectiveness of naphthalene acetic acid (NAA) or ethephon on thinning of red ‘Fuji’ apples. In particular the effectiveness of sprays at, before, or just after full bloom (FB) were assessed. Both NAA and ethephon were effective thinners at FB. The thinners differed in efficacy with time; NAA was less effective before and after FB, ethephon was efficient before FB but this thinning power declined over time linearly, becoming ineffective 5-6 d after full bloom (DAFB) Thinning effectiveness was reflected in improved mean fruit weight and size. With NAA, increasing the concentration from 10 to 20 mg P1 did not improve fruit thinning. Whereas NAA sprays 5-10 DAFB reduced pip numbers to fewer than the controls, ethephon sprays did not result in such large reductions. It was concluded that the most effective time to thin red ‘Fuji’ was at or near FB. This recommendation is firm for ethephon but further clarification is needed for NAA. This conclusion supports some work for other cultivars. However, the problem of the preoccupation with fruit set by many orchardists in countries with climates cooler than Australia is recognized as a continuing problem for the recommendation for such general early use of thinners.