Effect of fruit thinner on fruit set and quality in ‘Hongro’ and ‘Fuji’ apples

Benzyladenine (BA, 99% purity), MaxCel® (1.9% BA), Fruitone (3.5% NAA), MaxCel® + Fruitone, and simazine were applied postbloom as fruitlet thinning agents to mature 'Hongro' and 'Fuji' apple (Malus domestica Borkh.) trees. BA and MaxCel® were applied at 100 mg·L⁻¹ a.i. while Fruitone at 0.1 mg·L⁻¹ a.i. and simazine at 400 mg·L⁻¹ a.i. All PGRs were applied at 8 days after full bloom (DAFB, 6 mm fruit diameter) in both cultivars, while simazine was treated twice at 7 and 14 DAFB. In 'Hongro', the number of total fruit set per flower cluster in terminal buds was 1.67, 1.84, and 1.81 in MaxCel® + Fruitone, MaxCel®, and simazine applications, respectively, when compared with 2.35 of water control. These reductions in fruit set were mainly attributed to the increased ratio of defruited clusters by the thinning agents. In 'Fuji' apple, the number of total fruit set per flower cluster in terminal buds was 1.29, 1.60, and 1.76 in MaxCel® + Fruitone, Fruitone, and MaxCel®, respectively, when compared with 2.56 of water control in 'Fuji' apple. The addition of Fruitone to the MaxCel® promoted the thinning efficacy in both cultivars, compared to MaxCel® only. The thinning efficacies were similarly observed with lateral flowers in both cultivars. A significant increase of fruit weight by the postbloom thinning treatments was observed only in the BA application in 'Hongro', while the effect was observed in BA and MaxCel® in 'Fuji'. While the soluble solids content increased in the BA, MaxCel® and MaxCel®+Fruitone treatments in both cultivars, other fruit quality attributes were not affected by the application of post-bloom thinning agents.