Effects of Different Chemical Thinning Strategies on Yield Performance, Fruit Quality, and Multivariate Responses in Intensive Apple Production

Optimizing crop load through chemical thinning is essential for yield stability and fruit quality in intensive apple production, yet the relationships between tree growth, yield components, and fruit quality remain insufficiently characterized. This three-year study (2022–2024) evaluated four thinning strategies (control, ammonium thiosulfate [ATS]+ethephon, benzyladenine [BA]+ethephon, and ATS+BA+ethephon) across parameters of the apple cv. ‘Gala Schniga Schnitzer’: trunk cross-sectional area (TCSA), fruit yield (Y), fruit number per tree (FNT), crop load-number (CLnm), crop load-kg (CLkg), fruit size (FS), fruit weight (FW), water-soluble solids content (SSC), and fruit firmness (FF). Chemical thinning and year significantly but differentially affected yield components and fruit quality traits. In the ATS+BA+ethephon treatment, FS in 2023, FW in 2023 and 2024, and Y in 2024 significantly exceeded the control treatment in the respective years. Correlation analysis revealed strong positive relationships between FNT–Y, CLnm–CLkg, and FW–FS, with the strongest association observed between FW and FS (r = 0.98 in 2022 and 2023). Regression analyses confirmed significant linear relationships between FW–FS, FNT–Y, and CLnm–CLkg across thinning treatments. PCA explained 66.93, 54.89, and 50.91% of total variance by PC1 and PC2 in 2022–2024, respectively, with FS and FW showing the longest vectors in all years. The PCA structure varied among years, reflecting differences in the relative contributions of yield, crop load, tree growth, and fruit quality traits. Overall, chemical thinning should be considered not merely for reducing fruit number, but for regulating crop load and balancing productivity and fruit development, with effects varying by trait and year.