Apple’s (Malus × domestica Borkh.) tendency towards biennial bearing necessitates management strategies such as fruit thinning and vigor control. Applications of plant growth regulators to manage crop load and vigor overlap in the spring. A precursor to ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC; Accede®) was recently registered as a chemical fruit thinner in the USA. Prohexadione-calcium (P-Ca) is a gibberellin biosynthesis inhibitor widely used to manage vigor. It is suspected that P-Ca may interfere with ACC precursors, decreasing thinning efficacy of ACC. We evaluated the effects and interactions of ACC and P-Ca on shoot growth, ethylene evolution, leaf area, and yield responses of mature ‘Golden Delicious’. The experiment was conducted in 2023, in Mills River, NC, USA. Treatments included ACC (400 mg∙L-1), P-Ca (250 mg∙L-1), ACC P-Ca, and an untreated control. Application of P-Ca occurred on three dates. ACC was applied when average fruit diameter was ~20 mm. Across all measured responses, there were no interactions between ACC and P-Ca. P-Ca had a negative main effect on shoot growth rate (48.7–75.4% reduction). ACC reduced leaf number of spurs 4 and 9 days after treatment (DAT). Ethylene evolution in fruit and spurs was increased by ACC 1, 4, and 9 DAT (spur: 1.1e4%, 5300%, and 515%, respectively; fruit: 9200%, 725%, and 222%, respectively). ACC had a negative effect on fruit number, yield, and crop density (60.1%, 56.5%, and 60.3% reduction, respectively). These results indicate that ACC and P-Ca can be used simultaneously to manage crop load and vigor, respectively
