Young apple ( Malus × domestica Borkh.) orchards are intensively managed to ensure successful orchard establishment. Use of growth retardants may aid in disease management; however, concerns regarding reduced bearing surface and increased time to full production are disincentives for use in young orchards. Prohexadione–calcium (P-Ca) is a gibberellin synthesis inhibitor used in apple to manage vigor and fire blight ( Erwinia amylovora ). Acibenzolar- S -methyl (ASM) is a systemic acquired resistance inducer used to induce an immune response in plants to protect against fungal and bacterial pathogens. The objective of the study was to apply different suggested rates of P-Ca and ASM alone and in combination to determine their impacts on shoot growth rate, tree size, canopy infill, and crop load. In 2021 to 2024, the study was conducted in a newly planted experimental ‘Gala’ orchard with a factorial treatment structure of two factors P-Ca (0, 42.5, and 125 mg·L −1 ) and ASM (0 and 37.5 mg·L −1 ). Treatments were applied twice each season, on the same plots each year to determine cumulative effects. As P-Ca rate increased, shoot growth rate and tree height decreased. Even with a decrease in tree size, canopy infill and linear bearing surface were maintained regardless of P-Ca or ASM rates. Fruit set was increased and return bloom decreased as P-Ca rate increased, but the addition of ASM lessened these effects. These results indicate that repeated applications of P-Ca on young orchards, in high-vigor locations, may reduce tree size, but not to a level that limits orchard establishment. P-Ca may increase fruit set, but ASM may have utility in lessening this effect and managing crop load.
Vogel et al. (Wed,) studied this question.