The efficacy of 1-aminocyclopropane-1-carboxylic acid (ACC) in thinning apples under Chilean conditions

In Chile, apple trees commonly show high fruit set: ∼2.0–2.5 fruits/cluster; therefore, strong chemical thinning strategies are needed to avoid or reduce hand thinning. 1-aminocyclopropane-1-carboxylic acid (ACC; Accede®), an ethylene precursor, has been registered as flower or fruitlet thinner for different fruit crops, but it must be validated according to species, cultivars and weather conditions. The aim of this study was (i) to evaluate the efficacy of ACC concentrations since bloom up to the timing of ‘rescue’ treatments, later in the season, and (ii) to evaluate it as part of commercial chemical thinning programs (CCTP). Six trials in ‘Brookfield Gala’’ and 5 in ‘Fuji Raku Raku’ were done during four seasons. Across the seasons and cultivars, ACC, alone or as part of a CCTP, reduced fruit set in a dose-moment-cultivar dependent manner. For 'Gala', ACC at full bloom (224 and 448 mg L⁻¹) was highly effective in thinning flowers, equivalent to the effect of 12 mg L⁻¹ of NAA. From petal fall up to fruits of 10–12 mm in diameter, 448 mg L⁻¹ reduced fruit set; higher concentration (672 mg L⁻¹) was required when fruits reached between 18 and 25 mm. At the same period 672 mg L⁻¹ or higher concentrations with a coadjuvant or oil showed slight to moderate leaf drop, depending on the season. The thinner efficacy of 400 mg L⁻¹ of ethephon was similar to all ACC concentrations (448–672 mg L⁻¹) at 25 mm; however, it showed lower fruit weight and yield than ACC treatments. For 'Fuji', higher ACC concentration was required; 672 mg L⁻¹ at full bloom reduced fruit set from 2.5 to 1.5 fruits/cluster, which is considered good effect for 'Fuji'. Between fruits of 4 mm up to fruits of 10–12 mm in diameter and under good weather conditions for thinning (i.e., projected less carbohydrate availability), 448–672 mg L⁻¹ reduced fruit set, but not under “bad” conditions for thinning (i.e., cold nights and hot sunny days). Similar results were observed for "rescue" treatments, when only 896 mg L⁻¹ reduced fruit set until targeted fruit set (0.8–1.0 fruits/cluster), while medium rates (448–672 mg L⁻¹) plus coadjuvant generated a moderate thinning effect, but only under favorable weather conditions. In conclusion, ACC is a promising thinner for full bloom and also as a "rescue" thinner for apples, particularly at the 18–25 mm stage when other thinners are ineffective.