Commercial-Scale Use and Influence of 1-aminocyclopropane-1-Carboxylic Acid as a Rescue Thinner on Fruit Set and Early Fruit Growth in Apples

Only a limited number of post-bloom thinners is available for apple (Malus domestica Borkh.) at the present time. The influence of 1-aminocyclopropane-1-carboxylic acid (ACC) in two strains of ‘Gala’, one strain of ‘Delicious’, and one strain of ‘Fuji’ apples, when fruitlet diameter was about 20 mm, on fruit set and fruit size at hand-thinning in Southwest Idaho in the Intermountain West region, USA was studied. Applications of ACC reduced fruit set in ‘Schlect Spur Delicious’ in 7 and 30 days after application. Acc applications reduced fruit set in ‘Gala’ and ‘Fuji’ apples 30 days after application. Fruit weight after 30 days of ACC application (hand thinning time) in trees receiving ACC was higher than those in Untreated Control and the difference was significant only in ‘Schlect Spur Delicious’ apple. Application of ACC at 300 mg·L−1 or 365 mg·L−1 in this study did not have any advese effects on foliage or fruit skin in any of the tested cultivars in this project.