Thinning apples with 1-aminocyclopropane carboxylic acid: influence of cultivar, rate and timing

Crop load management of apple trees is necessary to improve fruit size, quality of apples and reduce biennial bearing. As an alternative to hand thinning, a new commercial thinning compound, 1-aminocyclopropane-carboxylic acid (ACC), has been in development for over a decade. A series of eight experiments were conducted to evaluate the efficacy of 1-aminocyclopropane- carboxylic acid (ACC) as a full bloom (FB) or post-bloom thinner for Ambrosia, Gala, Honeycrisp, and Crimson Crisp apple trees. Trees were treated with ACC at rates from 200-400 mg.L -1 ; different timings from FB to 20 mm; single, combination or sequential sprays of ACC with 6-benzyl adenine (6-BA) or 1-naphthaleneacetic acid (NAA); and two ACC formulations. Treatments were compared with untreated trees, hand thinned control (HTC), and grower control sprays of carbaryl, NAA, 6-BA, or combinations thereof. In all studies, single applications of ACC up to 400 mg.L -1 for fruitlet sprays and 600 mg.L -1 for full-bloom sprays failed to reduce fruit set or crop load compared to untreated trees. However, regression analyses indicated a significant linear or quadratic reduction in fruit set and crop load with increasing rates of ACC applied post-bloom in three of the six experiments. Ethylene emission peaked in fruits 1d after ACC was applied at 8-10 mm but was not detected when applied after 15-20 mm. Trees treated with carbaryl control sprays had comparable crop loads as the HTC and effective as a fruitlet thinner, but not in all experiments. ACC was not a sufficient substitute for carbaryl when used alone or tank mixed with 6-BA or used as a sequential spray. Further studies evaluating higher rates of ACC, using ACC in combination or sequence with other thinning agents, as well as understanding the reasons for its inconsistent thinning response are warranted.