Manejo de la carga en manzanos `Early Red One´ y `Rosy Glow´

 Apple trees set more fruit than desired, thus it is necessary to adjust the crop load to achieve a better fruit size and an adequate return bloom, thereby avoiding biennial bearing. The objective of this thesis was to evaluate different chemical products with a thinning effect. Applications were made of: calcium polysulfide (calcium sulfide mixture/sulfocalcic mixture), Metamitron, naphthaleneacetic acid (NAA), and 6-benzyladenine (6-BA). Two trials were conducted in the province of Canelones, Uruguay, during the 2021/2022 season on two apple cultivars: 'Early Red One' and 'Rosy Glow'. In 'Early Red One' apple, five treatments were evaluated: 1) trees without application (control); 2) calcium polysulfide at 3% concentration (2 applications, 60% of open flowers and four days later); 3) 100 mg L^-1 Metamitron applied between 8-12 mm of the fruit; 4) 200 mg L^-1 Metamitron applied between 8-12 mm of the fruit; and 5) NAA at 10 mg L^-1 at full bloom along with NAA at 7.5 mg L^-1 + 6-BA at 100 mg L^-1 at petal fall. In 'Rosy Glow' apples, five treatments were also evaluated: 1) trees without application (control); 2) calcium polysulfide at 3% concentration (3 applications, 80% of open flowers, two and five days after the first application); 3) 200 mg L^-1 Metamitron applied between 8-12 mm of the fruit; 4) 400 mg L^-1 Metamitron applied between 8-12 mm of the fruit; and 5) NAA at 10 mg L^-1 at full bloom along with NAA at 10 mg L^-1 + 6-BA at 100 mg L^-1 at petal fall. In all treatments, manual thinning was carried out around 50 days after full bloom. The following parameters were evaluated: percentage of fruit set, amount of thinned fruit, time spent in crop load adjustment (manual thinning), number of fruits per plant, average fruit weight, yield per plant, estimated yield, yield efficiency, fruit diameter and height, height/diameter ratio (constructed variable), percentage of red color in the fruit, russeting, penetration resistance (flesh firmness), starch index, total soluble solids, and number of seeds. The efficiency ranking of the treatments varied according to the cultivar. In 'Early Red One', the most efficient treatment was the application of NAA at full bloom along with NAA + 6-BA at petal fall, and in 'Rosy Glow', it was the application of 400 mg L^-1 Metamitron. The shortest thinning time required to adjust the load was observed in the NAA treatments at full bloom together with NAA + 6-BA at petal fall and 400 mg L^-1 Metamitron ('Early Red One' and 'Rosy Glow' cultivars respectively). In 'Early Red One', no differences in fruit size were observed between treatments. In 'Rosy Glow', the treatments that presented larger fruit diameter were: 3% calcium sulfide mixture/sulfocalcic mixture applied at 80% of open flower, 200 mg L^-1 Metamitron at 8-12mm, and NAA at 10 mg L^-1 at full bloom + NAA 10 mg L^-1 + 6-BA 100 mg L^-1 at petal fall. In 'Early Red One' estimated yield was not affected in any of the treatments compared to the control, but in 'Rosy Glow', there was a difference of approximately 18 tons ha^-1 between untreated control trees and the other treatments. The cost to carry out the load adjustment of the most efficient treatment in 'Early Red One' was USD 978.31 ha^-1, and in 'Rosy Glow' it was USD 437.78 ha^-1. In both cultivars, most of these costs were affected by the manual load adjustment (required in any of the evaluated treatments to reach the final number of desired fruit tree^-1/ the desired final number of fruits per tree).