Chemical Thinning of 'Gala' Apples with Treatments that Combine Sprays of NAA or BA during Flowering with Metamitron in Postbloom

During the 2012–13 season, a trial was performed in ‘Gala’/M7 apple trees in Quinta de Tilcoco (VI R), Chile, in order to determine the effect of chemical thinning treatments that either received or did not receive naphthalene acetic acid (NAA) (10 mg/L) or benzyl adenine (BA) (228 g/ha, 114 ppm) at bloom, combined with metamitron sprays (188 mg/L, 376 g/ha; Goltix® 70WG) at petal fall, 11 mm fruit size or both; additionally it was performed a control treatment, without any spray, and a treatment combining NAA at bloom (10 ppm) with carbaryl at petal fall and 11 mm fruit size (640 ppm; 1280 g /ha). Fifty days after full bloom, the effect of thinning was determined in two selected branches and expressed as fruit per cluster (F/C) and also as tree crop load, expressed as fruit per cm² trunk cross sectional area (F/TCSA). During harvest, yield efficiency (kg/TCSA) and mean fruit weight per tree was evaluated. All treatments differed from the absolute control (0.9 F/C, 10.71 F/TCSA). The more thinned trees were those which received two metamitron sprays (between 0.14 and 0.32 F/C; 2.72 and 4.13 F/TCSA). Metamitron sprays at petal fall (between 0.31 and 0.48 F/C, 4.11 and 6.44 F/TCSA) were generally more effective than sprays at 11 mm fruit size (between 0.42 and 0.59 F/C; 7.02 and 8.63 F/TCSA). According to the analysis of covariance, performed to remove the effect of crop load at harvest, fruit size and yield efficiency were about 30% higher in those more thinned treatments, showing the importance of early fruit removal in ‘Gala’ apples.