The Effect of Timing and Rate of Metamitron Sprays on Chemical Thinning of 'Brookfield Gala' Apple Trees

During the 2012–13 season a trial was performed in four 'Brookfield Gala'/M9 orchards, located along the Chilean apple production area: Quinta de Tilcoco (VI R, 34°21'S, 70°57'W), Chimbarongo (VI R, 34°42'S, 71°02'W), Yerbas Buenas (VII R; 35°45'S, 71°34'W), and Renaico (IX R; 37°48'S, 72° 3'W), with the aim of determining the effect of chemical thinning of different rates and application timing of metamitron (Goltix® 70WG). In each orchard, two trees were randomly selected and applied at 0, 85, 170 or 340 ppm of metamitron (0, 146.4, 291.6, or 582 g/ha), either at petal fall or 11 mm fruit size. Minimum and maximum temperatures and solar radiation were registered in each location in a period of 3, 6, and 9 days after the metamitron sprays. Response curves were established for each orchard, assessing the thinning effect in two selected branches fifty days after full bloom, and expressing it as fruit per cluster considering the control treatment as 100%. In all orchards the fruit thinning increased with metamitron concentration, reaching 50% of control, even though higher response rate was observed between 0 and 170 ppm. The most effective treatments were always those applied at petal fall (between 3% and 17% more than at 11 mm fruit size). Much less thinning was obtained in Renaico, and no climatic characteristic could be associated to this for the 9 days period after the metamitron sprays.