395 Effect of Concentration, Application Volume and Addition of a Surfactant on Response to Benzyladenine as Thinning Agent in ‘Williams’ Pears M. Curetti, R. Rodríguez, C. Magdalena and A. Rodríguez Instituto Nacional de Tecnología Agropecuaria Estación Experimental Agropecuaria Alto Valle CC 782, General Roca (8332), Río Negro Argentina Keywords: Pyrus communis, ‘Bartlett’, crop load, fruit set, fruit size, cytokinin, BA Abstract Benzyladenine (BA) is an effective thinning agent for ‘Williams’ pear, the most important cultivar in Argentina. The aims of this research were: to find the minimum effective BA rate for increasing fruit size in ‘Williams’ pear, and to improve application efficiency by modifying volume rate and adding a silicone surfactant. Trials were carried out over three seasons (2007-2010). In the two first seasons different rates were evaluated: 50, 100, 150 and 200 mg/L. Applications were made when fruit diameters were 10-12 mm. In the third season, two application volumes (450 and 1300 L/ha) and the addition of a silicone surfactant (Silwet ® ) were evaluated with BA at a rate of 100 mg/L. Variables determined were: coverage percentage, fruit drop, fruit number per 100 flower clusters (fruit set), fruit number per branch cross sectional area, fruit number per tree, fruit weight and distribution of yield depending on fruit size. No significant differences between 150 and 200 mg/L BA were found. These treatments reduced fruit set by 20-40% and fruit number per tree by 10-35% compared to the control. At harvest, fruit weight was improved by 10-20%, yield of small fruit (diameter <65 mm) was reduced by 40-65% and the yield of large fruit (diameter >70mm) increased by 60-100%. The 100 mg/L BA rate was less effective with only 10-20% reduction in fruit set. At harvest, this treatment showed 5% higher fruit weight, 10% less yield of small fruit and up to 30% higher yield of large fruit. The lowest BA rate (50 mg/L) did not stimulate a thinning effect and/or increase fruit size at harvest. In the third season, the surfactant addition improved the percentage coverage and efficacy of BA on fruit size at harvest. INTRODUCTION Fruit size, a crucial parameter of fruit quality in ‘Williams’ pears, is negatively correlated with crop load. Fruit thinning is the most important technique used to improve fruit quality (Link, 2000). Several cytokinins are currently being evaluated as fruit thinners, especially benzyladenine (6-benzylaminopurine, BA) (Wertheim, 2000). BA is considered to be a natural plant compound that provides very little risk to the environment because its toxicity to mammalian, bees and arthropod species is low (Bubán, 2000). Several mechanisms have been suggested to explain the thinning action of BA, based on an intensified competition among fruit, and between fruit and vegetative growth (Bubán, 2000; Chabikwa, 2008). BA inhibited net photosynthesis of apple leaves and reduced the carbohydrates available to developing fruit (Yuan and Greene, 2000; Stopar et al., 2001). BA stimulated vegetative growth (Elfving and Cline, 1993; Dal Cin et al., 2007), reduced the auxin export of fruit (Schröder and Bangerth, 2006) and increased ethylene production in both leaves and fruit (Greene et al., 1992; Emongor and Murr, 1994; Dal Cin et al., 2007). BA is an effective fruit thinning agent in apple (Greene and Miller, 1984; Greene and Autio, 1989; Greene et al., 1990, 1992; Elfving and Cline, 1993; Emongor and Murr, 1994; Bound et al., 1997). Effective concentrations of BA employed cover a range of 50 to 200 mg/L and depends on the cultivars to be treated (Bubán, 2000; Wertheim, 2000). The best application time coincides with a fruitlet diameter of 8 to 12 mm (Greene, 1993; Proc. 11 th International Pear Symposium Eds.: E. Sánchez et al. Acta Hort. 909, ISHS 2011