Acta Hortic. 1119. ISHS 2016. DOI 10.17660/ActaHortic.2016.1119.1 XXIX IHC – Proc. Int. Symposia on Abscission Processes in Horticulture and Non-Destructive Assessment of Fruit Attributes Eds.: J.N. Wünsche et al. 1 Managing fruit abscission in apple T.L. Robinson 1 , A.N. Lakso 1 , D. Greene 2 , G. Reginato 3 and A. De R. Rufato 4 1 Department of Horticulture, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456, USA; 2 University of Massachusetts, Amherst, MA, USA; 3 University of Chile, Santiago, Chile; 4 EMBRAPA, Vacaria, Brazil. Abstract Apple trees naturally set many more fruits than desired thus requiring active crop load management to achieve optimum fruit size and to ensure adequate return bloom. Chemical thinning is the primary method used to reduce crop load but despite 50 years of experience, it remains an unacceptably unpredictable part of apple production with large variation from year to year and within years. Our research suggests the variability in chemical thinner efficacy is related both to stage of fruit development and carbohydrate availability to support fruit growth. There is low sensitivity to chemical thinners when fruits are small at petal fall (about 4 mm diameter) followed by high sensitivity of rapidly growing fruits between 8-15 mm and then low sensitivity once fruits reach 20 mm. The basis for the differing sensitivity is not clear. A second source of variability is the availability of carbohydrates to support fruit development. Weather has strong effects on carbohydrate production and utilization. We have estimated carbohydrate supply and demand for fruit growth using the Cornell MaluSim carbohydrate prediction model and have related the carbohydrate balance to chemical thinning efficacy. Simulations over several years showed that there are often periods of particularly negative or positive carbon supply:demand balance, which were associated with severe thinning or mild thinning. We have also related the growth rate of fruits to fruit abscission. We have developed an integrated method to more precisely manage chemical thinning that utilizes estimated carbohydrate supply to the fruits and actual fruit growth rate measurements to provide real time information to fruit growers to manage thinning. Keywords: Malus × domestica, preharvest fruit drop, chemical thinning, precision crop load management INTRODUCTION Fruit abscission in apple orchards is actively managed at 2 times during the growing season. Early in the growing season between bloom and 4 weeks after bloom, and then later in the season near harvest. Early in the season fruit growers attempt to induce fruit abscission to reduce crop load via one or more sprays of chemical thinning compounds (Greene, 2002) while late in the season growers attempt to prevent preharvest fruit abscission via chemical drop control compounds. In this report we will only discuss management of early season fruit abscission. A discussion and model of the late season fruit abscission was published by Robinson et al. (2010). Apple trees produce many more flowers than needed and then shed a significant number in the first 30 days after anthesis (Robinson et al., 2014). Some of the flowers abscise due to lack of pollination or fertilization while others abscise after fertilization but before fruits are 30 mm in diameter. The central flower of each floral bud (king flower) is least likely to abscise while the upper laterals are most likely to abscise (Goffinet and Lakso, 2014). In some years almost all of the laterals abscise while in other years only 1 or 2 of the 4-5 lateral fruitlets abscise. But in almost all years, too many fruits remain on the tree without intervention for commercially optimum fruit size and good return bloom. Typically 90% of the fruitlets must be eliminated to allow the remaining 10% to achieve optimum commercial size. Prior to the emergence of chemical thinning compounds apple growers used hand thinning to reduce crop load; however, this practice which is done mid-season