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

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... nt 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.