Apple crop load management with special focus on early thinning strategies: A US perspective

There have been several reviews of apple crop load management practices, but few have focused on crop load management strategies prior to petal fall. While use of chemical thinners has been the primary method of managing crop load in modern fruit production, cultural practices such as pruning have been evaluated for use as primary or secondary methods to reduce cropping potential, and initial results appear promising. A significant research effort to screen and develop blossom-thinning products occurred after the loss of Elgetol in 1990. At least 150 chemicals have been evaluated, and the bulk of this review summarizes and catalogs apple blossom thinners. Despite these efforts, the number of chemical blossom thinners that can be legally applied in the USA is limited. However, the development of predictive models to optimize the timing of blossom-thinning applications may dramatically improve the consistency of chemical blossom thinning. Multiple mechanical thinning devices have been developed and are generally effective in reducing cropping potential. However, mechanical thinning technology has some limitations. Since the most recent review on apple crop load management two decades ago in 1999, there have been no new registered postbloom thinners released for commercial use in the USA. However, some new thinning products and decision-making aids show promise, and these are discussed.