Precision Chemical Thinning—A Useful and Practical Guide for Apple Growers

Precision thinning is a strategy to increase the efficacy of chemical thinning. The overall concept is to define the optimum fruit number per tree and try to thin to that specific number through the precision thinning program. The method relies on five main steps: 1) identifying the target number of fruit per tree; 2) using the Cornell Apple Carbohydrate Thinning model to predict thinning response; 3) applying the thinners; 4) using the Fruit Growth Rate model for early assessment of thinning response; and 5) re-applying the thinners if needed. Beginning in 2012 we organized a statewide New York group effort in order to manage chemical thinning of 'Gala' and 'Honeycrisp' more precisely. In practice, growers were advised to follow a sequence of thinning sprays to achieve their target crop load beginning with a spray at bloom, followed by a spray at petal fall. Then, if needed (defined by fruit growth rate model) another spray at 12 mm fruit size and if needed a final spray at 18 mm fruit size. The carbohydrate model was used to guide the rate of chemical and timing while the fruit growth model indicated the percentage of thinning achieved. In 2013 and 2014 most 'Gala' blocks did not thin enough showing a very heavy crop. In these conditions starting the thinning program with bloom sprays was a valuable tool and reduced the amount of hand thinning required. Most 'Honeycrisp' blocks did not thin enough in 2013 but some slightly overthinned in 2014. Bloom thinning sprays were quite effective in 2013 but less so in 2014. Bloom sprays of Maxcel showed great efficacy. The 10–12 mm spray was not effective in 2013 partially due to the positive carbohydrate balance, however it gave good thinning results in 2014. The sequential sprays provided excellent crop load control. The carbohydrate model can be a guide to let growers know whether or not to spray, how much to spray, what days to avoid spraying. However, the fruit growth rate model has been the best method to physically access, in a more precise way, thinning efficacy. The use of both models together in the precision thinning program is a useful tool to optimize crop load and fruit size and a great way to improve profitability.