Precision crop load management: the practical implementation of physiological models

T.L. Robinson, A.N. Lakso, D.W. Greene
The economic impacts of achieving the proper crop load each year are large (often $ 12,000-25,000 ha-1) and justify a more intense effort to manage crop load to achieve the optimum fruit number tree-1 each year. Precision crop load management (PCLM) is a new strategy we have developed that begins with defining the optimum fruit number tree-1 (target fruit number) then applying sequential chemical thinning sprays with rates and timing guided by the Cornell carbohydrate balance thinning model to predict thinning efficacy and subsequently by the fruit growth rate model to assess thinning efficacy in time to allow re-treatment when needed with the goal of reducing fruit number tree-1 in a step wise manner to the target fruit number to optimize crop value and reduce hand thinning costs. The apple carbohydrate thinning model exists on a web server at Cornell University, which is available over the internet at the NEWA site ( It is linked to on-farm weather stations from which the model uses temperature and sunlight data from the date of bud-break in the spring to daily calculate tree carbohydrate balance. The web version of the carbohydrate model also uses weather forecasts for prediction of carbohydrate balance 7 days into the future. The web site allows apple growers or consultants to run the model and receive predictions in real time of carbohydrate balance and suggested chemical thinner doses. The fruit growth rate model is used to rapidly assess the effect of each chemical thinning spray within 1 week after each spray. The PCLM program was implemented in 2013 with more than 20 fruit growers, consultants and extension field staff in New York, Massachusetts, Vermont, and New Jersey with 'Gala' and 'Honeycrisp' apple. The results of fruit diameter measurements made after petal fall thinning sprays around May 19 or 20 showed that the bloom and petal fall sprays provided significant thinning on 'Gala' and 'Honeycrisp' but that additional thinning was still needed. This suggested another spray at the 10-12 mm fruit size stage. Results of fruit diameter measurements after the 10-12 mm spray indicated another final spray was needed at 18-20 mm fruit size stage.
Robinson, T.L., Lakso, A.N. and Greene, D.W. (2017). Precision crop load management: the practical implementation of physiological models. Acta Hortic. 1177, 381-390
DOI: 10.17660/ActaHortic.2017.1177.55
Malus × domestica, fruit thinning, fruit growth rate, carbon balance, carbohydrate

Acta Horticulturae