A predictive model for Malus × pumila Borkh. 'Honeycrisp' to reduce storage risk in eastern New York State, USA

Malus × pumila Borkh. 'Honeycrisp' is a cultivar of major economic significance in the Northeastern U.S., but suffers severe losses to bitter pit (BP), whose incidence varies by growing region, year, and individual orchard block (specific cultivar × rootstock combination) and is known to increase during refrigerated storage. A three-year study (2016-2018) was established in eastern New York State orchards to identify variables associated with BP development with the objective of developing an economical BP prediction model for producers, consultants, storage operators and marketers to identify orchard blocks acceptable for storage. Thirty-six blocks with six trees each as experimental units, representing a wide range of BP incidence and rootstocks (M.9, B.9, M.26) were identified and studied in the Hudson Valley and Champlain Valley regions. Fruit sampled at 35 days pre-harvest was peeled, keeping the calyx and stem halves separate, and mineral content was analyzed. At harvest, mature fruit were sampled from three weekly commercial grade picks and maintained in regular storage at 2.2°C for 60 days with no preconditioning. BP incidence was evaluated at 60 days of storage. The ratio of magnesium to calcium was chosen as the most reliable predictor of BP. A quadratic regression model was constructed based on the Mg/Ca ratio modified by adjustment factors for region and rootstock and tested against an independent validation data set of 36 orchard blocks over two consecutive years (2017 and 2018). Using a 60-day storage threshold of 10% BP incidence or less, the model recommended the correct storage decision 72% of the time on an absolute basis, and in 100% of cases protected the decision-maker from storing fruit that would have exceeded 16.7% BP losses. Model performance was considered adequate for commercial trialing across all three growing regions of New York State in 2019, with continued performance improvements planned.