Rootstock and crop load affect maturity and quality of ‘Honeycrisp’ apples under Washington State growing conditions

The apple cultivar ‘Honeycrisp’ is difficult to grow in semiarid environments, such as the apple growing area of Washington State, USA, where horticultural practices need to be carefully managed to obtain optimal fruit quality. Rootstocks from the Geneva^Ⓡ series have become popular throughout the USA and in apple-growing regions worldwide, but knowledge about their performance (e.g., mineral uptake, impact on fruit quality, photosynthetic efficiency, etc.) under Washington State growing conditions is still limited. This research was designed to assess fruit maturity and quality of ‘Honeycrisp’ grafted on five rootstocks and three crop load levels. ‘Cameron Select^Ⓡ Honeycrisp’ trees grafted on rootstocks ‘B.9’, ‘M.9-T337,’ ‘G.935’, ‘G.41’ and ‘G.11’ were selected in 2019–2020 in a commercial orchard in Quincy, WA, and hand thinned to the target crop loads of 3, 5, and 7 fruit/cm² trunk cross-sectional area. At commercial maturity, the crop from all trees was harvested, sized, and assessed for bitter pit incidence. A subsample of fruit was set aside to conduct quality analyses one month after regular air storage. Rootstocks affected fruit maturity and quality characteristics. ‘B.9’ produced more mature and better colored apples, with lower firmness and titratable acidity. When thinned to 3 fruit/cm², ‘G.11’ had the highest bitter pit incidence among rootstocks over the two years of the trial. Across rootstocks, yield increased with increasing crop load, and fruit size was optimal (∼ 200–220 g) with crop loads 5 and 7 fruits/cm². Maturity and quality of apples from the crop load levels tested in this study confirmed results from previous trials on ‘Honeycrisp’.