The current understanding of apple stem cold hardiness is very broad- there are certain cultivars generally accepted to be cold hardy or tender. There has been no comprehensive study of cold hardiness of apples at multiple timepoints over multiple years. In this study, cold hardiness was evaluated for 21 rootstock and 23 scion genotypes monthly throughout the dormant season over 4 years. Cold hardiness was measured using the electrolyte leakage method and the temperature at which 25% tissue damage occurs (LT25) was used as the cold hardiness phenotype. The four study winters varied drastically in intensity and had a significant effect on genotype cold hardiness. Contrasting phenotypic responses in cold versus mild winters demonstrated clear environmentally driven differences between inferior and superior genotypes. Another major finding from the study is the rapid deacclimation of rootstocks compared to scions in late winter/early spring- we observed that differences in mean hardiness between groups exceeded 6 °C in some years. Overall, this study represents the largest apple cold hardiness experiment to date in terms of cultivars used and timepoints examined, resulting in both novel implications for how grafted apples navigate cold hardiness during dormancy and which cultivars are best suited for unusual but increasingly common winter conditions.
