Apple trees suffer from lethal and sublethal injury when severely cold temperatures occur following a warm spell or after trees have deacclimated (Howell and Weiser, 1970; McArtney and Obermiller, 2011). In some apple cultivars, tree trunks are up to several degrees less hardy than young shoots and scaffold branches (Quamme and Hampson, 2004), making them particularly vulnerable to cold temperature injury after freeze-thaw cycles (Barritt et al., 2004; McArtney and Obermiller, 2011). When the trunk is damaged by cold temperature injury, every part of the tree is stressed by the reduced functioning of vascular tissues. This type of injury may increase in occurrence as winter and early spring temperature fluctuations occur more frequently or become more severe (Rochette et al., 2004), and with concomitant reductions in orchard yield and life span.
Apple and peach rootstocks affect scion cold hardiness, particularly in the lower trunk, with varying degrees of hardiness among different genotypes (Layne, 1994; Layne et al., 1977; McArtney and Obermiller, 2011; Simons, 1970; Westwood and Bjornstad, 1981). However, in most cases, hardiness characterizations have been based on external trunk injury and tree survival, which may be confounded with infection by pathogens (Holubowicz et al., 1982), particularly in long-term studies (Barritt et al., 2004). Identifying and selecting genotypes with slow deacclimation in response to warming temperatures in late winter could potentially prevent injury and death of trees and limbs. Many promising apple rootstocks that possess good shoot hardiness in midwinter may lack the genetic capacity to adequately retain cold tolerance during temperature fluctuations in late winter and early spring (Caprio and Quamme, 1999).
Selection of adapted genotypes is an effective method of preventing cold temperature injury, and a large number of new genotypes are available from breeding programs in New York (Geneva series); Ontario, Canada (Vineland series); and Russia (Budagovsky series). Among the Malling rootstocks, ‘M.9’ had greater injury than ‘M.7’, ‘M.26’, and ‘MM.106’ from natural freeze events (Wildung et al., 1973). A field experiment performed in the Champlain Valley, NY, with ‘Honeycrisp’ and ‘McIntosh’ as the scions showed that ‘M.7’ and ‘MM.106’ had very poor survival (<30%) following a midwinter cold event in 2004 (Robinson et al., 2006). In the same study, trees on ‘O.3’, ‘V.1’, ‘V.3’, ‘G.16’, ‘G.30’, and ‘Mark’ displayed the highest survival (>90%), whereas trees on ‘B.118’, ‘M.9 T337’, ‘B.9’, ‘M.9 Nic 29’, ‘Supporter 4’, ‘M.26’, and ‘MM.111’ had only 50% survival. These midwinter mortality events usually follow a warm period when the tree may have partially de-hardened. A trait of ‘M.7’ rootstock that might be contributing to its tenderness is that it was found to reduce the endodormancy (chilling) requirement of grafted scions, and by itself had a low endodormancy requirement (Couvillon et al., 1984; Young and Werner, 1984, 1985) suggesting that the mechanisms for endodormancy might overlap sensitivity to cold damage. These studies have shown cultivar variability in cold hardiness at a point in time in late winter, rather than the amount of hardiness that is lost when shoots are exposed to warm temperatures. In controlled deacclimation studies, cultivars of apple scions (Coleman, 1985), sweet and sour cherry (Mathers, 2004), peach (Shin et al., 2015), and hydrangea (Pagter et al., 2011) vary in the rate of deacclimation.
Naturally occurring freezes used to characterize rootstock cold hardiness are limited to genotypes in cultivation and can be confounded by other factors such as prior sublethal cold temperature injury and subsequent invasion by pathogens (Domoto, 1991; McArtney and Obermiller, 2011). Controlled freezing has shown ‘M.9’ to be less hardy in spring than ‘M.7’, ‘M.26’, and ‘MM.106’ (Wildung et al., 1973), and ‘M.26’ to be as hardy in late winter as ‘Ottawa 3’ and CG.10 (Holubowicz et al., 1982), but most of these cultivars have become less important in production. Based on natural loss of hardiness in April, ‘M.9’ and ‘G.214’ are less likely to be vulnerable to injury in late winter than ‘G.41’, ‘G.30’, and ‘G.814’ (Moran et al., 2018). Additional research involving controlled deacclimation is needed to identify vulnerability among genotypes to cold temperature injury.
The objective of this research was to compare loss in cold temperature tolerance in select Geneva, Malling, and Vineland apple rootstocks following a 2-d exposure to warm temperatures in late winter.
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Summary of the factorial arrangement of the temperatures during the 2-day exposure period, shoot ages and genotypes measured each year of the study.