Phytophthora crown and root rot (PCRR) is among the most serious diseases of Persian walnut worldwide. In California, more than 10 species of Phytophthora have been implicated in the disease, but Phytophthora cinnamomi and P. citricola are especially aggressive and difficult to manage (Matheron and Mircetich, 1985a; Mircetich et al., 1998; Mircetich and Matheron, 1983). Incidence and severity of PCRR generally can be minimized by careful soil water management and use of PH rootstock (typically Juglans hindsii × J. regia) (Mircetich et al., 1998). Compared with NCB (J. hindsii) and Persian walnut (J. regia) rootstocks, PH is more resistant to several species of Phytophthora (Matheron and Mircetich, 1985b; Mircetich and Matheron, 1983). In practice, however, the resistance of PH has been insufficient to prevent losses caused by P. cinnamomi or P. citricola (Mircetich et al., 1998). Improved management strategies are needed for these two pathogens on walnut.
Chinese wingnut (Pterocarya stenoptera) (WN) has been explored to a limited extent as a rootstock for Persian walnut (Whitehouse and Joley, 1948; Zielinski, 1957). Seedlings of WN were reported to be highly resistant to P. cinnamomi and P. citricola (Browne et al., 2002; Matheron and Mircetich, 1985b), susceptible to Armillaria sp. (Serr and Rizzi, 1964), resistant to waterlogging injury (Catlin et al., 1977), and tolerant to Pratylenchus vulnus (Lownsberry et al., 1974). Although WN supported satisfactory scion growth in the Persian walnut cv. Eureka, it stunted the growth of other walnut scions (Serr and Rizzi, 1964). Chinese wingnut was hybridized with J. regia, and plants of the hybrids were produced using embryo rescue (McGranahan et al., 1986; Tulecke and McGranahan, 1985). The hybrids were highly resistant to P. citricola in vitro, but they did not survive in the field (McGranahan, unpublished data). Seedlings from different sources of P. stenoptera were reported to vary with respect to several vegetative growth characteristics (Li et al., 2001), but we are aware of no examinations of different wingnut seed families for resistance to Phytophthora or graft compatibility with current Persian walnut cultivars.
The objective of this study was to examine diverse open-pollinated seed families of P. stenoptera for: 1) relative resistance to P. cinnamomi and P. citricola; and 2) graft compatibility with key current cultivars of Persian walnut. It was hypothesized that superior seed sources of Chinese wingnut could be identified for use in breeding or propagating walnut rootstocks.
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