Persian (english) walnut (Juglans regia L.) orchards can incur serious economic loss from attack by several soilborne pathogens, including Agrobacterium tumefaciens Smith and Townsend (causal agent of crown gall disease), Armillaria mellea (Vahl:Fr.) P. Kumm (Armillaria root rot), numerous species of Phytophthora de Bary (Phytophthora crown and root rots), and several species of plant parasitic nematodes (nematode-incited root destruction or galling and debilitation). Rootstocks typically have been used for persian walnut cultivars to improve tree vigor, tolerance to salinity and waterlogging, and resistance to some pathogens (McGranahan and Catlin, 1987). Presently, about 80% of California walnut acreage is planted on Paradox seedling rootstock, which is mainly J. hindsii (Jeps.) R.E. Sm. × J. regia (Potter et al., 2002), while about 15% and 5% of the orchards are on Northern California black walnut (NCB) (J. hindsii) and Persian walnut seedling rootstocks, respectively. None of the rootstocks, however, is sufficiently resistant or tolerant to all of the important soilborne pathogens.
More than 10 species of Phytophthora have been implicated in Phytophthora crown and root rot on walnut, and among them P. cinnamomi Rands and P. citricola Sawada are especially aggressive and difficult to manage (Matheron and Mircetich, 1985a; Mircetich and Matheron, 1983; Mircetich et al., 1998). Several commercial walnut orchards in California have been decimated because of Phytophthora crown and root rot incited by P. cinnamomi (Browne et al., 2011a; Mircetich et al., 1998). Incidence and severity of Phytophthora crown and root rots generally can be minimized by careful soil water management, use of Paradox rootstock (Mircetich et al., 1998), and treatments with phosphonate (also referred to as phosphite) (Browne et al., 2011b), but rootstocks with improved resistance to the pathogens are needed. Compared with NCB (J. hindsii) and persian walnut rootstocks, Paradox is more resistant to several species of Phytophthora (Matheron and Mircetich, 1985b; Mircetich and Matheron, 1983), but in practice this resistance has been insufficient to prevent serious losses caused by P. cinnamomi or P. citricola (Browne et al., 2011a; Mircetich et al., 1998). All tested open-pollinated seed families of chinese wingnut (Pterocarya stenoptera C. DC.) were highly resistant to both pathogens, but they were not graft compatible with the popular cultivar Chandler and therefore had limited usefulness as walnut rootstocks (Browne et al., 2011a).
Advances in micropropagation and breeding technologies have enabled the exploration of diverse Juglans L. species and hybrids for the improvement of walnut rootstocks (Leslie and McGranahan, 1992). The objective of this study was to identify walnut rootstock germplasm that is graft compatible with persian walnut and has high resistance to P. cinnamomi and P. citricola.
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