Susceptibility of Juglans Species and Interspecific Hybrids to Agrobacterium

Crown gall, caused by the common soil-borne bacterium Agrobacterium tume- faciens, can be an economic problem in walnut nurseries and production orchards in California. The principal rootstocks used for commercial walnut production in California are the native Northern California black walnut, Juglans hindsii, and "Paradox," which are interspecific hybrids between a black walnut, primarily J. hindsii, as the maternal parent, and J. regia, the English walnut, as the paternal parent. Recent evidence has shown that some commercial black walnut trees producing Paradox hybrid seedlings are actually hybrids between J. hindsii and two other North American black walnut species, J. major and J. nigra. Here, we document that there was a higher incidence of crown gall on Paradox (J. hindsii ×J. regia) than on J. hindsii in three sites with natural soil inoculum. Paradox seedlings (with a female parent that was primarily J. hindsii with some J. nigra) inoculated with A. tumefaciens on the roots during transplanting had a higher incidence of crown gall than either J. hindsii or J. regia. When stems were inoculated with A. tume- faciens, J. hindsii ×J. regia populations had significantly larger galls than either J. hindsii or J. regia. Similarly, in stem inoculations on four out of six Paradox genotypes with a hybrid black walnut maternal parent, the progeny produced significantly larger galls than either J. hindsii or J. regia. However, two Paradox populations from black walnut hybrids that contained J. major, J. nigra, and J. hindsii produced galls that were no different in size than in the black walnut species and J. regia. Results suggest that J. regia and black walnut species are less susceptible to crown gall than most Paradox populations. Juglans regia Jeps., the "English" or "Per- sian" walnut, is the scion for commercial walnut production in California (Forde and McGrana- han, 1996). Rootstocks used for commercial walnut production in California may contain genetic material from several North American species of black walnut: J. hindsii (Jepson) a 3% incidence on J. hindsii (n = 29 orchards) (Epstein, unpublished). The economic impact of crown gall is greatest in young orchards in which trees with large or numerous galls tend to be stunted and predisposed to other stresses (Sinclair et al., 1987). In the California grower survey, 94% of growers reported reduced prof- its from orchards on Paradox in which there was a crown gall incidence of 20% or more. Despite its susceptibility to crown gall, Paradox performs better than J. hindsii in a wider variety of soils and is either more resistant or tolerant to root lesion nematode Pratylenchus vulnus (Lowensbery et al., 1974), root and crown rot caused by Phytophthora sp. (Matheron and Mircetich, 1985), high water table, flooding, and zinc deficiency (McGranahan and Catlin, 1987). Resistance to A. tumefaciens has been reported in other horticultural crops, particu- larly in grapes (Sule and Burr, 1998). However, crown gall on grapes is caused primarily by A. vitis (synonyms, A. tumefaciens biovar 3, Rhizobium vitis) (Young et al., 2001). Crown gall in walnut is caused by either A. tumefa- ciens (synonyms, A. tumefaciens biovar 1, A. radiobacter, R. radiobacter) or A. rhizogenes (synonyms, A. tumefaciens biovar 2, R. rhizogenes) (Moore and Canfield, 1996). In California, A. tumefaciens is the more common causal agent of crown gall on Paradox than A. rhizogenes (Kaur and Epstein, unpublished). A stem inoculation assay has been used to identify resistance to A. tumefaciens in roses (Marti et al., 1999; Reynders-Aloisi et al., 1998), Prunus spp. (Bliss et al., 1999), and grape (Ferreira and van Zyl, 1986; Stover et al., 1997; Sule et al., 1994). The present study was conducted to document the incidence of crown gall on Paradox and black walnut rootstocks in the field and to investigate the genetic variation of different walnut genotypes to infection by A. tumefaciens.