Susceptibility of Juglans Species andInterspecific Hybrids to Agrobacterium tumefaciens
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Crown gall, caused by the common soil-borne bacterium Agrobacterium tumefaciens, can be an economic problem in walnut nurseries and production orchards in Caliiornia. 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. Receat evidence has shown that some commercial black walnut trees producing Paradox hybrid seedlings are actually hybrids between Juglans 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 (Juglans hindsii x J. regia) than on Juglans hindsii in three sites with natural soil inoculum. Paradox seedlings (with a female parent that was primarily Juglans hindsii with some J. nigra) inoculated with A. tumefaciens on the roots during transplanting had a higher incidence of crown gall than either Juglans hindsii or J. regia. When stems were inoculated with A. tumefaciens, Juglans hindsii x J. regia populations had significantly larger galls than either Juglans hindsii or J. regia Similarly, in stem inoculations on four out of six Paradox gemotypes with a hybrid black walnut maternal parent, the progeny produced significantly larger galls than either Juglans hindsii or J. regia, However, two Paradox populations from black walnut hybrids that contained J. major, J. nigra, and Juglans 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.
McKenna, James R.; Epstein, Lynn 2003. Susceptibility of Juglans Species andInterspecific Hybrids to Agrobacterium tumefaciens. HortScience 38(3):435-439