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Role of Systemic Agrobacterium tumefaciens Populations in Crown Gall Incidence on the Walnut Hybrid Rootstock ‘Paradox’

October 2012 , Volume 96 , Number  10
Pages  1,415 - 1,421

L. E. Yakabe, S. R. Parker, and D. A. Kluepfel, United States Department of Agriculture–Agricultural Research Service, Crops Pathology and Genetics Research Unit, Davis, CA 95616

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Accepted for publication 22 February 2012.

Greater than 75% of English walnut production in the United States occurs on the walnut rootstock Juglans hindsii × J. regia ‘Paradox’, which is highly susceptible to infection by Agrobacterium tumefaciens. When seed were germinated and grown in the presence of A. tumefaciens, in the absence of wounding, 94% of the seedlings exhibited tumors while 89% contained systemic A. tumefaciens populations. When seedlings were wound inoculated, A. tumefaciens established endophytic populations in stem tissue and often migrated from the site of infection. Distribution of A. tumefaciens in the stem was random and may exhibit seasonal variation. A. tumefaciens populations in root tissue were more readily detected than in stem tissue and may serve as a reservoir for subsequent infection of the aerial portions of the tree. Importantly, 7% of inoculated, asymptomatic seedlings contained endophytic populations of A. tumefaciens. In all, 17% of seedlings inoculated as seeds developed galls at secondary stem-wound sites. These results provide an ecological and epidemiological foundation upon which to modify existing tree-handling practices in both nursery and orchard production environments to manage crown gall incidence.

This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012.