Evaluation of wild walnut Juglans spp. for resistance to crown gall disease
D. A. KLUEPFEL (1), M. K. Aradhaya (1), J. W. Moersfelder (1), A. E. McClean (1), W. P. Hackett (2), A. J. Dull (1)
(1) USDA ARS/University of California, Davis, CA, U.S.A.; (2) Department of Plant Sciences, University of California, Davis, Davis, CA, U.S.A.
Phytopathology 101:S92

Crown gall (CG) disease of walnut is caused by the ubiquitous soil-borne bacterium, Agrobacterium tumefaciens. The most widely used rootstock Paradox, an interspecific hybrid between Juglans hindsii and Juglans regia, is typically highly susceptible to A. tumefaciens. Identification of a durable source of resistance in wild Juglans species could be introgressed into commercially viable rootstocks, as an effective strategy for controlling crown gall in walnut. CG tolerant Juglans and Pterocarya spp. have been identified in a disease resistance screen conducted under greenhouse conditions. A wide range of variability in tumor formation was observed among different host genotypes. Even though CG resistance appeared to be rare in the germplasm accessions tested, Juglans microcarpa accessions were consistently the most resistant. Two J. microcarpa mother trees both generated open pollinated seedlings which exhibited increased tolerance to CG development. Rooted dormant cuttings from CG resistant selections were propagated, inoculated with A. tumefaciens, and continue to show CG resistance. These promising candidates are being further examined to confirm the stability of the observed resistance and to be used in directed crosses with commercially viable parents as a first step towards development of crown gall resistant rootstocks.