|MACCREE, MALENDIA - US Department Of Agriculture (USDA)|
|HACKETT, WESLEY - University Of California|
Submitted to: Walnut Research Conference
Publication Type: Proceedings
Publication Acceptance Date: 12/30/2010
Publication Date: 1/1/2011
Citation: Kluepfel, D.A., Aradhya, M.K., Moersfelder, J.W., Mcclean, A.E., Maccree, M.M., Hackett, W. 2011. Evaluation of wild Juglans species for crown gall resistance. Walnut Research Conference. 2010:239-252.
Technical Abstract: Paradox, the most widely used rootstock in CA walnut production, is highly susceptible to the causal agent of crown gall (CG) Agrobacterium tumefaciens. The bacterial pathogen induces the formation of large tumors around the crown of the tree resulting in a reduction in both vigor and yield. If left untreated, tumors can completely girdle the tree leading to premature death. Currently, crown gall disease in mature orchards is managed using surgery to remove the gall and adjacent infected tissues. Durable host resistance is the preferred form of resistance to all soil borne plant pathogens. This is important for crown gall disease given the fact that Agrobacterium spp. are found in all the walnut growing regions of California examined. The wild relatives of cultivated species are often a rich source of genes coding for resistance to insect pests, microbial pathogens, and abiotic stresses. Identification of a durable source of resistance to crown gall in the Juglans germplasm collection, that could be utilized directly or introgressed into commercially viable rootstocks, is likely to be an effective strategy for controlling crown gall disease in walnut. The walnut germplasm collection at the National Clonal Germplasm Repository, USDA-ARS in Davis, CA contains a wide range of intra- and interspecific diversity of the black walnuts and butternuts adapted to California conditions. The potentially useful black walnut species include J. hindsii, J. nigra, J. microcarpa, J. major, and some of their hybrids with cultivated species. Other members of the germplasm repository collection that could be used to develop both crown gall and nematode resistant rootstock include the Asian butternuts, J. ailantifolia, J.mandshurica, and J. cathayensis. In addition, wingnuts belonging to the genus Pterocarya have shown disease resistance characteristics that need to be exploited. Although wild Juglans species have contributed to walnut rootstock development programs, the range of genetic variation for crown gall resistance within and between these wild species has not been characterized. It is anticipated that a systematic evaluation of the Juglans germplasm will reveal a source of resistance/tolerance to A. tumefaciens and other plant pathogens. As a step towards development of crown gall resistant rootstocks, here we report on the identification of Juglans species exhibiting resistance to infection by A. tumefaciens EC1. Once identified, these novel sources of Agrobacterium resistance will be exploited in the ongoing U.C. Davis Walnut root stock breeding program to help reduce crown gall incidence in both nursery and production fields.