Submitted to: Walnut Research Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/1/2007
Publication Date: 1/1/2007
Citation: Stover, E.W., Maccree, M.M., Aradhya, M.K., Mcclean, A.E., Kluepfel, D.A. 2007. Evaluation of wild juglans species for crown gall resistance. Walnut Research Conference.
Technical Abstract: Crown Gall disease of walnut is caused by the ubiquitous soil-borne bacterium, Agrobacterium tumefaciens which is able to transfer a specific piece of its own DNA into the genome of the plant host cell. The result of this genetic transformation is the autonomous undifferentiated massive growth of infected plant cells which generates the most obvious symptom of this disease, plant galls or tumors. Paradox rootstocks are widely used in CA walnut production. These rootstocks are usually interspecific hybrids between J. hindsii and J. regia (Howard, 1945), which are typically highly susceptible to Agrobacterium tumefaciens. Extensive formation of tumors around the crown of the tree can often stunt the tree and result in reduced vigor and yields. If left untreated, tumors continue to grow and completely girdle the tree which contributes to premature death of the tree. 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 especially important for Crown Gall Disease given the fact that Agrobacterium spp are found in the soil in all the walnut growing regions of California examined. The wild relatives of cultivated species are often a rich source of genes coding for such desirable traits as, resistance to insect pests and 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 represents a wide range of intra- and interspecific diversity for some of the black walnuts and butternuts that are adapted to California conditions. The potentially useful black walnut species include J. hindsii, J. nigra, J. microcarpa, J. major, in addition to some of their hybrids with cultivated species. The Asian butternuts, J. ailantifolia, J. mandshurica, and J. cathayensis, which grow well in the germplasm collection, also could be used directly or in the development of Crown Gall resistant interspecific hybrids. Although wild species have contributed to walnut rootstock development programs, the range of genetic variation for crown gall resistance within and between these wild species has never been examined. It is anticipated that a systematic evaluation of the Juglans germplasm for crown gall resistance will unravel a hitherto unknown source of resistance/tolerance to crown gall disease 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/tolerance to infection by A. tumefaciens EC1. Once identified, these novel sources of Agrobacterium resistance can be exploited in the ongoing U.C. Davis Walnut root stock breeding program to help reduce the incidence of Crown Gall in both nursery and production fields. Objective Identify and characterize a novel source of Crown Gall (CG) resistance in the Juglans germplasm collection maintained in the USDA/ARS National Clonal Germplasm Repository in Davis, CA. Anticipated Outcome We anticipate the identification of a new source of Crown Gall resistance which will be useful in the development of Crown Gall resistant rootstocks in the UC Davis walnut breeding program. The germplasm thus identified also will be shared with other pathologists and horticulturists for further evaluation for resistance to other diseases, especially Phytophthora and to test for their ability to propagate vegetatively. Procedures Seedling germination and inoculation. Open pollinated seed