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United States Department of Agriculture

Agricultural Research Service

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Research Project: Development of Disease-Resistance Walnut Rootstocks: Integration of Conventional and Genomic Approaches

Location: Crops Pathology and Genetics Research

2013 Annual Report

1a.Objectives (from AD-416):
1)Identify and characterize Juglans germplasm resistant to key soil-borne pathogens. 2)Identify and develop SNP markers and the associated SNP genotyping platform. 3)Quanitfy economic impact of disease-resistant rootstocks on US walnut industry. 4)Conduct extension efforts that: a)deliver disease-resistant rootstocks to stakeholders, and b)assess emerging threats to walnut rootstocks.

1b.Approach (from AD-416):
Based on extensive preliminary disease resistance screening data, a diverse and promising panel of wild Juglans spp. germplasm selections has been assembled to provide novel sources of resistance to the major soil-borne diseases. Mother trees (MTs) will be selected from this panel based on progeny evaluation of resistance conducted in objective 1. Mother trees segregating for resistance to one or more disease will be used to produce biparental interspecific hybrid mapping populations for the construction of genetic maps used to map resistance genes relative to molecular markers, and to select the best hybrid genotypes for field testing (objective 2). To broaden the search for resistance genes in the germplasm collection, open-pollinated progeny of several MT's from each species that have been genotyped and evaluated for resistance, will be used in linkage disequilibrium analysis as explained in objective 2 to find association between molecular markers and resistance. Integrated extension efforts will reach and receive input from more than 4,800 walnut producers and nursery operators.Outreach efforts will promote informed adoption and refinement of targeted rootstocks and associated genomic technologies. This involves testing and demonstrating rootstocks in orchard trials and educating and engaging stakeholders through county-based grower meetings, extension newsletter, webcasts, trade magazines and websites.

3.Progress Report:

The agreement was established in support of objective 3, subobjective 3.3 of parent project, which is to improve management strategies for key soil borne diseases of tree fruit and nut crops, identify and characterize available walnut rootstock germplasm for resistance to key soil borne pathogens. The goal of this project is the generation and evaluation of Juglans spp. hybrids and paradox root stock selections for resistance to crown gall, Armillaria, lesion nematode, and Phytopthora spp. In addition, this project is designed to map the genetic loci which mediate disease resistance. This project has been actively funded by the walnut board for the last 7 years due to the long term nature of tree crop rootstock development. Open pollinated (OP) progeny, and directed crosses from, and between, several wild walnut species, maintained in the USDA, ARS, Davis, California, National Clonal Germplasm Repository, were identified with resistance to crown gall (CG), lesion nematode, and Phytopthora, and will continue to be examined here. We generated dormant cuttings from CG resistant seedlings.Approximately 20% of the seedlings propagated from dormant cuttings retained the original CG resistant phenotype. Disease resistant cuttings have then been clonally propagated using in vitro techniques. The Progeny from the walnut wild species, Juglans microcarpa (Texas Black) continue to generate resistant open pollinated progeny at a higher frequency than all other walnut wild species examined. Juglans major progeny exhibit the next highest level of CG resistance. To identify the genetic loci mediating CG resistance, F1 progeny generated from crosses between CG resistant female parents (Texas Black walnut) and CG susceptible pollen donors were screened for CG resistance. The progeny exhibited a 1:4 inheritance ratio of resistant: susceptible progeny. These CG resistant progeny have been placed into tissue culture. We have generated clonal copies which continue to show CG resistance. These clones will be examined for resistance to other major soil borne pathogens. We continue to examine open pollinated progeny from J. microcarpa mother trees in addition to making additional crosses in order to generate genetically diverse CG resistant genotypes for exploitation as a commercial walnut rootstock. Open pollinated seedlings from a close relative of walnuts, Pterocarya (Chinese Wingnut) exhibited a very high level of CG resistance and root well from the dormant cutting stage. Using the air layering technique, we have successfully cloned the mother trees being used in our walnut crosses. These clones are being examined for their disease resistance phenotype. This agreement is in support of a larger USDA, Specialty Crops Research Initiative (SCRI) grant obtained by the author of this report.

Last Modified: 7/30/2014
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