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United States Department of Agriculture

Agricultural Research Service

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

Location: Crops Pathology and Genetics Research

2013 Annual Report


1a.Objectives (from AD-416):
1) Assempble germplasm and generate mapping populations for discovery and mapping of novel genetic loci that mediate resistance to major soil-borne diseases of walnut. 2)Characterize Juglans species half-sib and interspecific hybrids for resistance to the soil-borne pathogens; Agrobacterium tumefaciens (Crown Gall Disease, CG), Phytophthora spp. (crown and root rot, PHY), Armillaria mellea (Armillaria root disease, ARD), Pratylenchus vulnus (Root-lesion nematode, NEM); and threatening rootstock pathogens Geosmithis morbida (Thousand canker disease, TCD) and the walnut strain of cherry leaf roll virus (walnut blackline disease, WBD). 3)Develop and map SNP markers associated with disease resistance. 4)Quantify economic impact of disease-resistant rootstocks on US walnut industry. 5)Conduct bi-directional extension efforts that: a)deliver disease-resistant rootstocks to stakeholders, and b)assess emerging threats to walnut rootstocks.


1b.Approach (from AD-416):
Our multi-disciplinary team will build on a conventional breeding approach with pathogen screening techniques, novel plant propagation technology and modern genomic tools for marker-assisted selection, in combination with educational outreach, to develop, characterize, and demonstrate walnut rootstocks with resistance to all major walnut rootstock pathogens. We will identify genetic markers associated with the resistant phenotypes, and these markers will be the first of their kind for Juglans. Integrated bi-directional extension efforts will insure that emerging diseases and other issues affecting adoption of new walnut rootstocks are identified and marketable products of the research are deployed and evaluated in commercial orchards.


3.Progress Report:

This project was established in support of objective 1 of the in-house project, which is to characterize the etiology, biology, and ecology of key phytopathogenic agents and their interactions with economically important tree and grapevine species. The goal of this project is to develop disease resistant walnut rootstocks using conventional and genomic approaches.

This is the first year of our USDA, National Institute of Food and Agriculture (NIFA) Specialty Crops Research INitiative (SCRI) supported project during which we established five Assistance Type Cooperative Agreements (ATCA’s) with university collaborators across the country. In addition we conducted four Principal Investigator (PI) research progress meetings and one industry oversight committee meeting in conjunction with the Walnut Research Conference in Bodega Bay, California. Initial steps in this project have focused on the generation of a genetically diverse walnut germplasm collection which is being propagated using a variety of traditional and in vitro techniques. Using embryo rescue techniques, we have been able to place recalcitrant walnut species into in vitro culture from which we have generated clonal copies. Clonal material from all our propagation efforts have resulted in the generation of thousands of trees which are currently being examined for resistance to crown gall, Phytophthora spp., lesion nematodes and Armillaria. All initial disease resistance screening is being conducted under greenhouse conditions with the exception of our screen for lesion nematode resistance which is occurring in the field. In addition to walnut (Juglans) evaluations, we generated and screened Chinese wingnut species (a distant walnut relative) obtained from open pollinated seeds for resistance to crown gall. The attractiveness of numerous Juglans species to the 10,000 cankers walnut twig beetle is being assessed under field conditions. In addition, we have screened target Juglans species for their susceptibility to the walnut twig beetle under both laboratory and field conditions. A statewide survey of the sensitivity of walnut rootstock Paradox clones to 10,000 (10K) cankers disease is under way. Genetic characterization of the 10K cankers disease causing agent, Geosmithia morbida, is being conducted along with an analysis of its interaction with the walnut twig beetle during transmission. We have isolated and identified a number of potential phytopathogenic agents which are associated with the newly emergent disease of Paradox walnut rootstocks known as lethal canker disease. Experiments to confirm Koch’s Postulates of these isolates are in progress at a field location in the state.


Last Modified: 9/29/2014
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