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Title:   Author  item
                                            Kluepfel, Danielitem
                                            Browne, Gregitem
                                            MCKENRY, MICHAEL - University Of Californiaitem
                                            HASEY, JANINE - University Of California Agriculture And Natural Resources (UCANR)item
                                            LESLIE, CHUCK - University Of Californiaitem
                                            Aradhya, Mallikarjunaitem
                                            HACKETT, WES - University Of Californiaitem
                                            Preece, Johnitem
                                            BOSTOCK, RICK - University Of Californiaitem
                                            SEYBOLD, STEVE - US Department Of Agriculture (USDA)
  Submitted to: Walnut Research Conference
                                    Publication Type: Proceedings
                                    Publication Acceptance Date: 1/15/2014
                                    Publication  Date: 1/15/2014
                                    Citation: Kluepfel, D.A., Browne, G.T., Mckenry, M., Hasey, J., Leslie, C., Aradhya, M.K., Hackett, W., Preece, J.E., Bostock, R., Seybold, S. 2014. Development of disease-resistant walnut rootstocks: Integration of conventional and genomic approaches. Walnut Research Conference. 77-87.Interpretive Summary:
                                Technical Abstract: English walnut (Juglans regia) is not typically grown on its own roots in commercial orchards due to its high susceptibility to key soil borne pathogens.  Also, it is sensitive to excessive soil moisture and salinity, which are common abiotic stresses in CA soils. Northern CA black walnut (J. hindsii) became the rootstock of choice in the early 1900s due to its tolerance of waterlogged soils, but its Phytophthora susceptibility caused the industry to explore the use of Paradox hybrids (J. hindsii x J. regia). Paradox hybrids were first developed by Luther Burbank in 1893 and were later recognized by Smith et al. in 1912 to possess superior horticultural qualities, including strong vigor, enhanced yields and graft-compatibility with English cultivars. The walnut industry currently relies on three rootstocks (% acreage): Paradox (80%), northern CA black walnut (15%), and English walnut (5%). Despite its popularity and relative advantages, standard Paradox seedling rootstock is susceptible to Agrobacterium tumefactions (crown gall, CG) and moderately susceptible to Phytophthora spp. (Phytophthora root & crown rot, PHY), and Pratylenchus vulnus (lesion nematode, NEM).  All of these pathogens cause serious losses to the walnut industry.
Our long-term goal is to develop, evaluate, and deploy walnut rootstocks with superior resistance to each of the major soil-borne/rootstock pathogens listed above and Geosmithia morbida (thousand cankers disease, TCD, and emerging threat). Our objectives are: 1) identify and characterize diverse Juglans germplasm resistant to key soil-borne pathogens, 2) develop genomic tools to facilitate efficient breeding of superior rootstocks, 3) quantify the economic impact of pathogen-resistant walnut rootstocks on the US walnut industry, 4) deliver disease resistant rootstocks to stakeholders, and 5) assess newly emerging threats to walnut rootstock health.  This research addresses the highest priority needs identified by the Production Research Advisory Committee (PRAC) of the CA Walnut Research Conferences and county-wide walnut days.  The most widely used central measures for major soil borne diseases of walnut in nurseries and orchards has been soil fumigation using methyl bromide, but this treatment is rapidly being phased out. Our objectives are centered on development of a) disease-resistant genotypes and b) genomic technologies to facilitate rapid development and release of commercially viable rootstocks.  Our outreach efforts will promote informed adoption and refinement of the targeted rootstocks and associated genomic technologies.  The outreach will involve testing and demonstrating rootstocks in orchard trials; identifying and assessing emerging rootstock threats TCD and Paradox canker disease by orchard surveys; and educating and engaging stakeholders through county-based annual grower meetings, extension newsletters, webcasts, trade magazines and websites.


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