Development of disease-resistant walnut rootstocks: Integration of conventional and genomic approaches (SCRI-match Year 3)

Authors: Kluepfel, Daniel; LESLIE, CHARLES - University Of California; Aradhya, Mallikarjuna; Browne, Greg; HASEY, JANINE - University Of California - Cooperative Extension Service; POPE, KATHERINE - University Of California - Cooperative Extension Service; MCKENRY, MICHAEL - University Of California - Cooperative Extension Service; WESTPHAL, ANDREAS - University Of California - Cooperative Extension Service; HACKETT, WES - University Of California; BOSTOCK, RICHARD - University Of California; SEYBOLD, STEVE - Forest Service (FS); DVORAK, JAN - University Of California; LUO, MING-CHENG - University Of California; COGGESHALL, MARK - University Of Missouri; SCHLARBAUM, SCOTT - University Of Tennessee; DANDEKAR, ABHAYA - University Of California; NEALE, DAVID - University Of California; LANGLEY, CHUCK - University Of California; Preece, John; Baumgartner, Kendra

Submitted to: Walnut Research Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/10/2016
Publication Date: 1/27/2016
Citation: Kluepfel, D.A., Leslie, C., Aradhya, M.K., Browne, G.T., Hasey, J., Pope, K., Mckenry, M., Westphal, A., Hackett, W., Bostock, R., Seybold, S., Dvorak, J., Luo, M., Coggeshall, M., Schlarbaum, S., Dandekar, A., Neale, D., Langley, C., Preece, J.E., Baumgartner, K. 2016. Development of disease-resistant walnut rootstocks: Integration of conventional and genomic approaches (SCRI-match Year 3). Walnut Research Conference. Walnut Research Reports 2015: 87-91.

Interpretive Summary:

Technical Abstract: Walnuts are grown on almost every continent with total world-wide production estimated at over 4 billion in-shell pounds. California walnut growers, who produce 99% of the US walnut crop, produced an estimated 1.2 billion pounds on approximately 310,000 bearing acres with a farm gate value of approximately $1.8 billion. Walnuts are the fourth largest U.S. specialty crop export which places the U.S. as the world's largest exporter of walnuts with over 50% sold on the export market. Walnuts are a nutrient-dense whole-food source of the short-chain omega- 3 polyunsaturated fatty acid that protects against cardiovascular disease. As a result, walnuts have been placed on the "super food" list by the US Food & Drug Administration. In addition, walnut is an important hardwood with an estimated standing timber value of over $539 billion. Most commercially cultivated walnut trees, grown for nut production, are grown on rootstocks with the cultivar Paradox representing the industry standard. Unfortunately, Paradox is susceptible to four key soil borne pathogens (Agrobacterium tumefaciens, Phytophthora spp., Pratylenchus vulnus, and Armillaria mellea) and two emerging diseases: thousand cankers disease (TCD) and lethal Paradox canker (LPC). Combined, these diseases (new and old) cause an estimated 18% annual loss worth $191 million to the CA walnut industry. The availability of new pathogenresistant rootstocks is critical for the long term viability of the walnut industry. We are using an integrated approach encompassing, disease resistance-screening, plant propagation, genomics, and conventional and novel breeding strategies to develop, characterize, and deploy clonal walnut rootstocks with improved resistance to the four major soil borne pathogens sited above. In this project we are exploiting the Juglandaceae germplasm collections in the USDA-ARS National Clonal Germplasm Repository in Davis, CA and the Missouri Horticulture and Agroforestry Research Center in New Franklin, Missouri. Together, these collections comprise the largest assemblage of wild Juglans spp. in North America. In this process we will identify genes conferring resistance to these diseases and develop SNP markers for their selection in segregating populations and integration into commercial rootstocks. Emerging walnut rootstock diseases, such as Thousand Canker Disease and Paradox Canker, are also being examined and monitored.