Non-technical summary from 2013 annual progress report submitted to USDA NIFA SCRI programopen pollinated Juglans ailantifolia germinating

In 2010, 4,800 growers in the US produced 503,000 tons of walnuts on 227,000 acres with a farm gate value of $1.061 billion. Walnuts are the fourth largest US specialty crop export which places the US as the world's largest exporter of walnuts with over 50% sold on the export market. Walnuts are among the most nutrient-dense whole-food sources of the short-chain omega-3 polyunsaturated fatty acid that protects the heart against cardiovascular disease and have been placed on "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 grafted on rootstocks to limit root diseases which currently cause an estimated 18% annual loss worth $191 million to the industry. The availability of new pathogen-resistant rootstocks is critical for the viability of the walnut industry and is ranked by the US walnut industry as its top research priority. We will use 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 four major soilborne diseases caused by Agrobacterium tumefaciens (crown gall), Phytophthora spp. (crown rot), Pratylenchus vulnus (root lesion nematode), and Armillaria mellea (oak root fungus). This work will exploit the Juglandaceae germplasm collections in the USDA-ARS National Clonal Germplasm Repository in Davis, Calif. and the Missouri Horticulture and Agroforestry Research Center in New Franklin, Mo. 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, will be monitored by integrated outreach. A comprehensive economic analysis will optimize nursery propagation strategies and quantify the value to the industry of using pathogen-resistant clonal rootstocks developed here. Our project will enhance collaboration, communication, and exchange of information among the walnut research community and the broader US tree crop industry.