2017 ASHS Annual Conference

In 2015, California produced 603,000 tons of walnuts on 315,000 acres with a farm-gate revenue of almost $1 billion. California is the world’s largest exporter of walnuts with over 60% of domestic production shipped abroad, largely to Europe and the Pacific Rim. Most commercially cultivated walnut trees, grown for nut production, are grafted onto rootstocks that are susceptible to root diseases that cause an estimated 18% annual yield loss worth $241 million to the industry. The central goal of our project is to identify, characterize and ultimately deploy disease resistant walnut rootstocks. To achieve this goal we are using an integrated approach that encompasses disease resistance -screening, in vitro and traditional plant propagation, genomics, and conventional breeding strategies. Our target pathogens (diseases) include Agrobacterium tumefaciens (crown gall), Phytophthora spp. (crown/root rot), Pratylenchus vulnus (root lesion nematode), Armillaria mellea (oak root fungus) and two emerging diseases: Thousand Cankers disease and Lethal Paradox Canker. In this work we have exploited the Juglandaceae germplasm collection in the USDA-ARS National Clonal Germplasm Repository in Davis, California which is the largest assemblage of wild Juglans spp in North America. In this collection we identified several Juglans species whose open pollinated progeny exhibited a high level of disease resistance to the key soil borne rootstock pathogens being examined in this project. We also generated, and clonally propagated, a breeding population of 600 interspecific hybrid progeny from two J. microcarpa x J. regia crosses. This breeding population has entered our disease resistance screening pipeline for crown gall, crown/root rot and root lesion nematode resistance. Initial resistance screening data has revealed that the breeding population segregates for resistance to at least two of the pathogens examined. We also produced reference quality genome sequences for both J. microcarpa and J. regia and employed them in Genotyping-by-Sequencing (GBS) of each of the 600 progeny. Using the GBS data, we generated a genetic map for both J. microcarpa and J. regia containing ~50 SNP markers for each of the 16 chromosomes and have tentatively identified one major QTL for A. tumefaciens resistance on chromosome 11. To date we have identified 15 experimental walnut genotypes which exhibit enhanced levels of resistance to one or more of the pathogens cited above. These genotypes, and their potential to serve as rootstocks, are in various stages of propagation and/or field-testing at five sites across the walnut growing region of California.