105 The Quest to Identify Disease Resistance in the USDA-ARS Juglans Germplasm Collection D.A. Kluepfel 1, a , M.K. Aradhya 2 , G.T. Browne 1 , M.V. McKenry 3 , C.A. Leslie 4 , A.E. McClean 1 , J. Moersfelder 2 , D. Velasco 4 and K. Baumgartner 1 1 Crops Pathology and Genetics Research Unit, USDA-ARS, Department of Plant Pathology, University of California, Davis, One Shields Ave., Davis, CA 95616, USA 2 National Clonal Germplasm Repository, USDA-ARS, One Shields Ave., Davis, CA 95616, USA 3 Department of Nematology, University of California, Riverside, Kearney Agricultural Center, 9240 S. Riverbend Ave., Parlier, CA 93648, USA 4 Department of Plant Sciences, University of California, Davis, One Shields Ave., Davis, CA 95616, USA Keywords: rootstocks, disease resistance, Juglans, walnut breeding, crown gall resistance, phytophthora resistance, nematode resistance Abstract Ninety-nine percent of the U.S. Persian walnut crop is produced in California, USA, where walnut trees are typically grown on the hybrid rootstock Paradox (Juglans hindsii × Juglans regia). However, despite their popularity and relative advantage, Paradox seedling rootstocks are susceptible to Agrobacterium tumefaciens (crown gall), root-lesion and root-knot nematodes, Armillaria mellea (Armillaria root disease) and Phytophthora spp. (Phytophthora root and crown rot). Each of these diseases can cause serious loss to the California walnut industry. Therefore, continued walnut rootstock improvement is needed to identify disease- resistant, commercially-acceptable rootstocks. Towards this end we are examining the Juglandaceae collection at the USDA-ARS National Clonal Germplasm Repository (NCGR) in Davis, CA (USA). This collection contains the largest assemblage of wild Juglans spp. in North America with >550 accessions of black walnut, Asian butternut, Persian walnut, and wingnut (Pterocarya). Our preliminary evaluations of this Juglans germplasm collection has revealed resistance to A. tumefaciens and Phytophthora spp. among the J. hindsii, J. major, J. microcarpa and Pterocarya accessions, and root-lesion nematode resistance in J. cathayensis. Preliminary evaluations of these initial findings provide a foundation for continued examination of Juglans wild relatives to identify novel genotypes, which may contain valuable genes for disease resistance and environmental adaptation. INTRODUCTION Walnuts are members of the family Juglandaceae, which includes Juglans (walnut), Platycarya, Engelhardia, Alfaroa, Oreomunnea, Carya (pecan and hickory), and Pterocarya (wingnut). Juglans contains approximately 21 extant deciduous wind- pollinated species distributed across North, Central, and South America, the West Indies, and southeastern Europe to eastern Asia and Japan (Manning, 1978). Juglans regia (Persian or English walnut) is the basis of commercial walnut production, whereas J. nigra (Eastern black walnut) is the most commonly used timber species. In 2010, 4,600 growers in the US produced 456,320 mt of walnuts on 91,900 ha with a farm gate value of US $1.1 billion (http://www.nass.usda.gov/Statistics_by_State/ California/Publications/ Fruits_and_Nuts/201109walom.pdf.). The US is the world's largest exporter of walnuts with over 50% sold on the export market. Northern California black walnut (J. hindsii) became the rootstock of choice in California during the early 1900s due to its tolerance of waterlogged soils, but its susceptibility to Phytophthora caused the industry to explore the use of Paradox hybrids a dakluepfel@ucdavis.edu Proc. I st IS on Wild Relatives Subtrop. & Temp. Fruit & Nut Crops Eds.: M.K. Aradhya and D.A. Kluepfel Acta Hort. 948, ISHS 2012