THE CALIFORNIA WALNUT IMPROVEMENT PROGRAM: SCION BREEDING AND ROOTSTOCK DEVELOPMENT

The goals of the University of California-Davis Walnut Improvement Program are to develop improved scion and rootstock cultivars for the California walnut industry and to utilize emerging genomic tools to increase breeding efficiency. Scion breeding objectives include new cultivars with early harvest dates, light kernel color, and resistance to blight and the development of scions with resistance to Cherry leaf roll virus. Rootstock development is directed at genetic resistance to soil-borne pathogens including Phytophthora, nematodes, and crown gall. Advances in clonal propagation methods, particularly micropropagation, have allowed replicated testing of genotypes of interest and have facilitated an expanding commercial production of improved clonal rootstocks. RX1, a Juglans microcarpa × Juglans regia hybrid with resistance to Phytophthora, and VX211, a very vigorous Juglans hindsii × J. regia hybrid exhibiting nematode tolerance, have been released for commercial use. Phenotyping, DNA mapping, and bioinformatic analyses are being combined to generate markers for traits of interest, including lateral bearing, kernel color, and harvest date, and will be used to improve selection efficiency. The Walnut Improvement Program is an integrated effort that depends on research cooperation with pathologists, molecular biologists, and UC Extension farm advisors, and relies on the assistance of commercial nurseries and walnut growers for planting and evaluation of statewide trials.