Walnut breeding in California

During the last 70 years, the University of California-Davis Walnut Improvement Program has developed, and released for commercial use, a series of improved walnut scion cultivars and several clonal rootstocks exhibiting pathogen resistance. This breeding and evaluation effort has been a cooperative and integrated undertaking with the participation of UC Extension farm advisors, university and USDA researchers, and the walnut growers and nurseries of California. Scion breeding objectives have included harvest dates earlier than ‘Chandler’, precocious high yield, extra light kernel color, and genetic resistance to blight, husk fly, Botryosphaeria, and Cherry leaf roll virus. Rootstock improvement has been directed primarily at resistance to Phytophthora crown and root rots, root lesion nematode, and crown gall disease. In recent years, the program has released three new scion cultivars – ‘Ivanhoe’, ‘Solano’, and ‘Durham’ – with a range of earlier harvest dates, and two hybrid (Paradox) clonal rootstocks – ‘RX1’, (Juglans microcarpa × J. regia) for Phytophthora resistance, and ‘VX211’, (Juglans hindsii × J. regia) for vigor and nematode tolerance. Improvements in micropropagation and greenhouse acclimatization methods have facilitated replicated testing of an expanded set of additional rootstock candidates and contributed to an expanding commercial production of pathogen-resistant clonal rootstocks. Current work in the program is directed at developing phenomic tools for measuring scion and rootstock traits of interest, and at incorporating genomic information into the selection of parents and progeny.