Advances in genetic improvement of walnut at the University of California, Davis

This is a review of the recent advances in genetic improvement of walnut cultivars and rootstocks. There has been an active walnut breeding program at UC Davis since 1948. 'Chandler', 'Howard', 'Vina', 'Tulare' and 'Serr' are examples of cultivars released by the program. Recently, three new cultivars, ('Sexton', 'Gillet' and 'Forde'), were released (patent pending). These cultivars, developed through controlled pollinations, are characterized by high early yields, low blight scores, large light-colored kernels and harvest dates before Chandler. Several cultivar selections for hypersensitivity to the cherry leafroll virus, causal agent of blackline disease, have also been made. These are derived from two to three generations of backcross breeding from Paradox (Juglans hindsii x J regia) with J. regia as the recurrent parent. Superior clonal Paradox rootstock selections have been identified and are in retesting or field trials. These include an extremely vigorous individual that appears to tolerate nematode (Pratylenchus vulnus) infestation, but is not resistant per se, 5 selections of a vigorous blackline-tolerant hybrid (J hindsh x J. regia) x J regia, three clones that are less damaged than others by Phytophthora citricola in greenhouse tests, and one clone that exhibits limited gall formation when challenged by (Agrobacterium tumefaciens) in preliminary tests. Genetic engineering through gene silencing has also resulted in clones that do not exhibit crown galls after inoculation in vitro. Many different researchers have been involved in the efforts to genetically improve walnuts.