Orchard Validation of Resistance to Agrobacterium tumefaciens in Experimental Clonal Walnut Rootstocks

Most commercial walnut trees in California use Paradox hybrid rootstocks. None of the rootstocks has sufficient resistance to the spectrum of major soilborne walnut pathogens, which includes Agrobacterium tumefaciens ( At; cause of crown gall disease), Phytophthora species ( Pspp; causes of crown and root rots), and phytopathogenic nematodes. Recent greenhouse-based testing identified resistance to At and Pspp in experimental walnut rootstock clones, but the resistance was not orchard validated. Here, we report on orchard evaluations of resistance to At among experimental rootstock clones ‘11-991’, ‘29JM 8’, ‘STJM4’ (all Juglans microcarpa × Juglans regia, putatively resistant to At or Pspp), and ‘K3’ ( J. microcarpa, putatively resistant to At). Seedling Paradox (typically Juglans hindsii × J. regia), clonal ‘VX211’ and ‘Vlach’ (both J. hindsii × J. regia), and clonal ‘RX1’ ( J. microcarpa × J. regia) were included as commercial standards. The rootstocks were propagated with ‘Chandler’ scions and planted in four orchard trials throughout California in 2016. Rootstocks were assessed for tree survival, crown gall incidence, and growth. Tree mortality, mainly resulting from failure to establish, not infectious disease, was very low on the standard clonal rootstocks and experimental ‘11-991’ rootstock. Paradox seedling had greater crown gall incidence compared with all clonal rootstocks. The experimental rootstocks had very low incidence of crown gall across all trials. Therefore, based on our orchard validations of crown gall resistance and acceptable survival and growth, experimental rootstocks ‘K3’, ‘11-991’, ‘29JM 8’, and ‘STJM4’ offer good potential for commercial evaluation.