Identifying walnut rootstocks with resistance to multiple soil-borne plant pathogens

Abstract

Most edible consumption walnuts in the U.S. are produced on the deep soils of California's Central Valley. Conditions favorable for tree growth are also conducive to the infection of susceptible walnut roots by soil-borne pathogens. Traditional Paradox seedling rootstocks that are hybrids of Northern California black walnut and English walnut, tend to promote scion vigor but are susceptible to key soil-borne pathogens, including Agrobacterium tumefaciens, Phytophthora spp., and Pratylenchus vulnus. Walnut production has relied on soil fumigation to reduce preplant populations of phytopathogenic nematodes, but fumigation has generally not provided effective management of Agrobacterium or Phytophthora. Recent releases of the clonal rootstocks 'RX1' (resistant to Phytophthora spp.), ‘Vlach’ (high vigor), and 'VX211' (tolerance to P. vulnus) provide some protection from single soilborne disease problems. Clearly, integrated management of soil-borne walnut diseases would benefit greatly from rootstocks with improved genetic resistance to single and multiple pathogens. In the current project, ~300 accessions each from controlled crosses of two selected mother trees were tested for susceptibility to the three pathogens in greenhouse or field-testing programs. When compiling the data of all three testing systems, one accession expressed only minimal susceptibility to all three pathogens and grew at medium to high vigor under nematode-infested field conditions. Nine accessions showed greatly reduced susceptibility to two of the three pathogens under investigation. Orchard trialing has been initiated to validate the putative pathogen resistance and horticultural performance of multiple elite walnut rootstock selections that have resulted from this interdisciplinary project.