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

Authors: WESTPHAL, ANDREAS - University Of California, Riverside; MAUNG, ZINTHUZAR - University Of California, Riverside; BUZO, TOM - University Of California, Riverside; BROWN, PAT - University Of California, Davis; LESLIE, CHARLES - University Of California, Davis; Browne, Greg; OTT, NATALIA - University Of California - Cooperative Extension Service; McClean, Ali; Kluepfel, Daniel

Submitted to: European Journal of Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2024
Publication Date: 5/22/2024
Citation: Westphal, A., Maung, Z., Buzo, T., Brown, P., Leslie, C., Browne, G.T., Ott, N., McClean, A.E., Kluepfel, D.A. 2024. Identifying walnut rootstocks with resistance to multiple soil-borne plant pathogens. European Journal of Horticultural Science. 89(2):1-10. https://doi.org/10.17660/eJHS.2024/008.
DOI: https://doi.org/10.17660/eJHS.2024/008

Interpretive Summary: Most edible walnuts in the US 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, which are hybrids of Northern California black walnut and English (Persian) walnut tend to promote scion vigor but are susceptible to key soilborne pathogens, including Agrobacterium tumefaciens (cause of crown gall disease), Phytophthora spp.(crown and root rot), and Pratylenchus vulnus (root lesions). 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 Pratylenchus vulnus) provide some protection from single soil-borne disease problems. Clearly, integrated management of soilborne 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, some accessions expressed only minimal susceptibility to all three pathogens and grew at mid to high vigor under nematode-infested field conditions. Multiple 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.

Technical Abstract: Most edible walnuts in the US 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, which are hybrids of Northern California black walnut and English (Persian) walnut tend to promote scion vigor but are susceptible to key soilborne 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 Pratylenchus vulnus) provide some protection from single soil-borne disease problems. Clearly, integrated management of soilborne 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, some accessions expressed only minimal susceptibility to all three pathogens and grew at mid to high vigor under nematode-infested field conditions. Multiple 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.