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ARS Home » Pacific West Area » Davis, California » Crops Pathology and Genetics Research » Research » Research Project #435700

Research Project: Putting Phenotypic and Genotypic Tools to Work for Improving Walnut Rootstocks

Location: Crops Pathology and Genetics Research

Project Number: 2032-22000-016-48-R
Project Type: Reimbursable Cooperative Agreement

Start Date: Oct 1, 2018
End Date: Aug 31, 2020

Objective:
1. Determine resistance/tolerance to biotic (black line, crown gall, root-lesion nematode, and Phytophthora root rot) and abiotic (water deficit) stressors and pyramiding these characters in superior walnut genotypes. 2. Development of new, and continuation of existing, field trials examining performance of elite walnut rootstock germplasm.

Approach:
Crown gall evaluations: In the greenhouse, seedlings or clonal saplings will be transplanted into 1-liter pots and grown to a trunk diameter of >0.5 centimeter (cm). These plants will be inoculated near the crown and in the apical 1/3 of the main trunk with a chisel blade dipped in a suspension of California A. tumefaciens biovar1 at 109 colony forming units/millilitre(ml). The 2 millimetre (mm) incision delivering at least 106 cells into the wound will be wrapped with elastic film. Positive and negative controls will consist of susceptible Paradox clones inoculated with A. tumefaciens or water respectively. In this rapid and reproducible procedure, tumor size will be rated 8 and 12 weeks post inoculation, and on potentially resistant candidates again 1-2 months after dormancy following the inoculation. Phytophthora: Conduct inoculations with isolates of Phytophthora species and evaluations of resistance to them in walnut rootstock germplasm and elite selections, as appropriate; for purposes of: i) identifying, confirming and further refining quantitative trait loci (QTLs) associated with resistance to Phytophthora, ii) examining resistance to Phytophthora in new seedling populations that have been selected, according to QTL markers, for putative multiple-pathogen resistance (i.e., for “pyramided” resistance to Agrobacterium, Phytophthora, and plant parasitic nematodes); iii) facilitating metatranscriptomic examinations of gene expression associated with resistance to Phytophthora; and iv) exploring Phytophthora resistance and its mechanisms in walnut family members that may serve as valuable donors of new genes for resistance to major soilborne pathogens or tolerance to environmental stress (e.g., salinity, drought, waterlogging). Water Relations: Conduct water relations studies under greenhouse conditions. In collaboration with pathologists in the overall project, interactive evaluations on elite genotypes and different pathogen systems will be conducted. Validating greenhouse information by sampling of field trials on research farms, in collaborators’ fields or commercial production will be part of the efforts. Elite Rootstocks: Investigate the response of elite rootstocks to Cherry leaf roll virus infection. By graft experiments in the greenhouse or protected field environment, the disease response will be measured. Test material will include the currently known elites, and other genotypes as the opportunity arises. They will also develop a virus infectivity assay to facilitate screening of walnut lines in a shorter time frame. All teams will conduct and participate in replicated field trials in commercial and research orchard settings to: i) further evaluate resistance of elite rootstocks to selected soilborne pathogens; and ii) assess horticultural adaptations and performance of the rootstocks.