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ARS Home Pacific West Area Davis, California Crops Pathology and Genetics Research Research





- Browne, Greg

Research Project: **Location:
                            Crops Pathology and Genetics Research** Project Number: 2032-22000-017-019-T
                        
                        Project Type: Trust Fund Cooperative Agreement Start Date: Jan 1, 2021
                        
                        End Date: Dec 31, 2025 **Objective:**
                        
                        Objective 1 - Clonal propagation of commercially available and existing novel Prunus interspecific hybrids.
Objective 2 - Generation of high-quality disease resistance data for all three target pathogens and ID/retest promising novel and commercially available disease resistant rootstock selections.
Objective 3 - Evaluate key commercially available clonal rootstocks and the novel elite rootstock genotypes as “finished” trees (i.e. grafted to Nonpareil and Monterey) for their disease resistance phenotypes using supplemental pathogen inoculations under field/greenhouse conditions. **Approach:**
                        
                        We are conducting a comprehensive evaluation of key commercially available standard rootstocks. In addition, we are performing additional disease resistance evaluations of available elite genotypes before release to commercial evaluation blocks. As detailed in our previous annual report, we identified a large number of hybrids with disease resistance characteristics. Some genotypes exhibited resistance against two or three of the test pathogens, but few genotypes consistently exhibited high resistance to all three targeted pathogens. Additional resistance evaluations and horticultural characterizations are needed before elite selections are forwarded for commercial testing. In November/December 2019, 35 elite rootstock genotypes selected for growth characteristics and pathogen resistance were collected from the germplasm repository at UC Davis (“Armstrong”) and other locations if necessary. Semi-hardwood cuttings were made, and were transferred to Sierra Gold Nurseries for propagation. During 2020, successfully propagated genotypes will be grafted to scions to produce 25 complete trees each of ‘Nonpareil’ and of ‘Monterey’. The complete, potted trees will be available for field-testing at Kearney and at UC Davis. It is anticipated that 20 of the original 35 genotypes will yield sufficient numbers of trees for these field evaluations. These grafted trees are expected to be available for planting in the fall of 2020. To the extent possible, genotypes will be entered as rootstock plants into the respective pathogen testing pipelines. First-year data of these field evaluations will guide what genotypes will be used for the “grafted tree” evaluations.
Nematode evaluations at Kearney - In the fall 2020, a field with natural infestations with root lesion and root knot nematodes will be used to establish an experimental orchard. At row spacing of 18 ft and tree spacing of ca. 14 ft, genotypes will be planted in groups of four trees (two Nonpareil, two Monterey) as the main plots for the randomized complete block design with four replications (total: 8 Nonpareil plus 8 Monterey for each rootstock genotype). After establishment of the trees, additionally, every tree will receive supplemental inoculations with infested field soil with root lesion and root-knot nematodes. The trees will be grown through first, and possibly second harvest.
Phytophthora evaluations at Kearney - In the fall of 2020, favorable genotypes vetted from the initial field-testing of rootstocks for Phytophthora response will be planted as grafted trees with Nonpareil and Monterey scions. A similar field plot layout as used with nematode testing will be established.
Crown gall Crown gall evaluations at Armstrong/Greenhouse - The same genotypes used for nematode and Phytophthora resistance bioassays will be evaluated for crown gall resistance under either greenhouse or field conditions at “Armstrong” (UC Davis). Trees will be artificially inoculated with Agrobacterium tumefaciens.


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