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ARS Home » Pacific West Area » Davis, California » Crops Pathology and Genetics Research » Research » Research Project #444616

Research Project: Putting phenotypic and genotyplc tools to work for improving walnut rootstocks (2023-2024)

Location: Crops Pathology and Genetics Research

Project Number: 2032-22000-017-051-R
Project Type: Reimbursable Cooperative Agreement

Start Date: Jun 1, 2023
End Date: May 31, 2024

Objective 1. In vitro and traditional walnut rootstock propagation that supports continuing selection and trialing of elite walnut rootstocks. Objective 2. Intensive rootstock trialing that tests putative superiority of new elite rootstocks for commercial walnut production. Objective 3. Publication and extension outreach that scientifically summarizes confirmed advantages and potential limitations of new walnut rootstocks.

Objective 1. Micropropagation methods will be used to preserve and multiply elite walnut rootstock genotypes. Two key sources of elites will include: 1) a "pool" of J. microcarpa x J. regia 'Serr' clones selected for putative CG, NEM, or PHY resistance (or resistance to two or three of the pathogens) from trials with replicated clonal plants (the clones were from '31 .01' and '31.09' J microcarpa mother trees pollinated by 'Serr'); and 2) a pool of elite J microcarpa x J regia seedlings (initially one plant per genotype) selected for putative CG, PHY, or NEM resistance (the seedlings were from '29.1 l ', '31.0 l ', '31.03', '31.09' and '3 l.12' mother trees of J microcarpa open pollinated with J. regia). Elites from the former and latter sources will be micropropagated: 1) for short-term assessment of resistance phenotypes in all three pathosystems (CG, PHY, NEM; done in a Davis greenhouse for CG and PHY and in KARE field plots for NEM) and 2) for long-term orchard assessments of walnut tree performance on the experimental rootstocks (done in commercial and experiment station orchards). Short-term confirmations of CG and PHY resistance will use ungrafted rootstocks (for pot trials in greenhouse, for field planting at KARE), whereas long-term trials in orchards will use grafted trees finished by a commercial nursery. Each batch of the experimental elite clones rooted for the additional CG, NEM, and PHY testing, whether destined for short­ term or long-term evaluations, will be produced along with clonal Vlach, VX211, and RX1 rootstocks, which will serve as resistance standards for comparisons with the experimentals. Objective 2. Short-term trials will be conducted to confirm putatively high resistance to CG, PHY, and NEM and complete PRP data sets. These trials are considered necessary because putatively high levels of resistance were, for some clones, based on data sets with limited replication, and resistance phenotyping has, for some clones, been completed only for one or two of the three pathogen systems. The confirmatory and PRP trials will be conducted in a greenhouse at Davis (for CG and PHY) and in field plots (for NEM). Objective 3. Data obtained from trials conducted under objective 2 will be summarized and provide the basis for extension outreach. The outreach will occur through presentations at UCCE-and industry­ sponsored meetings, industry reports and trade publications, and peer-reviewed journal submissions. Although field meetings are not planned for each 2016 trial location in 2023, it is anticipated that some UC Advisors in counties hosting the walnut rootstock trials will offer field days or informal tours featuring them.