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ARS Home » Pacific West Area » Davis, California » Crops Pathology and Genetics Research » Research » Research Project #434763

Research Project: Genetic and Physiological Characterization of Disease Resistant Walnut Rootstock Genotypes

Location: Crops Pathology and Genetics Research

Project Number: 2032-22000-016-36-R
Project Type: Reimbursable Cooperative Agreement

Start Date: Apr 1, 2018
End Date: Jun 30, 2019

1) Traditional and in vitro propagation of a genetically diverse collection of Juglans species including creation and characterization of an enhanced mapping population from Juglans microcarpa x J. regia crosses. 2) Identification of resistance/tolerance to crown gall, root-lesion nematode, and Phytophthora root/crown rot in the diverse Juglans germplasm. 3) Genetic, physical, and functional mapping of disease resistance genes and deployment of molecular markers for rapid screening/identification of resistant genotypes. 4) Outreach and development of new, and existing, field trials examining performance of elite rootstock germplasm.

1) While this is not a hypothesis driven objective, it remains a key aspect to the overall success of this project. Using all the technologies previously described, we will continue to produce seedling and in vitro clonal propagants from our collections that will be made available for testing in the different contexts created by the plant pathologists and horticulturists. For example, as each new disease resistant genotype is identified, it will be immediately propagated under in vitro conditions in order to confirm its phenotype under a variety of conditions. 2) In established disease resistance-screening pipelines, response to the following will be measured: (a) Agrobacterium tumefaciens (crown gall, CG), (b) Phytphthora cinnamomi (Phytophthora root rot, PHY) and (c) Pratylenchus vulnus and Meloidogyne incognita (root lesion and root knot nematodes, NEM). 3) a) Refinement of preliminary QTL maps for the three-pathogen systems. In our current SCRI/CWB-supported project, extensive screening for resistance to CG, PHY and NEM identified superior germplasm in J. microcarpa, J. major and J. cathayensis. Balancing resistance to multiple pathogens with the ability to produce large number of hybrids with J. regia and potential suitability as commercial rootstocks, two MT’s (J. microcarpa 31.01 and 31.09) crossed with ‘Serr’ were chosen for genetic dissection of disease resistance. Preliminary QTLs for resistance to CG, and PHY were identified after phenotyping a limited number of mapping population members (see 2017 Walnut Research Report). 4) We propose to continue evaluation of our five ongoing orchard rootstock trials (one trial each in Glenn, Lake, Sutter, Solano and Tulare counties). All five of the elite selections, as well as commercial standards of Paradox seedling and VX211 and RX1 clonal rootstocks were grafted to scions of J. regia ‘Chandler’. We will continue to assess tree growth (i.e., annual tree trunk circumference measurements, percent of photosynthetically active radiation intercepted by tree canopy), disease incidence and rootstock suckering. Crown gall incidence will be visually assessed and roots from trees that decline or die will be examined to determine whether they are infected with Phytophthora spp. or lesion nematode. Tree performance data from all orchard trials will be subjected to analyses of variance and regression analyses as appropriate.