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United States Department of Agriculture

Agricultural Research Service

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Research Project: Development of Disease Resistant Walnut Rootstocks: Integration of Conventional and Genomic Approaches (Uc Davis Component)

Location: Crops Pathology and Genetics Research

2013 Annual Report


1a.Objectives (from AD-416):
1)Assemble germplasm and develop genetic and association mapping populations for mapping genetic loci mediating disease resistance. (Leslie) 2)Conduct studies to elucidate host selection behavior of walnut twig beetle the vector of Geosmithia morbida the causal agent of Thousand cankers disease of walnut. (Bostock) 3)Identify and develop Single-nucleotide polymorphisms (SNP) markers and the associated SNP genotyping platform (Dvorak, Dandekar) 4)Quantify the economic impact of disease-resistant rootstocks on the US walnut industry(Klonsky, Lampinen)


1b.Approach (from AD-416):
1)Will work with the USDA-ARS, National Clonal Germplasm Repository for selection of mother tree panels and assembly of OP (open-pollinated) seedlings, assembly of interspecific hybrid seedlings, and propagation techniques. Selected germplasm will include Juglans spp. currently used in walnut rootstocks as well as novel species that offer promising levels of disease resistance. 2)Will work with the USDA-ARS, National Clonal Germplasm Repository to evaluate mature trees by inoculating healthy, intact branches with local isolates of G. morbida. After 6 weeks, inoculated branches will be excised and evaluated for canker development. Will work with USDA Forest Service in Davis CA on Walnut Twig beetle. Evaluated WTB to understand the extent to which Thousand cankers disease epidemic is being driven by host preferences. 3)Walnut trees derived from crosses of resistant MTs with English walnut and OP trees of wild walnut species for association mapping will be genotyped using GBS. 4)Examine the economics of clonal rootstock use-the costs and income associated with orchard establishment for each rootstock. The price to growers of the clonal trees produced in pots, clonal trees field grown and conventional trees will be collected from the major nurseries supplying trees to the walnut industry. Harvest costs will be estimated based on the yields extrapolated to a 40 acre block. The effect of rootstocks on yield will be analyzed by monitoring canopy growth dynamics as determined by measuring interception of photosynthetically active radiation.


3.Progress Report:

This project was established in support of objective 1 of the in-house project, which is to characterize the etiology, biology, and ecology of key phytopathogenic agents and their interactions with economically important tree and grapevine species. The goal of the project is to develop disease resistant walnut rootstocks using conventional and genomic approaches.

Successful detection and development of disease resistant walnut rootstocks requires the creation of a genetically diverse plant germplasm. Towards that end, we have collected over 12,000 open pollinated walnut seeds from approximately 30 mother trees representing at least 5 different Juglans species. These seeds have been stratified, germinated, transplanted and cultivated in pots in the greenhouse. We have generated hundreds of trees for screening using this approach. In addition, we conducted embryo rescue of recalcitrant genotypes in order to propagate them in vitro. Clonal propagation of these genotypes from the in vitro cultures has resulted in the generation of 100’s of clonal seedlings. These clones have been transferred to pathologists on our team who are screening the trees for resistance to crown gall, Phytophthora, Armillaria, and lesion nematode. In addition, we incorporated the use of air laying techniques which allowed us to clonally propagate numerous mother trees. The mother trees used in air layering experiments are those from which we are collecting open pollinated seeds and are using for directed crosses. In addition, we selected numerous mother trees of the species, Juglans microcarpa (Texas black walnut) and Juglans cathayensis, which have been crossed with pollen from Juglans regia (English walnut) CV Serr. Nuts from these crosses have been collected, stratified and germinated or passed through invitro embryo rescue in order to generate clonal copies of these important genotypes. These clones are also being evaluated for disease resistance. In addition, we generated a large population of Pterocarya stenoptera (Chinese wingnut), a walnut relative, which has been screened for crown gall resistance and found to be highly resistant. To date, we have generated and screened thousands of genetically diverse walnut trees for disease resistance.


Last Modified: 11/23/2014
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