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United States Department of Agriculture

Agricultural Research Service

Research Project: Evaluation of Juglans Accessions from Mexico, Central and South America for Resistance to Crown Gall, Lesion Nematode, and Phytophthora

Location: Nat'l Clonal Germplasm Rep - Tree Fruit & Nut Crops & Grapes

Project Number: 2032-21000-020-11-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Aug 20, 2013
End Date: Aug 20, 2014

The immediate goal is to evaluate diverse accessions from Mexico, Central America, and South America, primarily tropical or sub-tropical accessions, for their response to several soil-borne pathogens of interest for commercial rootstock development. Emphasis will be placed on the least cold resistant accessions currently maintained as potted plants in a greenhouse, but we also will include several more temperate accessions growing in the field collection located at Winters, CA. Selected accessions will be propagated by tissue culture and clonal copies grown out in a greenhouse for use in screening for pathogen resistance. Depending on numbers of plants obtained, pathogens screened for each genotypes will include crown gall (Agrobacterium tumefaciens), nematodes (Pratylenchus vulnus), and Phytophthora. One or more plant of each genotype will be retained for use by the NCGR as clonal backups for the current collection and the in vitro cultures will be retained for maintenance as in vitro backups, potential cryopreservation, and use in further studies. The new 5 year plan of the NCGR includes the objective of backing up the collections using in vitro and cryopreservation techniques. This will provide a beginning with the Juglans collection.

A set of diverse Juglans genotypes representing accessions from Mexico, Central America, and South America (see Table 1) will be introduced into tissue culture by nodal cuttings taken from semi-soft current-year’s growth. The seedlings used initially will be selected to maximize diversity and the total number introduced will depend on ease of introduction and rate of successful multiplication of the initial choices. We will attempt a minimum of 20 individuals representing all listed species and add additional genotypes as results and resources allow. In addition, methods been developed for clonally propagating walnuts by rooted cuttings, forcing shoots from branches, hardwood cuttings, and a modified air-layering procedure. The later, in particular, would be suitable to this work. These methods generally produce fewer plants than micropropagation but can be employed relatively quickly, can produce a few plants for initial work, and will be used to supplement the in vitro approach, particularly if any genotypes prove difficult to culture. The appropriate size and condition of micropropagated or cutting-produced Juglans plants for use in the screening procedures for resistance to each pathogen have been previously established. As plants are generated by either method and reach appropriate size, they will be distributed to the participating plant pathologists. Testing and evaluation procedures for each of these pathogens have been established through previous work on other Juglans species and these procedures will be employed for this work or adapted if necessary.

Last Modified: 10/17/2017
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