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United States Department of Agriculture

Agricultural Research Service

Research Project: Development of Almond, Stone Fruit, and Walnut Rootstocks with Improved Resistance to Soilborne Pathogens

Location: Crops Pathology and Genetics Research

2011 Annual Report


1a.Objectives (from AD-416)
1) Identify and deploy rootstocks with improved resistance/tolerance to the major soil-borne pathogens of almonds, other Prunus spp and walnut rootstocks including: Agrobacterium tumefaciens (crown gall), Phytophthora, Armillaria spp, phytoparasitic nematodes, and orchard replant disorder. 2) ID and map genes/QTLs governing resistance to facilitate marker-based disease screening and selection strategies to rapidly integrate resistance genes into new rootstocks. 3) Develop association mapping populations for mapping and validating genes governing resistance. 4) Propagate and field test resistant rootstock genotypes identified in objective #1.


1b.Approach (from AD-416)
We will use conventional breeding, disease-screening, novel propagation techniques in combination with educational outreach to develop, characterize, and deploy walnut and almond/Prunus rootstocks with to soilborne pathogens. The following material will be examined, NCGRU germplasm, existing rootstocks used as standards, and novel genotypes created from controlled crosses. Promising genotypes will be propagated as seed or as clonal plants and used in replicated evaluations of pathogen resistance. Genetic markers associated with resistance will be exploited in rootstock breeding efforts. Rootstocks with superior resistance/tolerance will be grafted/budded with scion cv's and evaluated in commercial nursery/orchard trials.


3.Progress Report

The agreement was established in support of Objective 1 of the in-house project, the goal being to improve management strategies for key soil borne diseases of tree fruit and nut crops; subobjective 1B-Identify and characterize available walnut and almond rootstock germplasm for resistance to key soil borne pathogens. The goal of this project is to use conventional breeding, disease-screening, novel propagation techniques to develop, characterize, and deploy walnut and almond/Prunus rootstocks with resistance to soilborne pathogens.

Juglans microcarpa, J. major, J. cathayensis all crossed with J. regia (Serr) generated > interpsecific hybrids. In addition to the hybrid seeds which went into tissue culture via embryo rescue, a second group of hybrid seeds were stratified, germinated and are now three month old seedlings in the green house. This material will be inoculated with Agrobacterium tumefaciens in the next few weeks and crown gall (CG) incidence rated. The 2010-11 screening data showed open pollinated seedlings from J. microcarpa mother trees exhibited the greatest degree of CG resistance. Twenty five percent of tested seedlings show CG resistance 6 months post inoculation. Clonal material taken from these CG resistant seedlings (dormant cuttings)was rooted and cultivated under greenhouse conditions. Rooting efficiency ranged from 0% to 38%. Eight successfully rooted J. microcarpa accessions (2-18 plantlets/accession), grown under greenhouse conditions, were screened for crown gall resistance. Five J. microcarpa accessions from this group continued to exhibit crown gall resistance. Seven novel Juglans microcarpa × J. regia hybrids were micropropagated for greenhouse evaluations of resistance to P. citricola and P. cinnamomi in late summer 2011. Three trials were established to assess resistance of 22 diverse almond/ peach rootstocks to Phytophthora spp. and replant disease complex. The field trial to evaluate resistance to the replant complex was established at the old orchard site near Parlier (i.e., the site where replant soil was collected for the greenhouse trial). Half of the plots at the old orchard site were strip fumigated in fall 2010 with Telone C35, which is known to prevent replant disease. The other plots remained non-fumigated as a control. In the field trial, resistance to the replant complex will be evaluated according to severity of growth suppression in the non-fumigated soil, relative to plant growth in the fumigated soil. All of the trials use randomized complete block designs with 5 replicate blocks of multiple plants for each combination of rootstock and soil treatment. Complete data from the trials will be available at the end of summer 2011. Several of the rootstocks with plum parentage in their backgrounds are expressing relatively high resistance to P. niederhauseri.


Last Modified: 4/24/2014
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