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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

2012 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, which is 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. ARS Scientists at Davis, California, made additional crosses with CG-resistant female parents of Juglans microcarpa, J. major, J. cathayensis with the CG-susceptible pollen donor J. regia (Serr). These interspecific progeny were screened for CG resistance. We found 1 in 5 of the progeny were CG resistant. CG resistant genotypes have been placed into an invitro culture system and 100s of clonal copies generated for wide spread field testing. Pterocarya (Chinese wing nut), a distant relative of walnuts, exhibited a high in vitro level of stable resistance to crown gall disease and Phytophthora crown/root rot. Rooted dormant cuttings of CG resistant Pterocarya remained CG resistant. Once again, 2011-12 screening data showed open pollinated seedlings from J. microcarpa mother trees exhibited the greatest degree of CG resistance. Additional J. microcarpa (Texas Black Walnut) mother trees were crossed with pollen from cv “serr”. Nuts from these new crosses will be collected, stratified and germinated this fall. Ten seedlings of J. microcarpa genoytpes continue to exhibit CG resitance 1.5 years post inoculation with the CG pathogen demonstrating the stable nature of CG resistance. Armillaria screening of commercially available hybrid walnut rootstocks revealed Paradox hybrid AX1 exhibited high levels of CG resistance. One field trial examining the CG resistant nature of 40 different Prunus genotypes has been established and inoculations with 5 genetically diverse Agrobacterium isolates have been completed. Three ongoing trials established to assess resistance of 22 diverse almond/ peach rootstocks to Phytophthora spp. and replant disease complex is beginning to reveal diversity in replant susceptibility. All of the trials use randomized complete block designs with 5 replicate blocks of multiple plants for each combination of rootstock and soil treatment.

Last Modified: 4/19/2014
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