1a.Objectives (from AD-416):
1. Identify and characterize a novel source of crown gall (CG) resistance in Juglans microcarpa genotypes and associated hybrids maintained in the USDA/ARS National Clonal Germplasm Repository in Davis, CA.
2. Clonally propagate CG resistant genotypes using micro propagation and standard propagation techniques.
3. Characterize crown gall resistant phenotypes of clonally propagated cuttings from genotypes exhibiting crown gall resistance.
1b.Approach (from AD-416):
Open pollinated seeds collected this fall from selected black walnut and butternut accessions maintained at the Wolfskill Experimental Orchards in Winters, California, will be stratified, germinated and grown under glasshouse conditions. Once seedlings reach a trunk diameter of at least 0.5cm, the crown of the tree will be inoculated with A. tumefaciens. Selections that consistently show CG resistance for three years will be clonally propagated and tested for resistance to Phytophthora root rot, lesion nematodes and Armillaria root disease.
The agreement was established in support of objective 3, subobjective 3.3 of parent project, which is to improve management strategies for key soil borne diseases of tree fruit and nut crops, – identify and characterize available walnut and almond rootstock germplasm for resistance to key soil borne pathogens. The goal of this project is the generation and evaluation of Juglans microcarpa hybrids and paradox root stock selections for crown gall resistance in walnut. This project has been actively funded by the walnut board for the last 4 years due to the long term nature of tree crop rootstock development. Open pollinated (OP) progeny from several wild walnut species, maintained in the USDA-ARS Davis, California, National Clonal Germplasm Repository were identified with resistance to crown gall (CG)and will continue to be examined here. When a given mother tree produces CG resistant OP progeny we found typically 20% of the progeny are resistant. We generated dormant cuttings form CG resistant seedlings.
Approximately 20% of the seedlings propagated from dormant cuttings retained the original CG resistant phenotype. Progeny form the walnut wild species, Juglans microcarpa (Texas Black) generate resistant open pollinated progeny at a higher frequency than all other walnut wild species examined. To identify the genetic loci mediating CG resistance, F1 progeny generated from crosses between CG resistant female parents (Texas Black walnut) and CG susceptible pollen donors were screened for CG resistance. The progeny exhibited a 1:4 inheritance ratio of resistant: susceptible progeny. These CG resistant progeny have been placed into tissue culture. We have generated clonal copies which continue to show CG resistance. These clones are now being examined for resistance to other major soil borne pathogens. We continue to examine open pollinated progeny from J. microcarpa mother trees in addition to making additional crosses in order to generate genetically diverse CG resistant genotypes for exploitation as a commercial walnut rootstock. Open pollinated seedlings from a close relative of walnuts, Pterocarya (Chinese Wingnut) exhibited a very high level of CG resistance and rooted well from the dormant cutting stage.