1a.Objectives (from AD-416):
1. Identify and characterize a novel source of crown gall (CG) resistance in the species Juglans microcarpa germplasm and associated hybrids maintained in the USDA/ARS National Clonal Germplasm Repository in Davis, CA.
2. Characterize crown gall resistant phenotypes of clonally propagated cuttings from genotypes exhibiting crown gall resistance.
3. Produce clonal copies of promising hybrids and conduct "large scale" greenhouse and field evaluation to identify promising genotypes.
1b.Approach (from AD-416):
Open pollinated seeds will be stratified, germinated and grown under glasshouse conditions. Once the seedlings reach a trunk diameter of at least 0.5cm the crown of the tree will be inoculated with A. tumefaciens strain EC1. Observations on tumor development will be recorded at monthly intervals by noting first-appearance and then percent trunk/crown girdled.
The agreement was established in support of objective 1 of parent project, which is to improve management strategies for key soil borne diseases of tree fruit and nut crops, sub-objective 1B – 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. Open pollinated 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). 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 facilitating the generation of hundreds of clonal copies (i.e. seedlings) of this valuable genotype to be used for widespread field testing. 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.