Identification and development of crown gall-resistant walnut rootstocks

Globally, California in the United States is the second largest producer of walnuts (Juglans regia). Most commercially cultivated walnut trees are grafted onto rootstocks that are less susceptible to several pathogens than their English walnut scions. However, root diseases still cause an estimated 18% annual yield loss worth $ 241 million to the industry. The central goal of this project is to identify, characterize and ultimately deploy crown gall disease resistant walnut rootstocks. To achieve this goal we are using an integrated approach that encompasses disease resistance screening, in vitro and traditional plant propagation, genomics, and conventional breeding strategies. In this work, we exploited the Juglandaceae germplasm collection in the USDA-ARS National Clonal Germplasm Repository in Davis, California. We identified several Juglans species in this collection whose open pollinated progeny exhibited a high level of crown gall (CG) resistance. We also generated, and clonally propagated, a mapping population of GROTERDAN600 interspecific hybrid progeny from two J. microcarpa × J. regia crosses. Initial resistance screening of this mapping population has revealed that it segregates for resistance to Agrobacterium tumefaciens, the causative agent of crown gall. We also produced reference-quality genome sequences for both parents, i.e., J. microcarpa and J. regia. Using the genotyping-by-sequencing (GBS) data from the 600 hybrid progeny, we generated a genetic map for both J. microcarpa and J. regia. These maps contain ~50 SNP markers for each of the 16 linkage groups. By combining the CG phenotyping with the GBS data, we tentatively identified a major quantitative trait locus for A. tumefaciens resistance. To date we have identified ~10 experimental walnut genotypes that exhibit commercially viable levels of crown gall resistance. These genotypes, and their potential to serve as rootstocks, are in various stages of propagation and/or field-testing at five sites across the walnut growing region of California.