P-560 - Insights into Xanthomonas arboricola pv. juglandis in California: population genomics, copper resistance, and type III effectors

Xanthomonas arboricola pv. juglandis (Xaj), the causal agent of walnut blight, remains a threat to crop production throughout central and northern California. The combination of copper-based pesticides and mancozeb is the primary method for walnut blight management, but its effectiveness is compromised by widespread copper resistance in Xaj populations. Our research aims to characterize the genetic diversity, prevalence of copper resistance, and type III secretion system effector (T3E) repertoires in California Xaj populations. Whole-genome sequencing and de novo genome assembly were conducted using 115 strains isolated from 1994 to 2019 from 12 counties. Population structure and phylogenetic relationships were assessed based on core-genome alignments. Five genetic clusters were identified, but no correlation between cluster assignment and geographic origin was found. A pan-genome approach was used to detect the presence of copper resistance-related genes. A total of 77 Xaj strains harbored copper resistance-related genes, and complementary in vitro assays confirmed those strains were copper-resistant. Type III effectors were identified using a combination of tBLASTn and the pan-genome approach. The T3E repertoires consisted of 14 ‘core’ effectors present in all 115 strains, in addition to 8 variable effectors. Genetically distinct strains with varying T3E repertoires were screened for virulence in walnut inoculations. Differences in virulence reveal the ongoing evolution of Xaj and its potential to adapt to walnut cultivars developed by the UC Davis breeding program.