Xylella fastidiosa, the causal agent of Pierce's disease of grapevine, has been found in all major grape growing regions in California, USA. Large collections of X. fastidiosa isolates are available from these areas, which enable comparative studies of pathogen genetic traits and virulence. Due to the significant resource requirements for experiments with X. fastidiosa in grapevine, however, most studies use only a single isolate to evaluate disease, and it is not clear how much variability between isolates impacts disease development in experimental or natural settings. In this study, a comprehensive panel of X. fastidiosa isolates from all California grape growing regions were tested for virulence in susceptible grapevine and in the model host plant tobacco. Seventy-one isolates were tested, 29 in both grapevine and tobacco. The results of this study highlight the inherent variability of inoculation experiments with X. fastidiosa, including variation in disease severity in plants inoculated with a single isolate, and variability between experimental replicates. There were limited differences in virulence between isolates that were consistent across experimental replicates, or across different host plants. This suggests that choice of isolate within the Xylella fastidiosa subspecies fastidiosa Pierce's disease group may not make any practical difference when testing in susceptible grape varieties, and that pathogen evolution has not significantly changed virulence of Pierce's disease isolates within California. Location of isolation also did not dictate relative disease severity. This information will inform experimental design for future studies of X. fastidiosa in grapevine and provide important context for genomic research.
Keywords: Pierce's disease; Xylella fastidiosa; grapevine; virulence.