Novel PCR-Based Detection Methods for the Lettuce Bacterial Leaf Spot Pathogen, Xanthomonas hortorum pv. vitians Morinière et al., 2020

Bacterial leaf spot in lettuce is a sporadic but devastating disease that threatens lettuce production worldwide. Severe outbreaks have resulted in up to 100% crop loss, and even smaller outbreaks can cause a significant yield loss, as the affected tissue must be removed from lettuce heads prior to their sale. The pathogen, Xanthomonas hortorum pv. vitians (Xhv), has at least three races, with each defined by the disease or resistance phenotype it elicits in lettuce cultivars and accessions. Specific molecular detection of Xhv would facilitate the work of clinicians, growers, seed companies, and researchers in the lettuce industry. We present an Xhv-specific touchdown PCR method and progress toward race-specific methods. We used an alignment of 18 Xhv strains and 9 closely related, non-target strains to identify pathovar- and race-specific gene clusters as targets for PCR primers. We evaluated the specificity first using in silico methods and then empirically using a collection of Xanthomonas strains. Our protocol demonstrated Xhv-specific detection from two sample types, including genomic DNA extracts and bacterial suspensions. Additional research is required to refine the race-specific protocols.