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, <i>Xanthomonas hortorum</i> pv. <i>vitians</i> (<i>Xhv</i>), 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 <i>Xhv</i> would facilitate the work of clinicians, growers, seed companies, and researchers in the lettuce industry. We present an <i>Xhv</i>-specific touchdown PCR method and progress toward race-specific methods. We used an alignment of 18 <i>Xhv</i> 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 <i>Xanthomonas</i> strains. Our protocol demonstrated <i>Xhv</i>-specific detection from two sample types, including genomic DNA extracts and bacterial suspensions. Additional research is required to refine the race-specific protocols.