Characterization of Xanthomonas spp. Associated with Bacterial Spot Outbreaks in Watermelon

Florida is a leading watermelon producer. During the unusually wet spring of 2018, when Florida watermelon fields received twice the 15-year average rainfall, severely diseased plants with atypical symptoms were observed across production areas. Bacterial strains were consistently isolated from tissue and characterized using phenotypic and molecular tests. Seventeen strains were isolated, and phenotypic characterization along with 16S rRNA gene sequence analysis identified all as Xanthomonas spp. Multi-locus sequence analysis of 11 strains identified one, three and seven strains as X. cucurbitae, X. melonis, and X. arboricola, respectively. X. arboricola strains exhibited high intragenetic diversity in the population and clustered most closely with the type strain of X. arboricola CFBP 2528^T. The three X. melonis sequences were identical to each other but differed from the type strain LMG 8670^T, whereas the X. cucurbitae strain was identical to the type strain CFBP 2542^T. Pathogenicity assays conducted across five cucurbit hosts found that only X. cucurbitae consistently caused pathogenic reactions on all cucurbit hosts, whereas X. melonis caused mild symptoms and X. arboricola only caused disease under conditions of prolonged leaf wetness. These findings represent the first documented outbreak in Florida involving three distinct Xanthomonas species on watermelon. The emergence of X. cucurbitae on watermelon and host range shift of X. melonis and X. arboricola are discussed in this study. Further studies are needed to understand the impact and coexistence of pathogenic and opportunistic strains during disease outbreaks and their effect on driving pathogen evolution and host adaptation.