Characterization of <i>Xanthomonas</i> 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 <i>Xanthomonas</i> spp. Multilocus sequence analysis (MLSA) of eleven strains identified one, three and seven strains as <i>X. cucurbitae</i>, <i>X. melonis</i>, and <i>X. arboricola</i>, respectively. <i>X. arboricola</i> strains exhibited high intra-genetic diversity in the population and cluster most closely with the type strain of <i>X. arboricola</i> CFBP 2528^T. The three <i>X. melonis</i> sequences were identical to each other but differed from the type strain LMG 8670^T, whereas the <i>X. cucurbitae</i> strain was identical to the type strain CFBP 2542^T. Pathogenicity assays conducted across five cucurbit hosts found that only <i>X. cucurbitae</i> consistently caused pathogenic reactions on all cucurbit hosts, whereas <i>X. melonis</i> caused mild symptoms and <i>X. arboricola</i> only caused disease under conditions of prolonged leaf wetness. These findings represent the first documented outbreak in Florida involving three distinct <i>Xanthomonas</i> species on watermelon. The emergence of <i>X. cucurbitae</i> on watermelon and host range shift of <i>X. melonis</i> and <i>X. arboricola</i> are discussed in this study. Further studies are needed to understand the impact and co-existence of pathogenic and opportunistic strains during disease outbreaks and their effect on driving pathogen evolution and host adaptation.