Abstract
Xanthomonas vesicatoria and Xanthomonas axonopodis pv. vesicatoria, causal agents for bacterial spot of tomatoes and peppers, are difficult to distinguish from other xanthomonads found on field-grown plants. A genomic subtraction technique with subtracter DNA from nonpathogenic epiphytic xanthomonads was used to enrich for sequences that could serve as diagnostic probes for these pathogens. A 1.75-kb PstI-NotI fragment (KK1750) that preferentially hybridized to X. vesicatoria DNA and X. axonopodis pv. vesicatoria DNA was identified and cloned into pBluescriptII KS+. It hybridized to 46 (89%) of the 52 geographically diverse bacterial spot-causing xanthomonad (bsx) strains included in this study. The six probe-negative strains were genotypically and pathologically distinct from the other bsx strains studied. Two of these strains, DC91-1 and DC91-2, resembled X. campestris pv. raphani in that they also infected radish plants. X. vesicatoria strains gave stronger hybridization signals than did most X. axonopodis pv. vesicatoria strains. In a survey of 110 non-bsx plant-associated bacteria, including 44 nonvesicatoria phytopathogenic xanthomonads and 43 epiphytic xanthomonad strains, only 8 were probe positive, but the responses were weak. Further testing revealed that one of these strains was actually a tomato pathogen. Pulsed-field gel electrophoresis and Southern blot analysis of 46 bsx strains indicated that KK1750 sequences could be either plasmid-borne (10.9%), chromosome-borne (43.4%), or present on both replicons (45.7%). KK1750, unique in its ability to hybridize to both X. axonopodis pv. vesicatoria and X. vesicatoria strains, should facilitate disease diagnosis for these important plant pathogens.
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