Genomic characterization of two Xanthomonas phaseoli pv. phaseoli strains of Italian origin reveals novel TAL effectors and putative susceptibility gene targets

Common bacterial blight (CBB) is a major disease of common bean (Phaseolus vulgaris L.), caused by Xanthomonas phaseoli pv. phaseoli (Xpp) and Xanthomonas citri pv. fuscans (Xcf). Despite the relevance of CBB in Italy, genomic data for Italian Xpp strains are currently missing. Here, we present the first complete genome sequence of two Italian Xpp strains, USB771 and DEF699, generated using Oxford Nanopore Technologies (ONT). Long-read sequencing produced high-quality assemblies for both strains, with genome completeness exceeding 98.9%. Comparative genomic analyses placed USB771 close to Canadian Xpp strains, whereas DEF699 clustered with the BB075a strain, isolated in the USA. Pathogenicity assays confirmed that both strains cause typical CBB symptoms, with DEF699 showing significantly higher virulence than USB771. In-depth analysis of transcription activator-like (TAL) effectors revealed variation in tal gene number, type, and genomic localization among CBB agents. Notably, USB771 carried a single chromosomal tal gene (tal19B) and lacked plasmid-borne TAL effectors, while DEF699 harboured one chromosomal (tal18A) and one plasmid-encoded tal gene (tal20B). Overall, this study provides the first genomic characterization of Italian Xpp strains, expands current knowledge of TAL effectors diversity in CBB pathogens by analysing public genomic data.