Xanthomonas translucens pv. undulosa (Xtu) is the causal agent of wheat bacterial leaf streak (BLS), which has become an important disease in the Midwestern United States and elsewhere. We recently discovered that Xtu strains differ in their avirulence on the Aegilops tauschii accession TA10105. A diverse set of Xtu strains was phenotyped on this accession, and representative virulent and avirulent isolates were selected for whole-genome sequencing. Comparative genomic analysis identified a single type III effector gene present exclusively in avirulent isolates. Targeted disruption of this gene in an avirulent strain resulted in a susceptible reaction on TA10105, confirming its role in avirulence. To identify the corresponding resistance gene, an F₂ population derived from a cross between TA10105 and a susceptible A. tauschii line was developed and used for genetic mapping. The resistance locus was mapped to chromosome 5D. (...)