Genetic and phenotypic characterization of Xanthomonas species pathogenic of wheat in Uruguay

Abstract

Bacterial diseases affecting wheat production in Uruguay are an issue of growing concern yet remain largely uninvestigated in the region. Surveys of 61 wheat fields carried out from 2017 to 2019 yielded a regional collection of 63 strains identified by 16S rRNA gene analysis as Xanthomonas spp. A real-time PCR protocol using species-specific primers previously reported allowed the identification of 44 strains as X. translucens, the causal agent of bacterial leaf streak (BLS) in wheat and other cereal crops. Multilocus sequence analysis (MLSA) of four housekeeping genes (dnaK, fyuA, gyrB, and rpoD) revealed that these strains were most closely related to X. translucens pv. undulosa, the pathovar that is most commonly associated with BLS of wheat. Multilocus sequence typing (MLST) was applied to examine the genetic diversity among X. translucens strains. Strains were assigned to four different sequence types, three of which have been previously reported globally. Additionally, 17 Xanthomonas strains not belonging to X. translucens were obtained from diseased wheat leaves. Phylogenetic analysis showed that these strains are closely related to Xanthomonas prunicola, and clustered together with previously uncharacterized Xanthomonas strains isolated from wheat in Minnesota, US. In planta pathogenicity assays carried out on a BLS susceptible wheat cultivar showed that X. translucens pv. undulosa strains caused brown necrosis symptoms typical of BLS, while non-translucens Xanthomonas sp. strains elicited an atypical symptom of dry necrosis. These findings suggest that local wheat fields are affected by X. translucens pv. undulosa, and by a new wheat pathogen within the Xanthomonas genus.