Goss's wilt and leaf blight of maize is caused by Clavibacter nebraskensis and has reemerged as an important disease in North America. Despite its epidemiological relevance, this species remains poorly characterized in terms of population structure, functional diversity, and ecological differentiation, particularly among strains reported from Mexico. In this study, long-read whole-genome sequencing and phenotypic assays were used to characterize genomic diversity, virulence, and fitness-associated traits in C. nebraskensis. We generated 24 long-read genomes, including 20 contemporary Mexican isolates and four historical United States strains collected between 1969 and 1996, and compared them with publicly available genomes from North America and South Africa. Phylogenomic analyses confirmed that all strains cluster within the C. nebraskensis clade, and gene accumulation curves supported a closed pangenome with accessory gene variation linked to geographic origin and isolation period. Functional assays showed strain-level variation in virulence, enzymatic activity, bacteriocin antagonism, polysaccharide production, biofilm formation, and pigmentation. Cellulolytic activity was associated with disease severity, whereas pigment-related traits were linked to thiamine metabolism. Overall, these results indicate that C. nebraskensis comprises ecologically heterogeneous populations, structured around alternative survival and competition strategies. Integrating genome-wide comparisons with functional characterization of fitness-related traits provides a framework for understanding the biological factors underlying Goss's wilt. dynamics.