Goss's wilt and blight of maize, caused by Clavibacter nebraskensis, is an important disease in the USA and Canada. The re‐emergence of Goss's wilt in the mid‐2000s and the subsequent spread of this disease probably resulted from changes in crop management practices and possibly changes in the pathogen population. The objectives of this study were to determine the genetic diversity and phylogeny of strains of C. nebraskensis associated with recent and historic Goss's wilt epidemics in the north Central USA, to measure the effects of collection period on population structure, and to identify the phylogenetic status of avirulent strains of C. nebraskensis. Multilocus sequence analysis and typing were used to address these objectives. Bayesian analysis of the sequences of atpD, dnaK, and kdpA separated the type strain of C. nebraskensis from type or reference strains of three other Clavibacter spp. The 125 strains of C. nebraskensis used in this study formed a monophyletic taxon. Four lineages were identified. Clades I, II, and IV contained a high proportion of highly virulent strains from the Upper Midwest. Clade III contained a high proportion of strains from Nebraska, Colorado, and South Dakota collected between 1969 and 1998. A significant substructure was detected between subpopulations from historic outbreaks in Nebraska and Colorado and more recent outbreaks in Minnesota. Virulence or avirulence in maize was not correlated with a particular MLSA clade. The results support the hypothesis that genetic changes in the population of C. nebraskensis were correlated with recent area expansions of Goss's wilt.

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Clavibacter nebraskensis is the causal agent of Goss’s leaf blight and wilt, an important disease of maize in the United States and Canada. The epidemiology and ecology of this bacterial pathogen are poorly understood. Infested maize residue is often considered to be the primary source of inoculum for maize; however, the potential for many other plant species to be infected and serve as inoculum sources is unknown. The goal of this study was to determine if C. nebraskensis could infect, survive, and grow on common weed, crop, and grass species. Seedling leaves of 18 plant species that grow in maize production areas in the United States were inoculated with this pathogen in a controlled environment and in the field. Lesion development, bacterial streaming, and pathogen population size on leaves were then determined and used as criteria to evaluate host–pathogen interactions. Woolly cupgrass (Eriochloa villosa) and the native prairie grasses big bluestem (Andropogon gerardii) and little bluestem (Schizachyrium scoparium) developed lesions and bacterial streaming, identifying them as hosts and susceptible to infection. To our knowledge, this is the first report of these grass species being hosts of C. nebraskensis. Ten other grass species, including wheat and oats, were identified as potential sustaining hosts that maintained epiphytic or endophytic pathogen populations >106 colony forming units per leaf sample but displayed no evidence of infection. Five broadleaf species tested were nonhosts based on the three criteria. This study suggests that multiple plant species support infection and growth of C. nebraskensis and further elucidates the ecology of this pathogen and the epidemiology of Goss’s wilt.