A high level of genetic divergence between Exserohilum turcicum populations from maize and sorghum in Henan province of China revealed by SSR markers

Exserohilum turcicum is the causative agent of northern leaf blight of maize and sorghum. Although host specificity of E. turcicum has been observed from isolates collected from maize and sorghum, limited molecular evidence has been presented so far for the formation of formae speciales. In this study, 15 simple-sequence repeat (SSR) markers were used to study genetic diversity and population structure of 60 E. turcicum isolates from maize and sorghum in Henan province of China. A total of 54 haplotypes were identified from all isolates, in which 29 haplotypes were unique to the maize population and 25 were unique to the sorghum population. The phylogenetic tree showed that 60 isolates were clustered into two genetic groups; Group I consisted of 30 isolates from sorghum and Group II consisted of 30 isolates from maize. The formation of two groups was also supported by the result of principal component analysis. The analysis of population structure indicated that the main source of genetic compositions was completely different between the maize and sorghum populations. The fixation index (F-ST) and gene flow (Nm) between the two populations were 0.184 and 1.111, respectively. The analysis of molecular variance demonstrated that 27% and 73% of the total variation were distributed between and within the two populations, respectively. These findings reveal a high level of genetic divergence between E. turcicum populations associated with maize and sorghum and advance our understanding of the molecular basis underlying host specificity of E. turcicum.