Different species of Rhynchosporium , including R. commune , R. secalis , R. agropyri , R. orthosporum and R. lolii cause scald disease or leaf blotch on barley and some grasses which reduces the quantity and quality of the yeild. In this study, 204 isolates from barley and uncultivated grasses were collected from Gorgan, Miandoab, Eyvan and Baghmalek. Morphological and phylogenetic analyses of two ITS and beta-tubulin loci were used to identify Rhynchosporium species and their host ranges in Iran. Conidia of 68 isolates from barley and wild grasses were measured for morphological identification. The mean of total conidial lengths was 16.79 with standard error of the mean = 0.04 and varied between 15.00-22.53 µm among different isolates. The mean of their conidial widths were 3.06 with standard error of the mean 0.006 and varied between 2.73 – 3.50 µm among different isolates. Despite, the large variation observed for conidial dimensions, the phylogeny constructed from the ITS-rDNA region and -tubulin gene, revealed that all isolates from barley and wild grasses in Iran belong to R. commune . R. commune is reported for the first time from Hordeum murinum . glaucum , H. vulgare . spontaneum and Lolium multiflorum from Iran. This study is also a first report of Avena sativa (matrix nova) as a new host of R. commune . Cross inoculation of R. commune isolates from all hosts, uncultivated grasses and cultivated barley, showed that uncultivated grasses could be as an inoculum sources for cultivated barley epidemics and also contribute in pathogen evolution. Thus, management of uncultivated grasses in the vicinity of barley fields should be considered in managing the disease on cultivated barley. Also, in this study, the genetic structure of R. commune populations was investigated in Iran and Syria (using of the collected isolates by Linde in ANU university in Australia) using microsatellite marker. Previous studies examined structure of Syrian populations as a representative of cultivated barley’s centre of the origin, the Fertile Crescent using microsatellites and suggested that R. commune and Hordeum vulgare (cultivated barley) did not co-evolve in the host’s centre of origin. In the present study R. commune populations from Syria with those from Iran, which represents a secondary centre of origin for barley at the eastern edge of the Iranian Plateau were compared. For this purpose, 14 microsatellite loci were amplified and analyzed in Iranian and Syrian populations. The results of this study showed that genotype diversity of Iranian populations is less than that of Syrian populations, but they don’t have a significant difference in gene diversity. So, R. commune and barley did not co-evolve in the centers of origin of cultivated barley and it’s likely that the pathogen have been introduced recently into Iran through infected barley seed. Hierarchical analyses of molecular variance revealed that most genetic diversity for populations of Iran and Syria was distributed within populations, with only %14 among populations. Analyses of multilocus association, genotype diversity and mating type frequency suggest that Iranian populations reproduce predominantly asexually. Investigation of mating types of Iranian isolates showed that they have the ability to reproduce sexually on barley and uncultivated grasses due to the presence of both mating types on these hosts. Key words Morphology, Host, Syria, Evolution, Gene and genotype diversity, Dispersion