Blast disease is a notorious fungal disease leading to dramatic yield losses on major food crops such as rice and wheat. The causal agent, Pyricularia oryzae, encompasses different lineages, each having a different host range. Host shifts are suspected to have occurred in this species from Setaria sp. to rice and from Lolium sp. to wheat. The emergence of blast disease on maize in Iran was observed for the first time in the North of the country in 2012. We later identified blast disease in two additional regions of Iran, Gilan in 2013, and Golestan in 2016. Epidemics on the weed barnyard grass (Echinochloa spp.) were also observed in the same maize fields. Here, we showed that P. oryzae is the causal agent of this disease on both hosts. Pathogenicity assays in the greenhouse revealed that strains from maize can infect barnyard grass and conversely. However, genotyping with SSR markers and comparative genomics showed that strains causing field epidemics on maize and on barnyard grass are different, although they belong to the same previously undescribed clade of P. oryzae. Phylogenetic analyses including these strains and a maize strain collected in Gabon in 1985, revealed two independent host-range expansion events from barnyard grass to maize. Comparative genomics between maize and barnyard grass strains revealed the presence/absence of five candidate genes associated with host specificity on maize, with the deletion of a small genomic region possibly responsible for adaptation to maize.