Abstract:
Pyricularia oryzae is a species complex that causes blast disease on more than 50 species of poaceous
plants. Pyricularia oryzae has a worldwide distribution as a rice pathogen and in the last 30 years emerged as an
important wheat pathogen in southern Brazil. We conducted phylogenetic analyses using 10 housekeeping loci for
128 isolates of P. oryzae sampled from sympatric populations of wheat, rice, and grasses growing in or near wheat
fields. Phylogenetic analyses grouped the isolates into three major clades. Clade 1 comprised isolates associated
only with rice and corresponds to the previously described rice blast pathogen P. oryzae pathotype Oryza (PoO).
Clade 2 comprised isolates associated almost exclusively with wheat and corresponds to the previously described
wheat blast pathogen P. oryzae pathotype Triticum (PoT). Clade 3 contained isolates obtained from wheat as well
as other Poaceae hosts. We found that Clade 3 is distinct from P. oryzae and represents a new species, Pyricularia
graminis-tritici (Pgt). No morphological differences were observed among these species, but a distinctive
pathogenicity spectrum was observed. Pgt and PoT were pathogenic and highly aggressive on Triticum aestivum
(wheat), Hordeum vulgare (barley), Urochloa brizantha (signal grass), and Avena sativa (oats). PoO was highly
virulent on the original rice host (Oryza sativa), and also on wheat, barley, and oats, but not on signal grass. We
conclude that blast disease on wheat and its associated Poaceae hosts in Brazil is caused by multiple Pyricularia
species. Pyricularia graminis-tritici was recently found causing wheat blast in Bangladesh. This indicates that
P. graminis-tritici represents a serious threat to wheat cultivation globally.
