Pyricularia graminis-tritici, a new Pyricularia species causing wheat blast

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.

Gene Flow between Divergent Cereal- and Grass-Specific Lineages of the Rice Blast Fungus Magnaporthe oryzae

“…Using population and phylogenomic analyses of single-copy orthologous genes and whole-genome SNPs identified in M. oryzae genomes from multiple cereal and grass hosts, we provide evidence that M. oryzae is subdivided into multiple lineages preferentially associated with one host plant genus. Neither the reanalysis of previous data nor the analysis of new data using previous phylogenetic species recognition markers supports the existence of a wheat blast-associated species called P. graminis-tritici (24). Marker MPG1, which holds most of the divergence previously detected, does not stand as a diagnostic marker of the wheat-infecting lineage of M. oryzae when tested in other lineages.…”

“…All of the P. graminis-tritici isolates from the Castroagudin et al study were contained in a clade with 80% support. Inspection of the MPG1 marker that was reported to be diagnostic for P. graminis-tritici (Castroagudin et al [24]) revealed that all of the isolates in this clade contained the P. graminis-tritici-type allele (green dots) and should therefore be classified as P. graministritici (Fig. 4).…”

“…Therefore, to test for possible correspondence between the proposed P. graminis-tritici species and the Lolium lineage (or indeed the Triticum lineage), we extended the 10-locus analysis to the M. oryzae genome sequences used in the present study. For reference, we included the multilocus data for 16 isolates from the Castroagudin et al study (24), representing all the major clades from that study. Nine of the 10 loci were successfully recovered from 68 of our M. oryzae genome sequences.…”

“…To test the previous delineation of a subgroup of wheat-infecting isolates as a new phylogenetic species, we reanalyzed the Castroagudin et al data set (24), which mostly included sequences from Brazilian isolates. However, instead of using bootstrap support in a total-evidence genealogy inferred from concatenated sequences for species delineation, we applied the GCPSR test (25,26).…”

“…and other Poaceae hosts, belongs to a new phylogenetic species, Pyricularia graminis-tritici, which is well separated from other wheat-and ryegrass-infecting isolates, as well as pathogens of other grasses (24). However, this proposed split was based on bootstrap support in a genealogy inferred from multilocus sequence concatenation, and genealogical concordance for phylogenetic species recognition (GCPSR [25,26]) was not applied.…”