The lolium pathotype of Magnaporthe oryzae recovered from a single blasted wheat plant in the United States

Wheat blast is a devastating disease that was first identified in Brazil and has subsequently spread to surrounding countries in South America. In May 2011, disease scouting in a University of Kentucky wheat trial plot in Princeton, KY identified a single plant with disease symptoms that differed from the Fusarium head blight that was present in surrounding wheat. The plant in question bore a single diseased head that was bleached yellow from a point about one-third up the rachis to the tip. A gray mycelial mass was observed at the boundary of the healthy tissue and microscopic examination of this material revealed pyriform spores consistent with a Magnaporthe sp. The pathogen was subsequently identified as Magnaporthe oryzae through amplification and sequencing of molecular markers, and genome sequencing revealed that the U.S. wheat blast isolate was most closely related to an M. oryzae strain isolated from annual ryegrass in 2002 and quite distantly related to M. oryzae strains causing wheat blast in South America. The suspect isolate was pathogenic to wheat, as indicated by growth chamber inoculation tests. We conclude that this first occurrence of wheat blast in the United States was most likely caused by a strain that evolved from an endemic Lolium-infecting pathogen and not by an exotic introduction from South America. Moreover, we show that M. oryzae strains capable of infecting wheat have existed in the United States for at least 16 years. Finally, evidence is presented that the environmental conditions in Princeton during the spring of 2011 were unusually conducive to the early production of blast inoculum.