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

item FARMAN, MARK - University Of Kentucky
item Peterson, Gary
item CHEN, LI - University Of Kentucky
item VALENT, BARBARA - Kansas State University
item BACHI, PAUL - University Of Kentucky
item MURKOCK, LLOYD - University Of Kentucky
item HERSHMAN, DONALD - University Of Kentucky
item Pedley, Kerry
item BAVARESCO, JORGE - Embrapa

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2016
Publication Date: 4/18/2017
Citation: Farman, M.L., Peterson, G.L., Chen, L., Valent, B.S., Bachi, P.R., Murkock, L.W., Hershman, D.E., Pedley, K.F., Fernandes, M.C., Bavaresco, J.L. 2017. The Lolium pathotype of Magnaporthe oryzae recovered from a single blasted wheat plant in the United States. Plant Disease. 101(5):684-692.

Interpretive Summary: In May 2011, a single blighted wheat head was observed in a University of Kentucky wheat research plot at Princeton, Kentucky. Laboratory examination of the wheat head showed it to be infected by the fungal pathogen Magnaporthe oryzae. This led to concern because M. oryzae on wheat is a new and emerging fungal plant pathogen in Brazil, Paraguay, Bolivia and Argentina that can cause significant crop loss as well as possible export concerns. This disease, wheat blast, has never been reported in the United States. Sequencing of PCR amplified products of the isolate found in Kentucky showed the presence of an allele normally found in a M. oryzae strain that causes grey leaf spot of ryegrass (Lolium spp). Genome sequencing revealed that the the isolated found on wheat in Kentucky was most closely related to a local isolate found on Lolium multiflorum and not the pathogen causing wheat blast in South America. Further greenhouse pathogenicity studies have shown that some U.S. isolates of M. oryzae that cause grey leaf spot on ryegrass are capable of producing blast symptoms on wheat heads. The conclusion was that the infection found in the single wheat head in Kentucky was most likely of local origin and not an exotic incursion.

Technical Abstract: 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, Kentucky 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 the pyriform spores of Magnaporthe oryzae. Spore PCR led to the successful amplification of three M. oryzae molecular markers, thereby confirming the pathogens’ identity. PCR amplicon sequencing revealed that the suspect possessed a wheat blast allele for CH7BAC7 but contained an MPG1 allele normally found in strains causing gray leaf spot on Lolium species. Genome sequencing revealed that the US wheat blast isolate was most closely related to an M. oryzae strain isolated from annual ryegrass in 2002 but was very distantly related to M. oryzae strains causing wheat blast in S. America. Inoculation tests showed that the suspect isolate was pathogenic to wheat in growth chamber assays. We conclude that this first occurrence of wheat blast in the US was most likely caused by a strain that evolved from an endemic Lolium infecting pathogen and not by an exotic introduction from S. America. Moreover, we show that M. oryzae strains capable of infecting wheat have existed in the U.S. for at least 16 years. Finally, evidence is presented that the environmental conditions in Princeton during the spring of 2011 were unusually conducive to inoculum production.