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Summary: The fungal pathogen, Magnaporthe oryzae Triticum pathotype, causing wheat blast disease was first identified in South America and recently spread across continents to South Asia and Africa. Here, we studied the genetic relationship among isolates found on the three continents.Magnaporthe oryzae strains closely related to a South American field isolate B71 were found to have caused the wheat blast outbreaks in South Asia and Africa. Genomic variation among isolates from the three continents was examined using an improved B71 reference genome and whole‐genome sequences.We found strong evidence to support that the outbreaks in Bangladesh and Zambia were caused by the introductions of genetically separated isolates, although they were all close to B71 and, therefore, collectively referred to as the B71 branch. In addition, B71 branch strains carried at least one supernumerary mini‐chromosome. Genome assembly of a Zambian strain revealed that its mini‐chromosome was similar to the B71 mini‐chromosome but with a high level of structural variation.Our findings show that while core genomes of the multiple introductions are highly similar, the mini‐chromosomes have undergone marked diversification. The maintenance of the mini‐chromosome and rapid genomic changes suggest the mini‐chromosomes may serve important virulence or niche adaptation roles under diverse environmental conditions. [ABSTRACT FROM AUTHOR]

Wheat blast, caused by the Pyricularia oryzae Triticum lineage (PoT), first emerged in Brazil and quickly spread to neighboring countries. Its recent appearance in Bangladesh and Zambia highlights a need to understand the disease's population biology and epidemiology so as to mitigate pandemic outbreaks. Current knowledge is mostly based on characterizations of Brazilian wheat blast isolates and comparison with isolates from non-wheat, endemic grasses. These foregoing studies concluded that the wheat blast population lacks host specificity and, as a result, undergoes extensive gene flow with populations infecting nonwheat hosts. Additionally, based on genetic similarity between wheat blast and isolates infecting Urochloa species, it was proposed that the disease originally emerged via a host jump from this grass and that Vrochloa likely plays a central role in wheat blast epidemiology owing to its widespread use as a pasture grass. However, due to inconsistencies with broader phylogenetic studies, we suspected that these seminal studies had not actually sampled the populations normally found on endemic grasses and, instead, had repeatedly isolated members of PoT and the related Lolium pathogen lineage (PoLl). Re-analysis of the Brazilian data as part of a comprehensive, global, phylogenomic dataset that included a small number of South American isolates sampled away from wheat confirmed our suspicion and identified four new P. oryzae lineages on grass hosts. As a result, the conclusions underpinning current understanding in wheat blast's evolution, population biology, and epidemiology are unsubstantiated and could be equivocal. [ABSTRACT FROM AUTHOR]

This is a response to a recent Letter to the Editor of Phytopathology, in which Gupta et al. (2019) caution against the indiscriminate use of the MoT3 diagnostic assay that distinguishes isolates of Magnaporthe oryzae in the Triticum lineage from those that do not cause aggressive wheat blast. We confirm that the assay does reliably distinguish between wheat and rice isolates from Bangladesh and worldwide, as described in the original paper by Pieck et al. (2017). We have been unable to reproduce the equally intense amplification of WB12 and WB 12-like sequences reported in Figure 1 of the Letter. Other data presented by Gupta et al. (2019) support the specificity of the MoT3 assay. Therefore, cautions beyond those always associated with accurate reproduction of diagnostic assays are unwarranted. [ABSTRACT FROM AUTHOR]