A Multilocus Gene Genealogy Concordant with Host Preference Indicates Segregation of a New Species, Magnaporthe oryzae, from M. grisea

Magnaporthe oryzae is described as a new species distinct from M. grisea. Gene trees were inferred for Magnaporthe species using portions of three genes: actin, beta-tubulin, and calmodulin. These gene trees were found to be concordant and distinguished two distinct clades within M. grisea. One clade is associated with the grass genus Digitaria and is therefore nomenclaturally tied to M. grisea. The other clade is associated with Oryza sativa and other cultivated grasses and is described as a new species, M. oryzae. While no morphological characters as yet distinguish them, M. oryzae is distinguished from M. grisea by several base substitutions in each of three loci as well as results from laboratory matings; M.oryzae and M. grisea are not interfertile. Given that M. oryzae is the scientifically correct name for isolates associated with rice blast and grey leaf spot, continued use of M. grisea for such isolates would require formal nomenclatural conservation.