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Background    Armillaria  is a globally distributed mushroom-forming genus composed primarily of plant pathogens. Species in this genus are prolific producers of rhizomorphs, or vegetative structures, which, when found, are often associated with infection. Because of their importance as plant pathogens, understanding the evolutionary origins of this genus and how it gained a worldwide distribution is of interest. The first gasteroid fungus with close affinities to  Armillaria — Guyanagaster necrorhizus —was described from the Neotropical rainforests of Guyana .  In this study, we conducted phylogenetic analyses to fully resolve the relationship of  G. necrorhizus  with  Armillaria . Data sets containing  Guyanagaster  from two collecting localities, along with a global sampling of 21  Armillaria  species—including newly collected specimens from Guyana and Africa—at six loci (28S,  EF1α, RPB2, TUB, actin-1  and  gpd ) were used. Three loci—28S,  EF1α  and  RPB2 —were analyzed in a partitioned nucleotide data set to infer divergence dates and ancestral range estimations for well-supported, monophyletic lineages.    Results  The six-locus phylogenetic analysis resolves  Guyanagaster  as the earliest diverging lineage in the armillarioid clade. The next lineage to diverge is that composed of species in  Armillaria  subgenus  Desarmillaria . This subgenus is elevated to genus level to accommodate the exannulate mushroom-forming armillarioid species. The final lineage to diverge is that composed of annulate mushroom-forming armillarioid species, in what is now  Armillaria sensu stricto.  The molecular clock analysis and ancestral range estimation suggest the most recent common ancestor to the armillarioid lineage arose 51 million years ago in Eurasia. A new species,  Guyanagaster lucianii  sp. nov. from Guyana, is described.    Conclusions  The armillarioid lineage evolved in Eurasia during the height of tropical rainforest expansion about 51 million years ago, a time marked by a warm and wet global climate. Species of  Guyanagaster  and  Desarmillaria  represent extant taxa of these early diverging lineages.  Desarmillaria  represents an armillarioid lineage that was likely much more widespread in the past.  Guyanagaster  likely evolved from a gilled mushroom ancestor and could represent a highly specialized endemic in the Guiana Shield.  Armillaria  species represent those that evolved after the shift in climate from warm and tropical to cool and arid during the late Eocene. No species in either  Desarmillaria  or  Guyanagaster  are known to produce melanized rhizomorphs in nature, whereas almost all  Armillaria  species are known to produce them. The production of rhizomorphs is an adaptation to harsh environments, and could be a driver of diversification in  Armillaria  by conferring a competitive advantage to the species that produce them.    Electronic supplementary material  The online version of this article (doi:10.1186/s12862-017-0877-3) contains supplementary material, which is available to authorized users.

Resolved phylogeny and biogeography of the root pathogen Armillaria and its gasteroid relative, Guyanagaster