Poplar rust systematics and refinement of Melampsora species delineation

We present a review of previous taxonomic treatments of the Melampsora species occurring on poplar and describe the features associated with each spore stage in a typical poplar rust life cycle. The morphological, biological and ecological characters traditionally used for taxonomy are summarized for all Melampsora taxa, including 17 accepted species, 2 formae speciales and 2 hybrids, currently listed in the literature as pathogenic on Populus spp. We discuss the historical taxonomic decisions that led to nomenclatural and classification complications for this group of rusts, and highlight the lack of type material for three species known only from the original collections: M. cumminsii, M. multa, and M. osmaniensis. Even when all the traditional features are considered, the taxonomy of Melampsora poplar rust species remains uncertain and incomplete. An overview of recent molecular studies suggests the need for a comprehensive revision of species concepts, based on phylogenetic relationships. To that end, we propose a polyphasic approach, including the Genealogical Concordance Phylogenetic Species Recognition method, be used to build robust and meaningful systematic framework for the Melampsora poplar rusts.