Stripe smuts of grasses: One lineage or high levels of polyphyly?

Stripe smut of grasses, Ustilago striiformis s.l., is a complex of smut fungi widely distributed over temperate and subtropical regions. The disease results in the shredding and death of leaf tissue following the rupture of elongated sori. Nearly 100 different grass species in more than 30 genera are infected by stripe smut. During the last two centuries more than 30 smut taxa have been described from members of this complex. The present study attempts to clarify the taxonomy and phylogeny of stripe smuts on grasses by analysing both morphological and molecular data. More than 200 specimens from different continents and host plants were examined. DNA was extracted from teliospores of 23 specimens from different hosts collected in Europe, Asia, and North America. The ITS and LSU regions of ribosomal DNA were amplified and used in phylogenetic analyses. The results of Maximum Parsimony and Bayesian analyses demonstrated that there are several lineages of stripe smut fungi. Analyses of morphological characters assessed with light and scanning electron microscopy showed high support for the differentiation of two clades as distinct from U. striiformis s.l., i.e., U. nunavutica sp. nov. and U. bromina. Two additional clades, U. striiformis s.str. on Holcus and a clade containing specimens from Elymus, were identified with molecular data although morphological differences were not apparent. Descriptions are given for each species.