Reassessment of the Erysiphe aquilegiae complex (Erysiphaceae)—resolution of a widespread and economically significant group of plant pathogens

Species of powdery mildew in the Erysiphe aquilegiae complex infect a broad array of economically valuable plants. The increasing number of reports of infections from diverse hosts prompted the present comprehensive phylogenetic and taxonomic reassessment of the complex. Accurate species delimitation within the complex has been hindered by morphological similarity among members of the complex, their broad host associations, and insufficient resolution in rDNA (ITS+28S) analyses. As such, the species concepts within this complex have been controversial, ranging from recognition of a single widely circumscribed species, E. aquilegiae s. lat., to narrower concepts recognizing multiple species, usually based on associated host genera or host species. To better understand this complex, we have undertaken phylogenetic analyses based on eight loci: rDNA regions (ITS, 28S, IGS) and protein-coding genes (CAM, GAPDH, GS, RPB2, TUB). In total, we sequenced 163 collections from more than 20 countries, resulting in the deposition of over 600 sequences in GenBank. Based on these results, we have recognized two species with broad host ranges, E. aquilegiae s. str. (emend.) and E. hortensiae comb. nov., There are several host-specific species, including: E. aquilegiae-coeruleae sp. nov. E. clematidicola sp. nov. E. clematidis-montanae sp. nov., and E. thalictri sp. nov. The old name E. nitida is reintroduced for powdery mildew on Aconitum spp. and Delphinium spp. (Ranunculaceae tribe Delphinieae), which forms a well-supported clade outside the E. aquilegiae s. str. and E. hortensiae clade. Furthermore, E. parnassiae could be confirmed as a separate species belonging to the E. aquilegiae complex. The application of the rank forma for genetically established biological races forming subclades within the plurivorous species is discussed and conditions for the introduction of acceptable formae are outlined.