Phylogeny and taxonomy of the Erysiphe adunca complex (Erysiphaceae, Helotiales) on poplars and willows

Erysiphe adunca s. lat. is a common powdery mildew species, distributed almost worldwide, on hosts of Populus and Salix species. Based on its wide host range and distribution as well as its strong morphological variations, E. adunca s. lat. was previously divided into several species or, alternatively, into varieties. However, comprehensive phylogenetic examinations on the putative monophyly of this species or the existence of a possible E. adunca species complex have not been conducted based on combined morphological and molecular approaches. In this study, we retrieved rDNA (ITS and LSU) sequences of 22 powdery mildew specimens on Populus and Salix spp. collected in China, Germany, Iran, Japan, Russia, and the United States and conducted morphological examinations of these and additional specimens. The morphological and molecular data obtained clearly indicate that E. adunca is a species complex comprised of at least four species., viz., E. adunca s. str. (on Populus spp. in Asia, Europe, and North America), E. capreae, including E. pseudoregularis as a new synonym (on Salix spp., particularly on species in Salix subgen. Vetrix sect. Vetrix in Asia, Europe, and North America), the reinstated E. salicis (on Salix spp. belonging to various subgenera of Salix in Asia and Europe), and E. mandshurica (which has been confirmed as a species of its own on Populus spp., in Asia). In addition to the clear phylogenetic differentiation, the morphology of the asexual and sexual morphs of E. adunca s. str., E. capreae and E. salicis is easily distinguishable. Sequences retrieved from collections on Salix gracilistyla (Uncinula salicis-gracilistylae), a species that was previously considered a variety of Uncinula adunca and Erysiphe adunca, formed a small subclade within the E. salicis clade and are now referred to as Erysiphe salicis var. salicis-gracilistylae comb. nov. Alphitomorpha adunca; Erysiphe capreae and E. salicis are epitypified, and a neotype is designated for Alphitomorpha obtusata, with ex-epitype sequences and, in the latter case, an ex-neotype sequence.