Abstract
The genus Fraxinus (Oleaceae), known as ash trees, currently comprises 43 recognized species that are distributed in temperate and subtropical regions of the Northern Hemisphere. Two Phyllactinia species, P. fraxini and P. fraxinicola, have been known on Fraxinus spp. so far. In this study, powdery mildews belonging to Phyllactinia were collected on Fraxinus spp. from different areas of the world to make molecular and morphological analyses. These specimens are divided into four distinct molecular phylogenetic groups, which are distinguishable by their morphology and/or host preference. Two new species, viz. P. japonica occurring on F. sieboldina and F. lanuginosa f. serrata, and P. fraxini-longicuspidis on F. longicuspis, are proposed in this study. An epitype is designated for P. fraxini. This study indicates very high host specificity among the four Phyllactinia species on Fraxinus, suggesting that genetic isolation by host specificity played a more important role than geographic segregation in the speciation events of these Phyllactinia species. Evolutionary timing calculated by molecular clock analysis suggests that these powdery mildews diverged in accordance with host phylogeny after divergence of host plants.