Peronospora species on Myosotis – a showcase for the uncharted diversity of a highly diverse and specialised downy mildew genus

Peronospora is the largest genus of obligate biotrophic oomycetes, causing downy mildew disease on various cultivated and ornamental plants worldwide. Despite the known high host specificity in Peronospora, many host–pathogen combinations have not been thoroughly investigated with respect to their phylogenetic relationships. This is especially the case for groups in which only one species has been described on a host genus. In this study, we highlight the fact that Peronospora is still vastly underexplored by investigating 48 specimens of Peronospora parasitic on the ornamental plant genus Myosotis (Boraginaceae). Morphology and multigene phylogenetic relationships revealed six new Peronospora species parasitic on Myosotis, with one species parasitising the widely cultivated species Myosotis sylvatica, providing further evidence that Peronospora species are generally highly host-specific. As a consequence, Peronospora occurrences on the several hundred plant species that are not type host species for a described Peronospora species should be re-examined, as they might represent species overlooked in previous studies.