Abstract
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.