Abstract
Downy mildews are the most species-rich group of oomycetes, with more than 700 known species. The relationships within the main downy mildew lineages (i.e. the downy mildews with pyriform haustoria, the downy mildews with coloured conidia, and the brassicolous downy mildews) are increasingly well resolved, and 20 well-characterised monophyletic genera have been described. However, their relationships to each other, the various lineages of graminicolous downy mildews, and to the species subsumed in Phytophthora are still unresolved. Recent phylogenomic studies have suggested a polyphyly of the downy mildews, but with a limited taxon sampling within Phytophthora . As taxon sampling is crucial for inferring relationships between large groups, we have conducted a multigene analysis with a set of 72 Phytophthora species and included all known downy mildew lineages. In addition, we performed approximately unbiased (AU) testing as an additional approach to evaluate major nodes. Our analyses resolve the downy mildews as a monophyletic assemblage in all phylogenetic algorithms used. We thus conclude that the evolution of the obligate biotrophy characteristic of downy mildews was a singular event and that all downy mildew pathogens can be traced to a single ancestor.