For diagnostics and control of emerging plant diseases, accurate species determination of the causal pathogens is a prerequisite. Downy mildew disease, caused by an unknown species of Peronospora, has speedily spread throughout numerous gardens and nurseries of the ornamental plant Aquilegia in the UK, but so far does not seem to have reached continental Europe. Apart from cultivated Aquilegia, downy mildew from wild columbines has only been reported from East Asia, where natural populations of Semiaquilegia are affected by downy mildew. To resolve the phylogenetic relationships of the causal pathogens on Aquilegia and Semiaquilegia, a phylogeny based on nine loci was performed. In addition, detailed morphological comparisons were carried out to determine if the downy mildew agents on Aquilegia and Semiaquilegia are conspecific and can be discriminated from related downy mildew species. Strong evidence was found that the downy mildew pathogens on Aquilegia and Semiaquilegia are conspecific, but distinct from other species of Peronospora affecting Ranunculaceae, Papaveraceae, and Saxifragaceae. Thus, a new species, Peronospora aquilegiicola, is introduced. The quick spread of the pathogen throughout the UK and the current absence from continental Europe highlights the importance to consider quarantine measures to restrict the further spread of this pathogen.