Description of three widespread new Peronospora species parasitising Caryophyllales

Abstract Downy mildews are cosmopolitan obligate biotrophic parasites of flowering plants. They are fungus-like eukaryotes of the kingdom Straminipila , and most species are highly specialised, often parasitising only a single host species. Due to the prevalence of a broad species concept before the advent of molecular phylogenetics and the paucity of differentiating characteristics in some groups, many widespread species are still undescribed. This is also the case for some downy mildews in the clade containing the type species of Peronospora , Peronospora rumicis . Spurred by the discovery of the undescribed species parasitising Rumex acetosa at the Blávík research station in East Iceland, three new widespread species of Peronospora are introduced in this article: Peronospora blauvikensis parasitic to Rumex acetosa , P. boylei parasitic to Rumex thyrsiflorus , and P. spergulariae parasitic to Spergula marina . As the species are morphologically highly similar, a set of diagnostic bases is given for each species, allowing a quick identification via molecular barcoding.