Hyaloperonospora parasitica

Hyaloperonospora parasitica is a species from the family Peronosporaceae. It has been considered for a long time to cause downy mildew of a variety of species within the Brassicaceae family such as oilseed rape and cauliflower, on which the disease can cause economically important damage by killing seedlings or affecting the quality of produce intended for freezing. However, recent phylogenetic research has shown that Hyaloperonospora parasitica is restricted to Capsella bursa-pastoris as host plant. In fact, as a rule Hyaloperonospora species are genetically and biologically quite distant from each other. Each species of Hyaloperonospora is highly specialized and parasitises only few distinct host species or even only a single one. Accordingly, the former Hyaloperonospora parasitica has been split into a large number of species. For instance, the taxonomically correct name of the parasite of the well-known model organism Arabidopsis thaliana is Hyaloperonospora arabidopsidis, not H. parasitica, whereas the pathogen of Brassica has to be called Hyaloperonospora brassicae.