Hyaloperonospora arabidopsidis is a species from the family Peronosporaceae

Peronosporaceae

Peronosporaceae are a family of water moulds that contains 17 genera, comprising more than 600 species. Most of them are called downy mildews. The closest relative of Peronosporaceae is Phytophthora....

. It is an obligate parasite and the causal agent of the downy mildew of the plant model organism Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

. While H. arabidopsidis has for a long time been subsumed under Peronospora parasitica (now Hyaloperonospora parasitica

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...

), recent studies have shown that H. parasitica is restricted to Capsella bursa-pastoris as a host plant. Like the other Hyaloperonospora

Hyaloperonospora

Hyaloperonospora is a genus of downy mildews .Species include* Hyaloperonospora arabidopsidis Göker, Voglmayr, Riethm., Weiss & Oberw., 2003...

species, H. arabidopsidis is highly specialized, and it is so far known with certainty only from Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

as a host plant.