Peronosporaceae

Peronosporaceae are a family of water moulds that contains 21 genera, comprising more than 600 species. Most of them are called downy mildews.

Many species of Peronosporaceae are obligate biotrophic plant pathogens. They parasitise their host plants as an intercellular mycelium using haustoria to penetrate the host cells. The downy mildews reproduce asexually by forming sporangia on distinctive white sporangiophores usually formed on the lower surface of infected leaves. These constitute the "downy mildew". The sporangia are wind-dispersed to the surface of other leaves. According to the genus concerned, the sporangia may then germinate by forming zoospores, thus resembling Phytophthora, or by germ-tube. In the latter case, the sporangia behave as conidia and are often referred to as such. Sexual reproduction is via oospores.