Peronosporaceae are obligate biotrophic plant pathogens. They parasitize their host plants as an intercellular mycelium using haustoria to penetrate the host cells. The downy mildews reproduce asexually by releasing sporangia or conidia. These may collectively be referred to as Conidiosporangia. Sexual reproduction is via oospores.
Parasitized plants are angiosperms, most Peronosporaceae are pathogens of dicots. Some downy mildew genera have a more restricted host range, e.g. Basidiophora, Paraperonospora, Protobremia and Bremia on Asteraceae; Perofascia and Hyaloperonospora almost only on Brassicaceae; Viennotia, Graminivora, Poakatesthia, Sclerospora and Peronosclerospora on Poaceae, Plasmoverna on Ranunculaceae. The largest genera, Peronospora and Plasmopara, have a very wide host range.
Peronosporaceae of economic importance include those which have parasitic relationships with grapevines, i.e. Plasmopara viticola. Peronospora tabacina, the "blue mold", is a saprotroph on tobacco. This species has such a delicate spore that it times its spore release for sunrise, a time of high ambient moisture and dew accumulation, so that its spores are less likely to succumb to desiccation and light. Bremia lactucae is a parasite on lettuce, Plasmopara halstedii on sunflower.