According to the genus concerned, the sporangia may then germinate by forming zoospores, thus resembling Phytophthora, or by germ-tube.

The parasitised plants are angiosperms or gymnosperms, and most Peronosporaceae are pathogens of herbaceous 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, and Plasmoverna on Ranunculaceae.

The largest genera, Peronospora and Plasmopara, collectively have very wide host ranges.

Peronosporaceae of economic importance include those that infect grapevines (Plasmopara viticola) and tobacco (Peronospora tabacina; blue mould).