Downy mildew diseases are caused by oomycetes, a group of fungus-like organisms that also includes Pythium and Phytophthora species. Downy mildew pathogens are very different from powdery mildews. They attack different plants under different environmental conditions. Also, they are controlled by different classes of fungicides.

Most of the downy mildew fungi are very host specific and infect only one plant family. Pathogens include species of Peronospora, Pseudoperonospora, Bremia, Plasmopara, and Basidiophora. Downy mildews infect almost all ornamental plants as well as some indoor plants. Perennial hosts include aster, buddleia, coreopsis, geranium (not Pelargonium), geum, gerbera, lamium, delphinium, veronica and viola. Downy mildew is also caused on rose by Peronospora sparsa.

Source: UMass Extension