It’s not easy to stop these pathogens from spreading once established.
Downy mildew diseases present a challenge to growers because they can be present but not obvious and they are difficult to control with fungicides once established. The pathogens are very different from powdery mildews. They attack different plants under different environmental conditions, and are controlled by different classes of fungicides. Downy mildew diseases are caused by oomycetes, a group of fungus-like organisms that also includes Pythium and Phytophthora species.
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. All types of roses are susceptible: wild roses and all cultivated roses including shrub roses. In addition to roses, other hosts of this pathogen include caneberries (blackberries and raspberries), dewberries (Rubus spp.), and cherry laurel (Prunus laurocerasus).