Source: UMass extension. Photos: Lynn Ann Hyatt, Margery Daughtrey, Cornell University
Downy mildews thrive under the cool, wet conditions of spring and fall. The pathogens need wet leaves and high relative humidity (greater than 85%) to cause disease; under these conditions, disease can occur rapidly and is difficult to control. Downy mildew 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. 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).
