Condensed from Downy Mildew by X Xu and T. Pettitt, Encyclopedia of Rose Science:
Downy mildew is a serious disease of roses caused by a fungus that favors cool, humid conditions. On leaves, downy mildew causes marks that are purplish-red to dark brown, usually with a clearly defined edge. The marks will often end at the edge of a vein. In a serious infection, sections of tan or grey dead tissue appear. See the Photos tab for examples. Downy mildew can also infect buds.

Severe leaf drop is a typical sign of downy mildew, especially fresh, new leaves that drop at the slightest touch. When downy mildew is present, even health-looking leaves will drop because they are actually infected but do not yet show signs of downy mildew on the upper leaf surface.

For some years it was thought downy mildew overwintered on canes or as a systemic infection. Recent research, including the work by the authors, suggests otherwise, that the Peronospora sparsa does not enter views or canes or persist systemically after infected leaves drop.

Apart from the use of chemical fungicides, two practices help control downy mildew: very careful garden sanitation to remove all debris from disease outbreaks, and avoiding overhead irrigation.