Purplish discoloration caused by downy mildew, Peronospora sparsa, a plant-pathogenic oomycete infecting a rose leaflet (right of center).

Necrotic patches on surface of leaf infected with downy mildew. Credit: John F. Karlik
Purplish discoloration caused by downy mildew, Peronospora sparsa, a plant-pathogenic oomycete infecting a rose leaflet (right of center). Credit: John F. Karlik