Downy Mildew is a group of fungus-like diseases that are often confused with botrytis, Powdery Mildew and other foliage diseases. Downy Mildew often starts with white to bluish-white, fluffy growth on the undersides of the leaves, and yellow spots on the tops of the leaves. As the infected area dies, it can turn yellow to brown, purplish, reddish-black or bleached white, and the fluffy growth turns grayish. The spots or lesions are angular (black spot forms round areas surrounded by a yellow ring) delineated by leaf veins. Infected branches may become distorted, drop their leaves and die. On impatiens (flowering plants), leaves and flowers drop, leaving green stems, which eventually collapse and die. Roses can also be quickly defoliated, looking like they have been sprayed with an herbicide. This disease is difficult to control.
- When possible, plant resistant varieties.
- Remove and destroy infected annual plants.
- Practice good sanitation, cleaning up fallen leaves and prunings.
- Rotate plantings.