The wet weather has brought out some diseases we normally do not see in the landscape. The following is information on downy mildews in landscape plants.
Downy mildews that are rather uncommon in the area have been favored by the continued rainy, cool weather. Downy mildew of rose occurs worldwide but is rare here. Infected leaves develop purplish red to dark brown irregular spots and leaflets may turn yellow with ½-inch islands of normal green tissue interspersed. Leaf loss can be severe and symptoms may resemble pesticide toxicity burns. Sanitation is important, since the fungus overwinters in infected leaves. Rake and dispose of fallen leaves, stems and flowers. The other downy mildews have been diagnosed on sunflower and black-eyed Susan, both infected by Plasmopara halstedii. Usually this downy mildew appears in the fall with cool and wet conditions, but not this year. It causes yellow to dark blotchy areas on the upper leaf surfaces and grayish white fuzzy growth on leaf undersides. It can infect most species of Rudbeckia especially 'Goldstrum'. Preventative fungicides can help suppress or control downy mildew. A return to normal weather this week will slow disease development.
Downy mildew on rose. Photo from the Oregon State University Online Guide to Plant Disease Control.
Information from Bob Mulrooney, Extension Plant Pathologist, UD