Downy mildew of roses—Peronospora sparsa
Downy mildews, including Peronospora and Plasmopara spp., are pathogenic water molds (oomycetes) that primarily damage foliage. Hosts include caneberries, rose, various herbaceous ornamentals, and many fruit, grain, and vegetable crops.
Downy mildew causes pale green to yellow areas on the upper surface of leaves, and sometimes also discolors buds and stems. Discoloration may turn purplish red to dark brown and necrotic; its shape often depends on leaf veins, which limit the pathogen's spread. Infected leaves may drop prematurely.
Where downy mildew lesions occur on the upper surface of leaves, fluffy masses of brown, gray, or purplish spores form on the underside of leaves. Because under ideal, cool moist conditions spores persist for only a few days, often no spores are visible even though downy mildew is causing damage.
Downy mildew can be confused with powdery mildew, such as on roses. However, grayish downy mildew spores are almost always limited to the underside of leaves, and viewed under magnification its spores occur in branched stalks (sporangia) like tiny trees. Powdery mildew is common on both sides of the leaf and produces spores on chains, and its whitish to gray growth is typically more extensive and prominent than with downy mildew. Powdery mildews are fungi and downy mildews are oomycetes (not fungi), which partly explains why their biology and management differ.
Downy mildew spores are produced only on living plants and spread mostly with air movement. Spores landing on a host germinate and infect within 8 to 12 hours if the plant is wet. Downy mildews are favored by low temperatures (40° to 70°F) and require high relative humidity (≥90%) or wet foliage to produce spores and infect plants.
To control downy mildew, provide good air circulation and maintain low humidity; adequately space plants and prune host canopies and nearby and overhead vegetation. Avoid wetting foliage; use drip or low-volume microsprinklers instead of overhead irrigation where feasible. Promptly remove and dispose of infected foliage to reduce pathogen inoculum. Control with pesticides (mostly preventive fungicides) is difficult, and modifying environmental conditions can be more effective.
Purplish spots on rose leaf
Sporulation on leaf underside
Downy mildew sporangia magnified