Downy mildew
Peronospora manshurica

Upper leaf surface
 

Lower leaf surface
 

Upper leaf surface symptoms

Initial symptoms of downy mildew are pale green to light yellow spots or blotches on the upper surface of young leaves. The initial lesions may be quite small — appearing as faint yellow flecks across the leaf tissue. Under ideal conditions these areas may enlarge into pale to bright yellow lesions of indefinite size and shape.

Lower leaf surface symptoms

During periods of wet or humid weather, a gray to purple fuzz develops on the lower leaf surface beneath the diseased areas.

Canopy symptoms

Young leaves are more susceptible than older leaves to infection. The disease may be evident throughout the canopy of the plant, if weather conditions remain favorable for disease development.

Weather

Downy mildew is favored by high humidity and temperatures of 68-72 degrees Fahrenheit. Spores produced on newly infected leaves are primarily wind-spread.

Survival

The downy mildew fungus survives as oospores in infected leaf residues and on seeds.

Distribution in Missouri

Although downy mildew can occur anywhere in the state, it is seldom found in more than trace amounts. The 2004 season was atypical in that downy mildew was widespread. In most cases, symptoms occurred in the mid to upper canopy and individual lesions remained small.