Downy mildew is native to North America, but can now be found in grape regions all over the world that have wet growing conditions. The disease can cause yield loss and premature defoliation which can result in a reduction in brix and winter injury.
The tops of the leaves show yellow/light green patches 7-12 days after infection and white, downy spots then appear on the underside of the leaf. Later when the infected tissue dies, the spots become brown. Severely infected leaves can drop prematurely. Infected berries remain firm, but turn a mottled dull-green or reddish purple and fall easily from the cluster leaving a dry stem scar on the pedicel.
The first (primary) infections can occur 2-3 weeks prebloom through fruit set. After 5 or so leaves have emerged, infection can occur if temps are 52°F and there is about 0.1″ or more of rain (the overwintering spores need to splash up from the soil onto suckers or into the canopy). Disease spread for the rest of the year is caused by rain or heavy dew that release spores from infected leaves. Secondary infections occur when temperatures are greater than 55°F with continuous leaf wetness during at least four-hours of darkness. The disease can spread very quickly in warm, wet conditions.
In addition to early season sprays, canopy management and good air circulation play a large role in the incidence and severity of downy mildew. Phosphorous acid products have been shown to reduce spore production- they have both protective (7 day) and limited post-infection activities (3-4 days).