Pacific Northwest 1998 An Online Guide to Plant Disease Control

FactSheets offer superior print quality for the page you are currently viewing. Using Microsoft's Internet Explorer has given best printing results.
Glossary
Guide Home
Search

OSU Extension Office
 
Grape -- Downy Mildew
See Also: Boston Ivy -- Downy Mildew
 
Cause: The fungus-like organism Plasmopara viticola has not been reported from grape in the Pacific Northwest, but the disease has been reported from at least seven California counties. One OSU plant clinic specimen from a homeowner was diagnosed with downy mildew in 1966. Grapevines, Boston ivy, and Virginia creeper all are hosts of this organism. The organism was detected on Boston ivy plants in Oregon in 2001. Subsequent surveys in 2002 found the organism on both Boston ivy and Virgina creeper plants in wholesale and retail nurseries throughout grape-producing areas of Oregon. One landscape planting of Boston ivy without overhead irrigation had symptoms of the disease. This may indicate that western Oregon can sustain natural spread and expression of this disease.

It is unknown at this time how this might impact the grape industry. Weather west of the Cascades would be highly conducive to disease development if the pathogen ever is introduced into grape vineyards. It can be spread by transporting infected vines or nursery stock or within plant debris or soil that contains oospores of the organism. The organism can overwinter or be transmitted as mycelium in dormant buds.

Symptoms: Lesions can be on any green part of the vine. Lesions are yellowish and oily or angular, yellow to reddish brown, and limited by veins. In moist weather, a delicate dense, white growth is on the leaf underside opposite lesions seen on the upper side.

Severely infected leaves drop, and vines may show premature defoliation. Shoot tips can die back; immature berries are highly susceptible. Symptoms may be easily confused with the grape erineum mite damage which produces a white gall (that later turns darker in color) on the undersides of leaves.

See the following site to see a wide selection of color pictures showing the range of symptoms on different cultivars. Grape Downy Mildew Symptoms: A gallery of slides

Notice the chlorotic spots on the upper-surface of the leaves.

Notice the white, downy appearance of infections on the berries

There are numerous white areas on this leaf, but most of these are due to the grape erineum mite. Erineum mite damage is often confused with downy mildew. The larger, whitish more diffuse area on the right-side of the leaf is downy mildew.

 
Cultural control: Quarantines and scouting is the best way to keep it out of vineyards.

  1. Do not transport infected stock into the region. Do not bring infected plant debris or soil into the region.
  2. Scout vineyards for any sign of the disease. Check newly planted stock originating out of state. If it is suspected, get diseased plant samples to a public diagnostic laboratory as soon as possible to confirm the diagnosis.
  3. Scout for the disease on nearby plantings of Boston ivy or Virginia creeper. Remove if discovered.
  4. Avoid overhead irrigation or practices that maintain wet foliage for extended periods of time.
Chemical control: The Pacific Northwest does not have the disease in grapvines, so there is no need for chemical control applications.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
Top

In print since 1954 and on the web since 1996. Questions or comments, please contact us.