Grapevine Powdery Mildew (Erysiphe necator)

Fruit Fact Sheet

Grapevine powdery mildew is one of the most widespread and damaging fungal diseases of grapevine. Powdery mildew can destroy infected clusters, impair photosynthesis, and significantly decrease fruit yield and quality.

In this fact sheet

Signs and symptoms of powdery mildew
Powdery mildew disease cycle
Managing powdery mildew
Powdery mildew resources

Green grape leaves with a whitish-gray sheen on the leaf surface. — Foliar powdery mildew on chardonnay. Image courtesy of K. Kanaley.

Powdery mildew, caused by the fungal pathogen Erysiphe necator, infects leaves, rachises, and berries of all grapevine cultivars. Erysiphe necator is native to eastern North America but can be found in vineyards worldwide. Generally, Vitis vinifera cultivars and their hybrids are more susceptible to powdery mildew than North American cultivars. Though primary infection requires rainfall, secondary infection cycles can occur under dry conditions as long as susceptible grapevine tissue is present and temperatures are between ~55-85 degrees Fahrenheit.

Signs and symptoms of grapevine powdery mildew

Powdery mildew symptoms can appear shortly after bud break and persist through harvest. The fungus produces spores called conidia on the leaf surface and leaf underside, which give the leaves a “powdery” white or gray sheen. Rachises and berries can also be covered with fuzzy white or gray conidia. Infected berries may appear a dull gray or brown color and will sometimes split open. Stem infections typically produce brown or black lesions.

A green grape leaf with a grayish-white sheen.

Powdery mildew conidia on the upper surface of a grapevine leaf.

A cluster of green grapes covered in a grayish-white substance.

Powdery mildew conidia growing on a chardonnay cluster.

A cluster of grapes. Grapes are dark and gray colored with some cracks on the berry surfaces.

Severe powdery mildew infection causes berries to split.

A grapevine stem covered in brown spots.

Brown powdery mildew lesions along a grapevine stem.

A cluster of green grapes. Some grapes are covered in small black dots and have a grayish-white sheen.

Small, dark chasmothecia (fungal reproductive structures) on chardonnay berries.

A grape leaf is shriveled and covered in a grayish white, fuzzy substance.

Powdery grayish-white conidia cover the underside of a grapevine leaf.

Grapevine powdery mildew disease cycle

The disease cycle begins with ascospore release when there is >0.1 inch of rain and temperatures >50°F. Ascospores can infect young leaves, clusters, stems and rachises. Next, signs and symptoms appear on infected tissues. Grayish white conidia grow on infected leaves and berries and brown lesions appear on stems. Conidia spread via wind to infect new tissue. Chasmothecia form on infected tissue and overwinter in the vine bark. Chasmothecia release ascospores again in spring.

Grapevine powdery mildew management

Cultural control

Measures that promote air flow within the canopy and increase light penetration will help prevent powdery mildew. Recommended control measures include:

Shoot thinning to reduce canopy density
Shoot positioning in the trellis to increase air circulation
Leaf pulling in the cluster zone
When planting a new vineyard, avoid low, shaded, wet areas with poor air flow.
Planting less-susceptible varieties (i.e. native North American varieties) to reduce disease incidence and severity

Chemical control

Unlike other grapevine diseases, E. necator lives entirely outside of the plant, so pesticides do not need to penetrate the vine tissue in order to be effective. This means that many topical fungicides can control powdery mildew, though they may not offer additional protection against other co-occurring diseases.

The most critical control period for powdery mildew is 3 weeks pre-bloom to 3 weeks post-bloom. This is the interval when clusters are most susceptible and chemical control can be most effective. Although berries become resistant to infection at about 3 weeks post-bloom, failure to control infections during this critical period means that infected berries can later split, dry out, and/or and become vulnerable to late-season bunch rots. Young leaves remain susceptible throughout the season and pesticide applications may be required after the critical control window to prevent defoliation from late-season infections. Generally, the following practices are recommended for chemical powdery mildew management:

Early season (bud break - bloom): Broad-spectrum protectant fungicides can be effective at preventing primary infections pre-bloom. Broad-spectrum protectants should be applied when weather conditions are conducive to infection (temperatures above 60°F and humid). Failure to control powdery mildew in the early season can result in severe cluster infections later on, so it is important to monitor vines closely during this period.
Note: New federal regulations may impact the availability of broad-spectrum fungicides for powdery mildew. Be sure to read the product label and consult with a grape IPM specialist or viticulture extension agent if you’re unsure about a product’s legal status.
Immediate pre-bloom – 3 weeks post-bloom: Clusters are highly susceptible to infection during this period, so preventing disease at this stage can significantly reduce fruit loss and the risk of other bunch rots later in the season. If powdery mildew infections are already present and/or weather conditions are conducive, this is the time to apply products with specific activity against powdery mildew. These kinds of products have a high risk of resistance development and should not be applied more than once per season.
Post-bloom – veraison: Chemical control should be used during this window to prevent severe foliar infections that can weaken the vines, slow fruit development, and increase susceptibility to winter injury. Controlling late-season disease can also decrease the number of overwintering spores produced, reducing the inoculum load for the following season. Monitor weather and consider applying protectants in advance of warm, humid periods. Rotate all products (never apply a fungicide with the same FRAC code twice in a row) to avoid resistance development.

Biopesticides

Biopesticides can be very effective at controlling powdery mildew. To ensure adequate powdery mildew control, Cornell Grape Pathology recommends using biopesticides in rotation with other “conventional” chemical products.

For more information and detailed management guidelines see the Cornell Grape Disease Control Guide (available in English and Spanish).

Any time you use a pesticide, you must read and follow the label directions and comply with all applicable laws and regulations related to pesticide use. Also be sure that any pesticide used is approved for use in your country and state/province.

Authors

Kathleen Kanaley
Ph.D. candidate, Plant Pathology, Cornell AgriTech
Dr. Kyle Bekelja
Grape IPM Coordinator, Cornell Integrated Pest Management
Raúl E. Lemus Garza
Bilingual Project Specialist, Cornell Integrated Pest Management

Last updated: April 2025

Acknowledgements

Funding support for K. Kanaley was provided by a spring 2025 Extension and Outreach Assistantship.

Web design by Henry Zelenak, Cornell IPM.

References

W. Wilcox, “Grapevine powdery mildew”, NYSIPM Publications, 2022. https://hdl.handle.net/1813/43121.2

Photos and Illustrations

Foliar powdery mildew on chardonnay. Image courtesy of K. Kanaley.
Powdery mildew conidia on the upper surface of a grapevine leaf. Image courtesy of K. Kanaley.
Powdery mildew conidia growing on a chardonnay cluster. Image courtesy of K. Kanaley.
Severe powdery mildew infection causes berries to split. Image courtesy of Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Brown powdery mildew lesions along a grapevine stem. Image courtesy of Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Small, dark chasmothecia (fungal reproductive structures) on chardonnay berries. Image courtesy of K. Kanaley.
Powdery grayish-white conidia cover the underside of a grapevine leaf. Image courtesy of K. Kanaley.
Powdery mildew disease cycle. Images courtesy of: K. Kanaley, Cornell University; South African Fruit Journal; Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; University of Georgia Plant Pathology, University of Georgia, Bugwood.org; and Wayne F. Wilcox, Cornell Integrated Pest Management program.