# Powdery mildew

- URL: https://backyardgrapes.com/diseases/powdery-mildew/
- Scientific name: Erysiphe necator
- Category: Fungal
- Affects: Leaves, Shoots & canes, Fruit clusters
- Severity: high
- Favored by: Warm (20–27°C), shady, humid but dry leaves; poor air movement.
- Easily confused with: Downy mildew, Botrytis bunch rot (grey mould)

**Summary:** A dusty white-grey coating on leaves and berries — the most common grape fungus almost everywhere.

**What to look for:**
- Powdery white to grey patches on the upper (and lower) leaf surface.
- Leaves may curl, look distorted, or drop early.
- Young berries covered in grey dust; they stop growing and can split or scar.
- A faint musty smell on heavily infected clusters.

**Treatment — Cultural / prevention:**
- Prune and train vines for good airflow and sun exposure on the fruit zone.
- Remove and destroy heavily infected shoots and leaves.
- Avoid heavy nitrogen feeding, which drives dense, susceptible growth.

**Treatment — Organic / low-toxicity:**
- Sulphur sprays (not within ~2–3 weeks of an oil spray, and not in high heat).
- Potassium bicarbonate or fresh 1% milk sprays on light early infections.
- Horticultural or neem oil early, on a preventive schedule.

**Treatment — Conventional (chemical):**
- Rotate DMI, SDHI, or strobilurin fungicides labelled for grapes.
- Alternate chemical groups (FRAC codes) each spray to slow resistance.

Powdery mildew is caused by the fungus *Erysiphe necator* and is the single most
widespread disease of grapes. Unlike most fungi, it does **not** need free water on
the leaf to infect — only humidity — so it thrives even in dry climates.

It overwinters in dormant buds and in small fruiting bodies on the bark, then
releases spores in spring that spread on the wind. Berries are most vulnerable
from bloom until they start to soften; protecting the cluster during this window
is the key to control.

Catching it early matters: once berries are coated and cracked, secondary rots
often move in through the wounds.

**Image credits:**
- Grape cluster coated in grey-white powdery mildew, with split berries — [Photo: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org, Bugwood.org](https://www.ipmimages.org/browse/detail.cfm?imgnum=5608072) (CC BY-NC 3.0 US)
- Grape leaf covered by white powdery mildew mycelium — [Photo: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org, Bugwood.org](https://www.ipmimages.org/browse/detail.cfm?imgnum=5608073) (CC BY-NC 3.0 US)
- Mid-stage powdery mildew: dull grey-white powder over berries on the vine — [Photo: Lucyin, Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Maladeye_farineuse_roejh%C3%A9ns.jpg) (CC BY-SA 4.0)
- Scattered white powdery mildew colonies on a grape leaf, with a young cluster above — [Photo: Stanislav Cherepushkin, iNaturalist](https://www.inaturalist.org/observations/309180804) (CC BY-NC 4.0)
- Powdery mildew on grape leaves and young berries — [Bayer Crop Science New Zealand](https://www.cropscience.bayer.co.nz/pests/diseases/powdery-mildew---grapes)

**Sources:**
- [Powdery Mildew - Grape Pest Management Guidelines](https://ipm.ucanr.edu/agriculture/grape/powdery-mildew/) — UC IPM
- [Grapevine Powdery Mildew (Erysiphe necator) Fruit IPM Fact Sheet](https://cals.cornell.edu/integrated-pest-management/outreach-education/fact-sheets/grapevine-powdery-mildew-erysiphe-necator-fruit-fact-sheet) — Cornell Integrated Pest Management
- [Powdery Mildew of Grapes](https://www.extension.arizona.edu/publication/powdery-mildew-grapes) — University of Arizona Cooperative Extension
- [Grape: Powdery Mildew (Hortsense)](https://hortsense.cahnrs.wsu.edu/fact-sheet/grape-powdery-mildew/) — Washington State University Extension