Backyard Grapes Identify & treat grapevine problems
Fungal high severity

Downy mildew

Plasmopara viticola

Yellow "oil spots" on top of the leaf with white fuzz underneath — loves wet, warm weather.

Foliar grapevine downy mildew lesions Photo: K. Kanaley Cornell CALS / New York State Integrated Pest Management
Downy mildew symptoms on grapevine leaves SA Fruit Journal (South African Table Grape Industry)
Pale yellow leaf spots on upper surface of grape leaf caused by downy mildew Ohio State University Extension (Ohioline)
White downy sporulation on the underside of a grape leaf Photo: Cecil Smith iNaturalist CC BY 4.0
Angular, vein-bounded oil-spot lesions turning brown on the upper leaf surface Photo: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org Bugwood.org CC BY-NC 3.0 US

Description

Downy mildew (Plasmopara viticola) is the classic wet-climate grape disease. The “oil spot” on top of the leaf with matching white fuzz underneath is its signature and the easiest way to separate it from powdery mildew, which is powdery on the upper surface and needs no free water.

It spreads explosively after warm rain: spores splash up from the soil and infected debris, and a single humid night can trigger a new cycle. Because it moves so fast, prevention before rain beats trying to cure it afterward.

What to look for

  • Pale yellow, oily-looking blotches on the upper leaf surface ("oil spots").
  • Downy white cottony growth on the underside, directly below the spots.
  • Older spots turn brown; badly hit leaves drop, exposing fruit to sunburn.
  • Young berries turn grey-brown and leathery; shoot tips can curl and die.

Treatment options

Cultural & prevention

Start here — habits that stop problems before they spread.

  • Improve drainage and airflow; avoid overhead watering.
  • Remove fallen leaves in autumn, where the fungus overwinters.
  • Keep the fruit zone open so leaves dry quickly after rain.

Organic / low-toxicity

Gentler sprays and biologicals for active outbreaks.

  • Copper-based sprays (Bordeaux mixture) applied preventively before wet spells.
  • Reapply copper after heavy rain during the high-risk period.

Conventional fungicide / pesticide

For severe cases. Always follow the product label.

  • Systemic downy-mildew fungicides (e.g. phosphonates, or labelled products).
  • Alternate FRAC groups and follow the rain-based spray timing on the label.

Sources

This entry is based on the references below. Read them for full detail and for spray timings and rates valid where you live.

  1. Grapevine Downy Mildew (Plasmopara viticola) Fruit Fact Sheet — Cornell University / New York State IPM Program
  2. Downy Mildew of Grape — Ohio State University Extension
  3. Downy Mildew / Grape / Agriculture: Pest Management Guidelines — UC Statewide IPM Program (UC ANR)
  4. Downy Mildew of Grape (Home and Landscape) — UC Statewide IPM Program (UC ANR)