🧅 Onion How hard it is to control: ★★★☆☆ 3/5

Downy mildew of onion

Dangerous onion disease caused by an oomycete (fungal-like organism), occurring in cool, wet periods. Can cause complete yield loss.

Symptom photos

Downy mildew of onion — Leaf — fine, greyish-violet layer of spores on the surface — Leaf — fine, greyish-violet layer of spores on the surface Photo: Howard F. Schwartz, Colorado State University, Bugwood.org, licence CC BY 3.0 us

Symptoms

Pale green or yellow spots on leaves
Gray-violet coating
Leaf collapse and die-back

What it does to the crop

Leaf collapse
Bulb rot
Yield loss up to 50%

What encourages it

Cool, humid weather
Temperature 10-15°C
Morning dew

Prevention

Wider spacing
Planting in airy locations
Crop rotation 4-5 years

⚠️ Before you reach for a spray

This atlas is informational and still growing — different diseases can look almost identical. Confirm the diagnosis and read the product label before applying any plant protection product. If you are unsure, ask your local extension or advisory service.

Keep the disease atlas with you in the garden

In the app you can identify a disease from photos, work out the spray dose for your own bed and log the treatment in your task calendar. Free, no sign-up, works offline.

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