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

White rot on onion

A very serious soil-borne disease caused by Stromatinia cepivora. The fungus breaks down the base of the bulb and the roots, and its black sclerotia survive in the soil for up to 20 years.

Symptom photos

White rot — Onion base — white mycelium with dark sclerotia — Onion base — white mycelium with dark sclerotia Photo: Jochen Kreiselmaier, Pflanzenschutzberatung DLR Rheinpfalz, licence CC BY-SA 4.0

White rot — Young plants — yellowing and dying in patches across the bed — Young plants — yellowing and dying in patches across the bed Photo: Jochen Kreiselmaier, Pflanzenschutzberatung DLR Rheinpfalz, licence CC BY-SA 4.0

Symptoms

Leaves yellowing and wilting, starting with the oldest
White, cottony fungal growth on the base plate and roots
Small black sclerotia the size of poppy seeds within the growth
Soft, watery rot at the base of the bulb
Plants lifting easily out of the soil, with no roots

What it does to the crop

Plants collapsing in patches that grow year on year
The crop lost entirely on the affected part of the plot
Soil contaminated with sclerotia for 15-20 years
The site ruled out for allium crops

What encourages it

Cool soil at 10-20°C
Wet, heavy soils
Alliums grown as a monoculture
Sclerotia carried in soil on tools and footwear

Prevention

Rotate over a very long cycle — at least 8 years without alliums
Remove infected plants with a spadeful of soil and burn them
Wash tools and footwear after working on an affected site
Avoid moving soil between beds
Plant healthy, inspected onion sets

⚠️ 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

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