What diseases affect onion?

Bacterial soft rot, Downy Mildew, White mould (Sclerotinia), Rust

Onion pests and diseases: how to recognise them

Q: Which onion problems need quick action?

Bacterial soft rot: Whole roots, tubers, bulbs, heads or fruit can collapse within a few days, often leaving only the skin — in the garden and in storage. Downy Mildew: Badly affected leaves shrivel and turn brown or yellow and fall early; plants become stunted and some may die. White mould (Sclerotinia): Later, hard black seed-like bodies (sclerotia) 0.5–1 cm across in the mould and inside hollow stems.

Allium cepa

The pests, diseases and disorders recorded for onion in our library, each with a photo and its first symptom. Every page has the full list of symptoms, organic and chemical treatment and prevention.

At a glance

Problems in total	6
Pests	2
Diseases	4
Need quick action	3

How to grow onion · All pests and diseases

Pests

Onion flySeedlings collapse and die; one maggot can kill several neighbouring seedlings.Photo: Rasbak, CC BY-SA 3.0, Wikimedia Commons
WirewormsWorst in beds newly dug from lawn or grassland.Photo: Alton N. Sparks, Jr., University of Georgia, Bugwood.org, CC BY 3.0, Wikimedia Commons

Diseases

Which onion problems need quick action?

These spread or do lasting harm fast; check the plants and start treatment the day you see them.

Bacterial soft rot: Potato stems may turn black and rot from the base upwards (blackleg).
Downy Mildew: Pale green, yellow, purple or brown blotches on the upper side of leaves, often angular where they are bordered by the veins.
White mould (Sclerotinia): Roots, heads, pods and fruit can rot too, also in storage.

Plants with many of the same problems

Which plants each problem affects: sources

The links between these problems and the plant were checked against these pages.