Allium white rot
A soil-borne fungus that rots the base of onions, garlic and leeks, leaving white fluffy growth studded with small black resting bodies. Those resting bodies survive in soil for fifteen years or more, so a plot that gets it stops being a plot for alliums.
Caused by: Stromatinia cepivora · also called onion white rot
Nothing available to a home grower will fix this
There is no chemical or biological treatment available to a home grower that clears white rot from soil, and rotation does not outlast it — the resting bodies persist far longer than any practical rotation. Ground that has had it should be treated as unavailable for alliums.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Onion white rot
Checked 2026-08-13
What brings it on
Cool, moist soil and the presence of a host. Resting bodies germinate in response to chemicals released by allium roots, which is why the disease can lie dormant for years and then appear the season alliums return.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Onion white rot
- University of Minnesota Extension — University of Minnesota: White rot of onion and garlic
Checked 2026-08-13
Preventing it
Ordered by how much difference each measure actually makes, not by how often it is repeated.
- Do not import soil, plants or tools from an affected plot, and clean soil off boots and equipment between sites. Introduction on contaminated soil is how nearly every clean plot acquires it.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Onion white rot — avoiding introduction
Checked 2026-08-13
- Grow alliums from seed rather than from bought sets or bulbs where the disease is a concern locally.
Sourced guidanceDepends on conditionshigh confidence
Applies when: Reduces one introduction route. It does nothing about soil already infested.
- Royal Horticultural Society: Onion white rot
Checked 2026-08-13
Dealing with it once it is there
- Remove affected plants with the surrounding soil as soon as they are found, and do not compost them.
source
Sourced guidancehigh confidence
- Royal Horticultural Society: Onion white rot — managing an outbreak
Checked 2026-08-13
- On infested ground, grow alliums in containers of fresh compost instead. This is the only approach that reliably produces a crop.
source
Horticultural consensusmoderate confidence
- Royal Horticultural Society: Growing alliums on infested soil
Checked 2026-08-13
What it affects
Signs that lead here
Each of these lists the other things that produce the same sign, and how to tell them apart.
On sprays and treatments
GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.
Plant-specific guides
Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.