Onion White Rot is a serious disease that affects onions, garlic and leeks.
What does it look like?
Yellowing and wilting foliage, especially in dry conditions; root and bulb decay; and white fungal growth at the bulb base, later developing small black sclerotia (resting structures).
The white growth with black specks at the bulb base is the telltale sign; yellowing leaves on their own could be many things.
It lives in the soil and lies dormant until a host crop is planted, so there’s no particular season to watch for.
Both diseases force the same kind of long rotationMoving each crop family to different ground each year, on a multi-year cycle, so pests and diseases that build up in soil don't get a repeat host. Full definition → off affected ground, but not for the same length of time: this one's 15-plus years is far longer than onion downy mildew's five. See Onion Downy Mildew →
Why this matters in Scotland
Allotments are where it spreads, and it attacks the crops Scottish growers rely on.
Because it travels on soil, shared allotment sites, where soil moves easily between plots, carry the highest risk. And leeks and onions are Scottish mainstays: easy, dependable, and standing through winter when little else crops.
This one is effectively permanent, which changes what prevention means here.
Its resting bodies, the black sclerotia, can survive in the soil for at least 15 years, so crop rotation can't clear it once it's in.
Once it's established, that ground is effectively finished for onions, garlic, leeks and their relatives. Keeping it out is the only option.
It moves in contaminated soil, on tools and muddy boots. The sclerotia germinateA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition → when they detect chemicals given off by allium roots, then invade the roots directly.
The sclerotia sit dormant until an allium tells them it has arrived. That is why 15 years of not growing onions does not clear the ground: the pathogen is waiting, not dying.
Controls: what works
Prevent introduction: avoid bringing in contaminated soil, especially on tools and footwear
Keep contaminated soil out of your beds, including soil or plants from other gardens. This matters most on allotments.
Nothing stops it once it's in the soil, so this is the one control that counts.
Chemical control and soil sterilisation
There are no chemical treatments for the disease, or for sterilising the soil, available to UK gardeners. Treating soil with allium-based compounds, to trick the fungus into germinating with nothing to infect, has been trialled but isn't available to gardeners.
What to do instead: Keeping it out is the whole strategy.
Treat affected bulbs as compromised, not just cosmetically marked
Below ground, the fungus rots the roots and then invades the bulb itself.
So a bulb with the white growth at its base is affected inside too, not just on the surface. Don't trim it and keep it like a bruised apple.
Myths about onion white rot
“Rotating onions and garlic away from an infected bed for a few years will get rid of white rot”
Rotation doesn't cure white rot. White rot sclerotia survive so long in the soil that, once the disease is established, rotation is impractical and of limited use; on its own it will not control the disease. A standard three-to-five-year rotation, enough to break the cycle of most soil-borne diseases, is nowhere near long enough against sclerotia that persist for decades.
What to do: Once white rot is confirmed on a bed, stop planting alliums there altogether rather than rotating on a normal schedule; treat the ground as permanently retired from onions, garlic, leeks, shallots and chives.
In Scotland: On a shared allotment, one contaminated plot becomes background risk for the whole site for decades, since sclerotia travel on tools, boots and borrowed equipment regardless of what any individual plot holder rotates.
3 sources
- RHS: Onion white rotThe disease is effectively impossible to eliminate once introduced, and the long survival period makes crop rotation impractical.
- University of Maine Cooperative Extension, Bulletin #2062: White rot of garlic and onionsCrop rotation is of limited use in disease control owing to the long persistence of white rot sclerotia in the soil.
- University of Maryland Extension: White rot of onion and garlicRotation alone will not control white rot because sclerotia can survive in the soil for 20 to 40 years.
“All alliums, onions, garlic, leeks, shallots and chives, are equally at risk once white rot is in the soil”
Sources don't agree on this. University of Maine Extension states that all Allium-family plants can be infected but adds that 'onion and garlic are the most susceptible,' implying leeks and the others carry somewhat lower risk. University of California's statewide IPM programme instead describes fairly uniform vulnerability across the genus, with root exudates from any Allium species able to trigger sclerotia germination and no differential resistance flagged. RHS lists onions, garlic, leeks, chives and shallots as confirmed hosts without ranking them against each other.
What to do: Don't treat any allium as a safe substitute crop on infected ground. Even the source that flags onion and garlic as more susceptible still lists leeks, shallots and chives as capable of catching it.
3 sources
- University of Maine Cooperative Extension, Bulletin #2062: White rot of garlic and onionsAll Allium-family plants can be infected with white rot, but onion and garlic are the most susceptible.
- UC IPM: White rot, Onion and garlicThe disease affects garlic, onion and other Allium crops, and growers are advised not to follow any Allium crop with another Allium crop.
- RHS: Onion white rotConfirmed hosts include onions, garlic, leeks, chives and shallots.
“Digging chopped onion or garlic into infected soil tricks the fungus into germinating with nothing to infect, then starves it out”
The underlying idea is real, but the home-garden version of it isn't proven. Sclerotia are stimulated to germinate by the same volatile compounds alliums give off, and using this as a soil treatment 'has been under discussion for many years, but has not yet translated into the availability of any products on the UK amateur market.' A peer-reviewed trial using concentrated onion oil, garlic oil and allium waste got 88 to 99 percent sclerotia reduction in greenhouse conditions, but that fell to 50 to 71 percent in the field, and required repeated applications of industrial quantities, roughly 1000kg per hectare of solid material, three times over four months. That is a different proposition from digging kitchen onion peelings into a raised bed.
Our read The home version, chopping up scraps and burying them, doesn't appear to have been tested, and it's a very different thing from the trialled treatment.
What to do: Don't rely on chopped onion or garlic as a home treatment for infected ground; no consumer product or amateur-scale method has been shown to work, so prevention remains the only real option once white rot is confirmed.
2 sources
- RHS: Onion white rotUsing allium volatile compounds as a soil treatment to trigger sclerotia germination has been discussed for years but has not become an available UK amateur product.
- Reduction of Stromatinia cepivora inocula and control of white rot disease in onion and garlic crops by repeated soil applications with sclerotial germination stimulants (peer-reviewed study, PMC)Soil-applied germination stimulants derived from onion and garlic reduced viable sclerotia by 88 to 99 percent in greenhouse trials and 50 to 71 percent in field trials, using repeated industrial-scale applications.
“An onion or garlic bulb with white rot is fine to eat once you cut away the affected part”
There's no published food-safety research on this either way. Asked directly whether trimmed, white-rot-affected garlic is safe to eat, a Cooperative Extension plant pathologist answered: 'I could not find information on the toxicity of white rot in humans, but I would not personally eat the garlic.' That's an absence of evidence rather than evidence of harm, but it means nobody has tested it, and the expert who looked into it erred on the side of not eating it. RHS's own description of the disease also notes that the pathogen moves from the roots into the bulb itself rather than staying on the surface, which is part of why cutting off the visible white growth doesn't necessarily remove all the affected tissue.
What to do: Treat white-rot-affected bulbs as compost or waste rather than food; there's no safety data to fall back on, and the fungus doesn't stay confined to the part you can see and cut away.
2 sources
- Ask Extension: White rot on garlicA Cooperative Extension expert found no toxicity data on white rot in humans but said they would not personally eat affected garlic.
- RHS: Onion white rotBelow ground, the pathogen rots the roots and then invades the bulb itself.
“Onion white rot sclerotia can survive in the soil for 15 to 20 years or more”
RHS puts sclerotia survival at 'at least 15 years,' and that figure sits at the conservative end of what other sources report. University of Maryland Extension gives a range of 20 to 40 years, and University of Maine Extension states a field can stay unusable for garlic or onions for as long as 40 years even with no allium grown on it in that time. Whatever the exact figure, on a gardening timescale it's effectively permanent.
What to do: Treat any bed where white rot has appeared as permanently unsuitable for onions, garlic, leeks, shallots and chives, not just off-limits for a few seasons.
3 sources
- RHS: Onion white rotSclerotia can remain in the soil for at least 15 years, though they will not always live this long.
- University of Maryland Extension: White rot of alliums in the home gardenSclerotia can live in the soil for years without the benefit of host plants, and the pathogen can survive in soil for 20 to 40 years.
- University of Maine Cooperative Extension, Bulletin #2062: White rot of garlic and onionsWhite rot can render a field unusable for garlic or onion production for as long as 40 years, even without an Allium host.
“Garlic grown from cloves brings white rot onto a plot far more often than onion grown from seed”
Onion seed is not likely to carry sclerotia because the pathogen is not seedborne in onion, while on garlic 'the disease is commonly introduced into the field on seed cloves.' University of Maryland Extension backs this up, naming plant material, 'especially garlic cloves,' as a route sclerotia spread field to field, and specifically warning never to plant grocery-store garlic because it can be a symptomless carrier. Onion setsSmall immature bulbs planted directly rather than grown from seed, giving a faster, more reliable crop in a short growing season. Full definition → and transplants can still carry contaminated soil on them, but the seed itself isn't the vector that a garlic clove can be.
What to do: Buy garlic for planting only from a supplier selling certified, disease-free stock, never a supermarket bulb, and apply the same soil-hygiene caution to onion sets and transplants rather than assuming onion is automatically lower risk.
2 sources
- UC IPM: White rot, Onion and garlicOnion seed is not likely to carry sclerotia since the pathogen is not seedborne in onion, whereas on garlic the disease is commonly introduced on seed cloves.
- University of Maryland Extension: White rot of onion and garlicSclerotia spread through movement of soil, equipment or plant material, especially garlic cloves, so growers should plant only clean stock from known origins.
Frequently asked questions
What should I do first if I find onion white rot in my garden?
The first thing to try: Prevent introduction. Then: Treat affected bulbs as compromised, not just cosmetically marked.
Is there a spray that works?
No. There are no amateur chemical treatments available for onion white rot. The controls that work are cultural: the measures listed above.
How reliable is this advice?
Every control here is backed by good evidence.
Sources
Show the source
- RHS1: The fungus (Stromatinia cepivora), affected alliums, symptoms including the white growth and black sclerotia, survival in soil for at least 15 years, spread on tools and footwear, particular care on allotments, germination triggered by allium chemicals, no chemical treatment or soil sterilisation for gardeners