Onion White Rot is a serious disease that affects onions, garlic and leeks in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
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 diagnostic — foliage yellowing alone could be a dozen things.
This pest lives in the soil and lies dormant until a host is planted. A seasonal calendar would be misleading.
🌱 Plants at risk
Onion White Rot also affects these crops:
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
An allotment problem, and Scotland has a strong allotment culture.
The disease travels on contaminated soil carried by tools and footwear, which makes allotments a high cross-contamination risk. Leeks and onions are Scottish mainstays, easy and dependable, standing through winter when little else crops. So the crop Scotland most relies on faces a disease that is permanent, untreatable, and spread by borrowed spades. That makes tool and boot hygiene, normally the dullest possible advice, genuinely load-bearing here.
This one is effectively permanent, which changes what prevention actually means here.
RHS: 'Sclerotia can remain in the soil for at least 15 years', which makes crop rotation impractical once introduced. There are also 'no chemical treatments against this disease or for soil sterilisation available to UK gardeners.'
Once established, white rot effectively renders soil permanently unsuitable for growing alliums. There is no cure, no rotation long enough, and no treatment. Prevention is not the best option: it is the only option.
The fungus spreads via contaminated soil on tools and footwear. 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 volatile chemicals specific to alliums, then directly invade the roots.
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 actually works
All the methods below are backed by real evidence, not just repeated folklore.
Prevent introduction: avoid bringing in contaminated soil, especially on tools and footwear
Prevent it arriving in the first place by avoiding contaminated soil. That matters most on allotments, where the risk of cross-contamination is highest.
There is no second preventive or curative control — once introduced, nothing stops it. What to do with bulbs already affected is a separate, produce-safety question, covered below.
Chemical control and soil sterilisation
RHS: 'There are no chemical treatments against this disease or for soil sterilisation available to UK gardeners.' Experimental approaches using synthetic allium volatiles remain undeveloped for amateur use.
What actually works: Because nothing treats it once established, prevention is the whole strategy: keep contaminated soil off tools, boots and bought-in plants, particularly on allotments where it moves between plots.
Treat affected bulbs as compromised, not just cosmetically marked
RHS: 'Below ground, the pathogen rots the roots and then invades the bulb.'
This answers the question most gardeners actually have once they see the white growth at the base: is the bulb itself still good? The fungus does not stay at the surface — it moves from the roots into the bulb. Unlike a surface blemish, a bulb showing these symptoms should be treated as affected throughout, not trimmed and kept like a bruised apple.
Myths about onion white rot
6 common claims about onion white rot, checked: 1 busted1 disputed2 untested2 hold up
“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 No study we found tests the DIY version gardeners actually describe, chopping up scraps and burying them, so the gap between the trialled treatment and the home method matters here.
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 actually 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 ceiling, every source we checked treats this as effectively permanent on a normal gardening timescale.
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?
Start with prevent introduction. After that, treat affected bulbs as compromised, not just cosmetically marked. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
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 much of this advice is actually proven?
Every control listed above is evidence-based, not just repeated folklore. None of it is a guess.
Sources
1 source, recorded with what each was used for
- RHS1: Stromatinia cepivora, affected alliums, symptoms including basal white growth and black sclerotia, 15+ year sclerotia persistence making rotation impractical, spread via tools and footwear, volatile-triggered germination, no chemical treatment or soil sterilisation, allotment cross-contamination risk