Onion White Rot is a serious disease that affects onions, garlic, 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).
This pest lives in the soil and lies dormant until a host is planted. A seasonal calendar would be misleading.
Why this matters in Scotland
An allotment problem, and Scotland has a strong allotment culture — which the site already has a page for.
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, and our leek guide rates them 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.
The most consequential fact on any of our pest pages: this one is effectively permanent.
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 germinate 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
Every method below is graded by evidence, not by how often it gets repeated. Where a claim has been tested and failed, we say so plainly.
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.
Frequently asked questions
What should I do first if I find onion white rot in my garden?
Start with prevent introduction — avoid bringing in contaminated soil, especially on tools and footwear. After that, treat affected bulbs as compromised, not just cosmetically marked. The full list of evidenced controls is above, each graded by how much evidence supports it.
Is there a spray that works?
No. There are no amateur fungicides available for onion white rot. The controls that work are cultural: the measures listed above.
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