Mint Rust is a common disease that affects mint and oregano in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
What does it look like?
Dusty orange pustules on the stems and leaves, which may be followed by dusty yellow or black pustules. Large areas of leaf tissue die and plants may lose leaves. Symptoms run from spring until autumn.
Is this disease active now?
Symptoms can be expected from spring until autumn.
These are UK-generic timings. Scottish onset may differ. Use the Frost Date Calculator to adjust for your region.
Life cycle
Black resting spores form and contaminate the soil, which is why affected plants need to come out before they develop.
Controls: what actually works
All the methods below are backed by real evidence, not just repeated folklore.
Remove affected plants promptly, before the black resting spores form
Remove affected plants promptly, before the black resting spores form and contaminate the soil, and dispose of the material carefully rather than composting it: a home compost heap doesn't get hot enough to kill persistent disease spores, so burial, burning, or a local authority green waste collection are the safer routes.
The timing is the whole instruction. Once black spores reach the soil the bed is compromised — this is a window, not a general tidiness suggestion.
Rhizome heat treatment: wash rhizomes in early autumn, immerse in hot water at 44C for 10 minutes, cool in cold water, then plant
The full protocol runs like this. In an infected bed, locate any uninfected stems and carefully dig these out and move them elsewhere. Heat treatment means washing rhizomes thoroughly in early autumn and immersing them in hot water at 44C (111F) for 10 minutes, then cooling them in cold water and planting.
This is a genuinely precise protocol, with an exact temperature and duration rather than a rough guideline, so it's worth following as given rather than approximating.
Good cultivation, cultivarA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition → selection, garden hygiene, and encouraging natural enemies
Good cultivation comes first: avoiding pests, diseases and weeds through cultivation method, cultivar choice, garden hygiene, and encouraging or introducing natural enemies.
This is general good practice rather than mint-specific advice: healthy, well-spaced plants growing in good conditions simply have fewer disease problems to begin with.
Myths about mint rust
4 common claims about mint rust, checked: 2 busted1 disputed1 untested0 hold up
“Cutting mint right back to the ground and burning or binning the top growth clears a rust infection for good”
Mint rust overwinters as black resting spores that contaminate the soil and the outside of the rhizomes, not just the top growth. Chopping off and disposing of the visible orange-pustuled stems and leaves removes that season's symptoms, but if the rhizomes underground are already carrying spores, the plant simply reinfects itself from below the following spring. Clearing an infection for good needs the rhizomes dealt with too, either dug out and destroyed or lifted, washed and heat-treated, not just the leaves and stems.
Our read The control section lays out plant removal and rhizome removal as two separate, both-required steps for garden mint, with a distinct rhizome heat-treatment protocol offered as the alternative to digging the whole clump out. That structure only makes sense if top growth alone is not considered sufficient.
What to do: Treat cutting back as the first step, not the whole job: dispose of the top growth without composting it, then either remove the rhizomes entirely or wash and immerse them in 44C water for 10 minutes before replanting.
2 sources
- RHS: Mint rustResting spores in the soil or on the rhizomes infect new shoots in spring, so removal instructions for mint specify taking out infected rhizomes as well as affected plants, or heat-treating rhizomes at 44C for 10 minutes as an alternative.
- RHS: Disposing of diseased materialHome compost heaps do not reach the temperature needed to destroy persistent disease material, so infected plant matter should go by burial, burning or council green waste collection rather than the compost heap.
“Once mint shows rust pustules, none of the leaves from that plant are safe to eat, even the green-looking ones”
Rust fungi are plant pathogens, not human ones, and Puccinia menthae is not known to produce the kind of toxins that make some fungal contamination genuinely dangerous to eat. General food-safety guidance on garden produce from diseased plants is that it remains safe to eat, if not always appetising, and the real line to watch for is mould or rot rather than the pathogen itself. That said, no fungicide is available for mint rust that is approved for use on culinary herbs, which is a separate reason to keep chemical treatment off the menu rather than a statement that the rust itself is unsafe.
Our read The RHS mint rust page's note that no fungicides are cleared for culinary use on mint is about pesticide approval, not about the rust fungus being toxic; it does not say rust-affected leaves are unsafe to eat.
What to do: Pick around visibly pustuled leaves for texture and appearance rather than safety, wash mint as normal before use, and avoid any leaves that are also mouldy, slimy or rotten.
2 sources
- Iowa State University Extension and Outreach: Can Sick Plants Make People Sick?Garden produce from a sick plant is generally safe to eat, although it may not be desirable, and the produce to avoid is anything mouldy or rotten, since some fungi and bacteria produce toxic compounds.
- RHS: Mint rustNo fungicides with activity against mint rust are available for use on mint or other herbs that will be used for culinary purposes.
“Mint rust will jump straight from one mint variety to any other mint growing nearby”
Puccinia menthae is not a single uniform threat to every mint in the bed. Research on commercial mint growing has identified distinct host races within the fungus: a native-spearmint race that cannot infect peppermint, and a peppermint race that cannot infect native spearmint, while Scotch spearmint is susceptible to both. Separately, an oregano-derived strain of the same fungus can infect oregano, Greek oregano and sweet marjoram but not spearmint. So cross-infection between mint varieties happens, but which varieties are at risk from which infection depends on the race involved, not a blanket rule that any mint infects any other.
What to do: Do not assume a rust-free variety planted next to an infected one is safe by variety alone. Watch all mint, marjoram and oregano in the same bed for symptoms and isolate or remove affected plants regardless of which species started it.
2 sources
- Washington State University Extension: Management of Rust on MintOne rust race infects native spearmint but not peppermint and is called native spearmint rust; the other, peppermint rust, infects peppermint but not native spearmint; both groups of races infect Scotch spearmint.
- RHS: Mint rustMint rust infects several mint species and some related plants including marjoram and savory, confirming the fungus is not confined to a single mint variety.
“If part of a mint bed has rust, you can rescue it by digging up whatever stems still look clean and replanting them elsewhere to start a fresh, rust-free patch”
The RHS lists this as something to try, alongside hot-water treatment of rhizomes, but doesn't say how reliably it works or address whether the rust fungus can already be present in stems that look clean.
Our read GrowScotland's own reasoning: rust fungi are well known elsewhere for symptomless or latent infection in tissue that looks healthy, and the RHS's own hedge ('in an attempt to') reads as an acknowledgement it isn't a sure thing. Treat this as a low-cost experiment worth trying, not a guaranteed fix — don't be surprised if rust reappears in the new patch.
What to do: Worth attempting if you want to save a favourite mint variety, but keep expectations modest and keep an eye on the new patch for symptoms rather than assuming it's cured.
1 source
- RHS, 'Mint rust'In an infected bed, try to locate any uninfected stems and carefully dig these out and move to another location in an attempt to start a new, healthy colony.
Frequently asked questions
What should I do first if I find mint rust in my garden?
Start with remove affected plants promptly, before the black resting spores form. After that, rhizome heat treatment. 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 mint rust. The controls that work are cultural: the measures listed above.
When are mint rust most active in Scotland?
Check the seasonal activity section above. Scottish timings can run a few weeks behind the UK-generic figures.
Sources
3 sources, recorded with what each was used for
- RHS1: Puccinia menthae, hosts including marjoram and savory, orange then yellow/black pustules, spring-to-autumn symptoms, leaf death and defoliation, black resting spores contaminating soil, removal timing, disposal instruction, rhizome heat treatment protocol, RHS's first-line-of-control philosophy
- RHS1: verifying leek rust is a genuinely different organism (Puccinia porri, alliums only) — the reason this entry exists separately rather than being folded into leek rust
- RHS1: home compost heaps not reaching disease-killing temperatures; burial, burning or green waste collection as alternatives