Mint Rust is a common disease that affects mint, oregano and summer savory.
What does it look like?
Pale and distorted shoots in spring can be the first sign. Dusty orange pustules appear on the stems and leaves, later followed by dusty yellow or black pustules. In severe cases 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-wide timings, and Scottish onset may differ.
🌱 Plants at risk
Life cycle
The fungus completes its life cycle on the mint and produces resting spores to pass through winter. In garden mint it is generally thought to grow into the rhizomes and spend the winter there, though this is still debated, and resting spores in the soil or on the rhizome surface can infect new shoots in spring. Black resting spores contaminate the soil, which is why affected plants need to come out before they form. US commercial mint advice adds that the fungus overwinters on mint stubble and on wild and escaped mint.
Controls: what works
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. Home compost bins rarely reach temperatures sufficient to kill pathogens, so burn dry material or use a council green waste scheme, whose facilities should reach temperatures that kill most diseases. Don't bury material infected by soil-borne diseases.
The timing is the instruction: once black spores reach the soil the bed is compromised.
Rhizome heat treatment: wash rhizomes in early autumn, immerse in hot water at 44C (no higher) for 10 minutes, cool in cold water, then plant
In an infected bed, try to locate any uninfected stems, carefully dig them out and move them elsewhere to start a new colony; infected rhizomes must also be removed. Heat treatment is a nursery method that could be adapted for home gardeners: wash rhizomes thoroughly in early autumn, immerse them in hot water at 44C (111F) for 10 minutes, cool them in cold water and plant. 44C is very close to the temperature that kills the plant, so use an accurate thermometer.
Follow the temperature and time 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.
Myths about mint rust
“Cutting mint right back to the ground and burning or binning the top growth clears a rust infection for good”
Mint rust passes the winter as resting spores in the soil and on the rhizome surface, and in garden mint the fungus is generally thought to spend the winter in the rhizomes, though that is debated. Removing the visible stems and leaves removes that season's symptoms, but infected rhizomes can reinfect new shoots in spring. Clearing an infection needs the rhizomes dealt with too, either removed or lifted, washed and heat-treated.
Our read The RHS asks for infected rhizomes to be removed as well as affected plants, with heat treatment of the rhizomes as the alternative, which only makes sense if top growth alone is not sufficient.
What to do: Treat cutting back as the first step, not the whole job: dispose of the top growth by burning or council green waste rather than home composting, then either remove the rhizomes entirely or wash and immerse them in 44C water (no higher) for 10 minutes before replanting.
2 sources
- RHS: Mint rustResting spores in the soil or on the rhizomes infect new shoots in spring, so infected rhizomes must be removed as well as affected plants, or heat-treated at 44C (no higher) for 10 minutes.
- RHS: Disposing of diseased materialCompost bins rarely reach temperatures sufficient for killing pathogens, so infected plant matter should be burnt or go to a council green waste scheme, and material infected by soil-borne diseases should not be buried.
“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. General guidance on garden produce from diseased plants is that it is generally safe to eat, if not always appetising, and the thing to avoid is anything mouldy or rotten. No fungicide is available for mint rust that is approved for culinary herbs, which is a separate reason to keep chemical treatment off the menu, not a statement that the rust 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.
Our read Spread depends on which race of the fungus is present and which mint or oregano is nearby, so neither assuming it always jumps nor assuming a variety is safe is supported.
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”
This is listed as something to try (RHS), alongside hot-water treatment of rhizomes, but nothing says how reliably it works or whether the rust fungus can already be present in stems that look clean.
Our read The tentative wording (in an attempt to) suggests it isn't a sure thing, and stems that look clean may still carry the fungus. Treat this as a low-cost experiment, not a guaranteed fix, and 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?
The first thing to try: Remove affected plants promptly, before the black resting spores form. Then: Rhizome heat treatment.
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 should I watch for mint rust?
Symptoms can be expected from spring until autumn.
Sources
Show the 4 sources
- RHS1: Puccinia menthae, hosts including marjoram and savory, pale distorted shoots, orange then yellow or black pustules, spring-to-autumn symptoms, leaf death and defoliation, resting spores in soil and rhizomes, removal timing, digging out uninfected stems, the heat treatment protocol, no fungicides for culinary herbs
- Pacific Northwest Extension: the fungus overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → on mint stubble and on wild and escaped mint
- RHS1: confirming leek rust is a different organism (Puccinia porri, alliums only), which is why this entry exists separately from leek rust
- RHS1: compost bins rarely reaching temperatures that kill pathogens; burning dry material; council green waste facilities; not burying material infected by soil-borne diseases