Mint Rust is a common disease that affects mint 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 pest 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
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.
Remove affected plants promptly, before the black resting spores form
Remove affected plants promptly, before the black resting spores form and contaminate the soil.
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.
The most specific control protocol on any of our pest pages, with an exact temperature and duration. It is also the clearest demonstration that mint rust is not leek rust: nothing like this appears on the leek rust page, because rhizome treatment is meaningless for a crop grown from seed or sets.
Good cultivation, cultivar 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 a standing philosophy rather than mint-specific advice, and it is the same position that produced the biodiversity reclassification of slugs and aphids. Worth quoting once on the site as the stated approach rather than repeating it on every page.
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 — wash rhizomes in early autumn, immerse in hot water at 44c for 10 minutes, cool in cold water, then plant. 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 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
2 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, rhizome heat treatment protocol, RHS's first-line-of-control philosophy
- RHS1: contrast — leek rust is Puccinia porri on alliums, pustules on lower leaf surface