Mint rust is a disease caused by the fungus Puccinia menthae. Mint rust infects several mint species as well as some related plants including marjoram and savory. Mint rust symptoms can be expected from spring until autumn.

You may see the following symptoms:

• Pale and distorted shoots in spring
• Dusty orange pustules on the stems and leaves. These may be followed by dusty yellow or black pustules
• Large areas of leaf tissue die and plants may lose leaves

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

Remove affected plants promptly before the black resting spores are formed and contaminate the soil. In the case of garden mint it is also necessary to remove infected rhizomes. 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.

Heat treatment is a method used by commercial nurseries, but could be adapted for the home gardener. Wash rhizomes thoroughly in early autumn and immerse in hot water at 44ºC (111ºF) (no higher) for 10 minutes, then cool in cold water and plant. An accurate thermometer is required, because 44ºC (111ºF) is very near the lethal temperature for the plant, and it may be more profitable to spend the money on some new plants.

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the links and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

No fungicides with activity against mint rust are available for use on mint or other herbs that will be used for culinary purposes.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners).

Link

Chemicals: storing and disposing safely

The rust fungi are described as biotrophs; that is, they grow within the living tissues of the plant and extract nutrients from the cells without killing them. They are not able to survive in an active state on dead plant material, so must either alternate with a different, perennial host, or produce resting spores to pass the dormant season.

Puccinia menthae completes its entire life cycle on one plant host and produces resting spores to pass through winter.

In garden mint, the general consensus was that the fungus grows into the rhizomes, where it spends the winter, causing a systemic infection throughout the tissues of the plant.  However there is currently some debate as to whether this really is the case. It is known that resting spores present in the soil or contaminating the outside of the rhizomes can infect new shoots in spring.

The fungus has several spore stages, the first being the orange pustules that appear in spring or early summer, followed later by yellow spores and finally by black resting spores.