Mint rust is a common fungal disease of garden mint, but also affects marjoram and savory. The fungus causes dusty orange, yellow and black spots on leaves.
What is mint rust?
Mint rust is a disease caused by the rust 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 loose leaves
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.
No fungicides are available to gardeners to treat rust on mint used for food.
For ornamental species, fungicides containing difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Doff Systemic Fungus Control and other formulations), tebuconazole (Bayer Garden Multirose concentrate2) or triticonazole (Scotts Fungus Clear Ultra) may be used. They will be most effective at preventing new infections, but because they have systemic activity (are taken into the plant tissues) they may have some curative properties.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners).
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 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 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.
Common name Mint rust
Scientific name Puccinia menthae
Plants affected Garden mints, marjoram and savory
Main symptoms Orange, yellow and black spots on leaves
Caused by Fungus
Timing Spring until autumn