Plants affected
Mints (Mentha), common marjoram (oregano), winter savoury.
Symptoms
Infected shoots become pale, swollen and twisted and orange, yellow or black (depending on time of year) pustules develop on the undersides of the leaves and on the stems, which often die.
Cause
The mint rust fungus (Puccinia menthae) can enter the rhizomes (underground stems) and so permanently infect the plants. Overwintering spores are also produced which can survive on the soil or rhizome surface.
Non-chemical control
- Burning off old foliage in autumn with a flame gun may control the disease but not if it has penetrated the rhizome.
- The disease may not infect all the rhizomes, so propagation from some pieces, especially the youngest, may yield disease-free plants.
- Hot water treatment may cure the problem: immediately prior to autumn planting, wash plants thoroughly, immerse the rhizomes for 10 minutes in water kept at 44°C , then cool in cold water.
Chemical control
None is available to home gardeners.