Advice
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White rust of chrysanthemum
Symptoms
The early signs are yellow spots on the upper surfaces of the leaves. Later the spots turn brown and sunken; corresponding on the lower surface are raised buff pustules, which turn whitish as they mature. Eventually the leaves die back. Flowers may also be affected.
Cause
This most serious disease of Chrysanthemum originated in China and Japan and arrived in Britain in 1964. It is highly infectious and left unchecked it severely weakens plants and stunts growth.
The fungus (Puccinia horiana) thrives in cool damp conditions. Spores carried on air currents spread the disease from plant to plant. White rust survives only a few weeks on fallen leaves and the main source of infection is overwintered stools or cuttings. It is, therefore, important always to eliminate infection from overwintered plant material and not to bring in the disease on purchased plants.
Non-chemical control
Check plants regularly and remove infected leaves as soon as they are seen and destroy them (do not add to the compost heap). Badly affected plants should also be destroyed: either burn them or seal in a plastic bag and place in the dustbin.
Chemical control
Repeated applications of a fungicide at seven-day intervals may be required. Penconazole (Scotts Fungus Clear) and myclobutanil (e.g. Systhane Fungus Fighter) have some systemic activity. Alternate between these two fungicides to avoid resistance developing in the pathogen.
Note: Amateur growers are no longer required to notify the Department for the Environment, Food and Rural Affairs (Defra) of an outbreak of white rust, although suppliers of propagating material are still subject to some legal controls.
Further information
Search for suppliers of chrysanthemums in your area with the online RHS Plant Finder or search the Award Plants database for Award of Garden Merit species and cultivars.