Chrysanthemum White Rust
White rust on chrysanthemum is caused by the fungus Puccinia horiana P. Henn.
It is considered to be the most serious fungal diseases of the florists chrysanthemum, Dendrathema X grandiflorum (Ramat.) Kitam.
The disease was introduced into Europe from Asia through the movement of infected plant materials.
It spread rapidly, frequently causing the complete loss of glasshouse chrysanthemum crops throughout the whole of Europe.
Fungicide trials were conducted under field conditions in the UK, using the rust-susceptible cultivar Margaret found the following:
All fungicide treatments, which included mancozeb (1440 g a.i./ha) chlorothalonil(1100 ga.i./ha), fenpropimorph plus propaconazole (375 + 125 g a.i./ha), oxycarboxin (750 g a.i./ha) or myclobulaniul (60 g ai./ha), applied in 4 applications over approximately 3 months, significantly reduced CWR disease severity.
Mancozeb alone reduced disease severity by 51%, mancozeb with chlorothalonil by 81%,and mancozeb alternating with fenpropimorph plus propiconazole by 97%.Fepropimorph was phytotoxic at the rate used (1500 g a.i./ha) when applied to young plants, but not when used at the rate on plants 5 weeks before harvest.
The early signs are yellow spots on the upper surfaces of the leaves.
Later the spots turn brown and sunken, with corresponding raised buff pustules on the lower surface of the leaves, these turn whitish as they mature.
Eventually the leaves die back, and flowers may also become affected.
This most serious disease of Chrysanthemum species originated in China and Japan and arrived in Britain in 1964.
It is highly infectious and left unchecked it will severely weaken plants and stunt growth.
White rust is generally selective, i.e. it will appear on some varieties and not others, and is an airborne disease that can be carried on the wind up to a mile.
The fungus thrives in cool damp conditions.
White rust survives only a few weeks on fallen leaves and the main source of infection is over wintered stools or cuttings.
It is, therefore important always to eliminate infection from over wintered 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 seen and destroy them (do not add to the compost heap).
Badly affected plants should also be destroyed: either burn them or seal them in a plastic bag and place in the dustbin.
Repeated applications of fungicide at seven-day intervals may be required. Bupirimate + triforine (Miracle Nimrod-T), penconazole (Murphy Tumbleblite 2) and myclobutanil (Bio Systhane) have some systemic activity.
Alternating between two or more fungicides will avoid resistance developing in the pathogen.
Some people recommend using the above fungicides at double strength applied in June/July before it appears then again when boxing up stools for the winter.
One can use Tilt a commercial fungicide, but it is a preventative fungicide rather than a cure.
It is best applied as new growth appears on the stools in spring then again at planting out or potting on stage.
Amateur growers are no longer required to notify the Ministry of Fisheries& Food (MAFF) although suppliers of propagating material are still subject to some legal controls.
Source: O'Neill, T.M. and D. Pye. 1997. Evaluation of Fungicides for control of chrysanthemum white rust.
Further notes on the availability of chemicals can be found here.