Although generally quite a hardy flower; I have found that the pest which most often attacks my chrysanthemums is white rust caused by the fungus Puccinia horiana. As this is the most common chrysanthemum disease I have used it for the featured image at the top of this article.
Chrysanthemum White Rust Puccinia horiana
The picture of a single chrysanthemum leaf shows a close up of white rust at quite an advanced stage.
Watch out for the first sign of the disease – pale yellow spots on the upper leaf surface. If these aren’t treated they will develop into the brown sunken spots pictured. Underneath the leaf you will then be able to see light brown bumpy pustules filled with spores of the fungus waiting to burst and infect more plants.
White rust does not directly kill chrysanthemum plants but if left untreated it will severely weaken the plant and many of the leaves will eventually blacken and die and the plant may not survive.
It is interesting to note that white rust was not a problem in the United Kingdom until about 1963 when infected plants were imported from the far east where many new chrysanthemum strains are bred.
White rust thrives in cool wet conditions so is most problematic towards the end of the summer and during autumn – just when many chrysanthemums should be producing their best flowers!
Treatment of white rust
As is always the case – prevention is better than the cure so always buy nursery plants from reliable growers and carefully check new plants for any sign of disease. Do not over water your chrysanthemums and make sure there is adequate air circulation to reduce dampness – this is especially important if you are growing them in a crowded greenhouse.
Keep checking the leaves and at the first sign of infection spray with a fungicide. (The best one I’ve found is a boscalid and pyraclostrobin powder which can be bought from Chysanthemums Direct – it then needs mixing with water). Remove and destroy any severly infected leaves.
Chrysanthemums are also sometimes attached by brown rust Puccinia chrysanthemi which is closely related to the white rust fungus and the treatment is the same.
Chrysanthemum Eel Worm (nematodes)
These microscopic worms cause similar spots to those cause by fungus but they usually start on lower leaves and gradually spread upwards – this is because the eel worms are spread by spring rain fall and swim up from the bottom of the plant. They can survive in fallen leaves from year to year so ensure to remove all old leaves from around your plants. As mentioned in our last article you can treat trimmed stools before storing over winter with a warm (46ºC) water dip for five minutes – this will kill the nematodes.