Chrysanthemum White Rust (page
last updated 07 June 2006
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White Rust is likely to make
its customary appearance in the U.K. over the coming summer months so it's
wise to be on the lookout for this disease. This short (and by no means
definitive) article may help growers to recognise and deal with this
problem.
So, what
is white Rust?
White Rust is a fungal disease caused by Puccinia
horiana, The fungus produces two types of spores -
teliospores and basidiospores. Teliospores are produced in pustules and
remain in the pustules unless they are aggressively brushed off.
Under
moist conditions (96% to 100% relative humidity) for at least 3 hours,
teliospores produce basidiospores. Basidiospores can cause an epidemic if
conditions are right. They spread from plant to plant by splashing water
and they must have a film of water on the plant surface for infection.
Infection can occur in as little as 2 hours at the optimal temperature of
17C.
Basidiospores can also
travel short distances by wind currents during moist weather.
Infections occur when infected cuttings or viable spores are brought into
a greenhouse, thereby exposing any uninfected cuttings. Infected cuttings
may appear normal even though the fungus is present, waiting to manifest
itself when conditions become more favourable.
Chrysanthemum white
rust originated in eastern Asia and is now established in Europe, Africa,
Australia, Central America and South America. It is a notifiable disease
in the USA and Canada.
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What does it look like?
Chrysanthemum white rust can be
recognized by the small white to yellow spots, up to 4 mm wide, on
the upper surface of the leaf. These slightly dimpled spots become
brown over time. Pustules form on the underside of the leaf, beneath
the small spots. These are buff to pink-colored but become white as
they age. Pustules are most common on young leaves but can be found
on any green tissue and flowers. Infected plants do not always
display symptoms during hot and dry conditions.
In the early stages it is not so easy
to recognise as there are only small pale, slightly
raised, spots on the top of the leaves, but these develop quickly
and pinkish/ buff pustules, resembling blisters, soon
appear on the undersides of the leaves which are easily
identified. These blisters contain thousands of fungal spores, which
will soon be dispersed to infect adjacent plants, the complete cycle
can be as short as 7 - 10 days.
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How do I deal with it?
The most likely cause of infection is the arrival of new plants, so
it makes sense to keep a watchful eye on all new imports, and if
possible quarantine them for several weeks.
One factor that works for us is that white rust needs the right
conditions to spread, high humidity above 96% and temperatures of
about 17 °C - these optimum conditions can occur over the summer
months (and earlier and later under glasshouse conditions).
Prevention not cure
Inspect any newly acquired plants
and remove any infected leaves and spray all of the plants regularly for three or four
weeks with a chemical spray containing the active
ingredient myclobutanil, this is one of the most effective products
available to the amateur grower.
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Maintain good hygiene
Continuing the 'prevention not cure' theme, try to keep a clean site, use a good
general hygiene programme, by quickly disposing of old leaves and removed
shoots, spray plants regularly, say every two weeks with a suitable
chemical.
In the event that only
a small number of plants appear to be affected it may be worth
considering removing these plants from the plot as an alternative to
a chemical remedy.
End of season treatment
It is possible that while plants are not showing signs of infection they may
still be contaminated, and will need treatment at the end of the season if they
are to be used for propagation. Continued chemical spraying is one form of
ongoing protection.
And Finally ......
Unfortunately white rust is now
widespread within the U.K. but fortunately it is now a little easier to
deal with than it was a few years ago. If you are observant, identify the
problem early and act quickly then it's possible that it can be contained. It only effects
chrysanthemums, so it is limited in how easy it can spread.
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Comments from Frank Mastramano (USA) added
7th June
Dear Paul,
I found the new page on white rust to be very informative. Unfortunately,
I became aware of the problem when a new member of our society mentioned to a
fellow member that her plants looked strange. She was advised to get in
touch with me. I was no white rust expert but had read some various
articles describing it.
I hadn't taken two steps into her garden when I became fully aware she had an
unbelievable infection of white rust on all but one of her plants. One
point of interest is that in the U.S. when you encounter white rust you are
required to contact the local, county or state department of agriculture.
Even though they are a government agency their response is within a day.
If they detect it in your garden they will remove all plants and enclose them in
plastic bags. If you grow in pots they will take the plant, pot and soil
for their disposition of all.
While judging a chapter show, the gentleman who was judging with me and I
noticed two exhibits that appeared to be infected with white rust. The
show chairman thanked us for our observation and contacted the local
agricultural department. They arrived the same afternoon and made the
decision to close the show that evening and removed all exhibits from the
tables. They disposed of all materials and made a visit to the garden of
the exhibitor.
One final point is that there is a product called "Immuniox" that is
sold in some garden centers that contains myclobutanil and has been found to
control white rust. However, when the agriculture departments are notified
and find the infection they do not allow you to control the problem. They
dispose of all items; pots, stakes, shade cloths, and whatever else they find
that came in contact with the plants.
I forward these comments to you knowing that you will be able to determine if
they are appropriate for further distribution.
Frank
PS,
I should have mentioned that when I took just two steps into the garden the
problem was evident. I then turned around immediately and made sure I did
not brush against any of the plants with my clothing. That would have been
a certain way to bring the white rust home to my plants.
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With Thanks to Harry Lawson for use of
photos.
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