Chrysanthemum White Rust
Puccinia horiana
Sunshine is delicious, rain is refreshing, wind braces
up, snow is exhilarating;
there is no such thing as bad weather just different kinds of good weather.
- John Ruskin
Introduction
I love fungi, especially rusts, how these tiny organisms ever conceived
such complicated life cycles is a wondrous thing. Puccinia horiana is the causal organism of Chrysanthemum white rust (CWR). Although
its life
cycle isn’t as complicated as its relatives black stem rust of
wheat and white pine blister rust, it is still an incredible disease,
with disastrous consequences.
Biology
CWR is a microcyclic, autoecious rust which means that it has less than
four spore stages and its entire life cycle may be completed on a single
host species, which happens to be about 12 species of chrysanthemums.
The disease will not develop outside of a susceptible plant.
The two spores
that are produced are first, the teliospores, recognized by the whitish/pinkish
pustules found on the underside of the leaves.
On the upper-side of the leaves there will be a small, whitish dimpled
spot that will turn brown over time. The teliospores can survive
for
as long as 8 weeks if they remain in the pustules. Even if the leaves
are detached from the plant the teliospores will remain viable in
the leaf litter when the relative humidity remains around 50%. There
are
other rusts that transpire on chrysanthemums, so proper identification
is essential. CWR is identified based on the morphological features
of the teliospores. Suspect material should be sent to an appropriate
diagnostic
laboratory for microscopic examination and fungal identification.
In approximately three hours, with almost 100% humidity, the teliospores
will produce the second type of spore, the basidiospore. The basidiospores
are fairly frail but these are the guys that can cause an epidemic
if the environmental conditions are right. The right conditions
are a temperature
around 62.5oF (17oC), and infection can occur in as little as two
hours
but symptoms won’t necessarily manifest themselves for another
5-14 days. Symptoms will appear in cooler, wet weather. Basidiospores
will only survive for approximately an hour if the RH is from 81-90%.
Basidiospores can travel short distances by several means, wind currents
in wet weather, splashing water, and humans walking through a wet infected
crop. The basidiospores need free water on the surface of the plant
to infect.
Importance
CWR is a federally quarantined pest in the United States; therefore
the importations of several chrysanthemum species are prohibited
from several
countries. The introduction and spread of CWR could lead to serious
economic losses. This fungal rust can be spread rapidly throughout
a greenhouse
causing complete crop failure.
Spread
CWR was first reported in China and Japan in 1895. It originated in eastern
Asia and is now established in Africa, Australia, Europe and Central
and South America. There have been outbreaks here and in Canada, but
the pest was found and eradicated.
How can you help?
- Buy cuttings and plants from reputable sources and inspect them frequently, especially when the weather becomes cooler and wetter. In the Pacific Northwest weather change usually occurs in late August to mid September. Plants that are infected, but as of yet have shown no symptoms, will start to be symptomatic at this time.
- Talk to your garden center staff members and make sure they are aware of this disease. Remember that garden mums at home can be a source of contamination for garden center and nursery owners as well as greenhouse growers. Extreme care should be taken when moving between retail and home sites. Early detection is imperative.
- At your local garden center, look to see if they have a color poster or a brochure of the disease, so that you might recognize it more readily.
- Always use a foot bath at retail centers if one is provided. These prevent spores from traveling on the bottoms of shoes and boots.
- In your home garden always be aware of the visitors you have, whether they travel on two, four, six or eight legs and be friend, foe or innocuous.
The gardening season officially begins on January 1st
and ends on December 31st.
-Marie Huston