Thought this item from ProMED might interest you vector-watchers...as
well as the awn-heads among us (is that redundant? :^) .
Note once again the relationship between monocropping/single trait
resistance and inadvertant selection for virulence.
LEAF RUST, WHEAT - CANADA
A ProMED post
Date: Tue, 12 Oct 1999 09:52:51 -0400
From: "Marjorie P. Pollack"
Source: The Western Producer 7 Oct 1999 [edited]
Leaf rust runs rampant in AC Barrie wheat
Rust is eating away at Western Canada's most popular wheat
variety. AC Barrie, which accounted for almost one-third of prairie
wheat acreage last year, is proving susceptible to several new
races of leaf rust on the Prairies. That, combined with the late
seeding of this year's crop and ideal climatic conditions, has
allowed the disease to take a bite out of wheat growers'
"We got a lot of problems with leaf rust on Barrie this year," said
Brent McCallum, a plant pathologist at the Winnipeg Research
Centre of Agriculture and Agri-Food Canada (AAFC). McCallum
said a survey conducted by the Research Centre indicates that
leaf rust on all wheat, not just Barrie, is the worst in 20 years.
Farmers in rust-affected areas are suffering yield losses ranging
as high as 20 percent this year. Most years it's around 5 percent.
When Barrie was released in 1994, it was fully resistant to leaf rust,
thanks to a gene known as LR16 that was bred into a number of
Canadian and American varieties, including Columbus and AC
Majestic. But the cagey leaf rust pathogen has changed in the
intervening years and that gene no longer provides protection.
"Now it's able to attack AC Barrie," said McCallum. He indicated
that resistance in AC Barrie appeared to be breaking down over
the years but because leaf rust is much more prevalent this year,
diseased AC Barrie was more obvious.
Every year, rust spores are blown north from the United States,
eventually infecting fields in southern Manitoba and southeastern
The spores usually arrive in late May or early June. If moisture
conditions are right, the spores germinate and infect the leaf. After
about 10 or 12 days, small circular pustules which contain
thousands of spores rupture, releasing more spores, which, in turn,
infect more plants.
Because so much of this year's crop was seeded late, the disease
gained a much stronger foothold than usual, establishing itself in
the crop during the early stages of development. "Usually our
crops are getting fairly mature by the time the epidemic gets very
severe, but this year's crop was so late that it hit when it was the
most potent," said McCallum.
Several other factors conspired to make this a bad year for rust.
The amount of rust damage in Canada is driven to a large degree
by the situation in the USA.
This was the worst rust year in the past 25 years in the upper
Midwest, so a lot of spores came into the Canadian Prairies. In
addition, high moisture and humidity levels throughout much of
southern Manitoba and Saskatchewan provided ideal conditions
Leaf rust can be controlled with fungicide, but AAFC wheat breeder
Ron DePauw said at a cost of $20 an acre, it's not cost effective.
DePauw, who developed AC Barrie at the Swift Current, Sask.,
research centre, said farmers concerned about leaf rust may want
to consider other varieties that are still resistant to the new races
of the disease, including AC Elsa, AC Intrepid, AC Domain and AC
Plant breeders and pathologists are working to incorporate new
genes into popular varieties like Barrie to upgrade rust resistance.
"We knew this was a threat to happen and we've been working on
it," said DePauw. "It's something we always have to be building on,
to have as much genetic resistance built in as possible."
McCallum said that while this year's situation was unusual, it may
lead wheat growers in rust-prone areas to reassess what they plant
next year. "Farmers have to weigh the overall package ," he said.
"It may be poor for leaf rust but it may have other good quality
characteristics. They have to make the decision whether they're
worried enough about leaf rust to make the switch from AC Barrie
to another variety."
While acknowledging that many farmers were forced to seed late,
DePauw said this year's experience emphasizes the importance of
seeding early, especially if planting a variety that is susceptible to
new races of rust.
[By Adrian Ewins, Saskatoon newsroom]
[According to Don Harder, plant pathologist at the Winnipeg
Research Centre of AAFC, the new race of leaf rust has been
present for about 3 years, and has affected crops of AC Barrie
(and others containing the LR16 gene) quite severely in North
Dakota for the last two years. It is most likely that virulence against
LR16 has been present in existing populations of the fungus and
has been selected for by the widespread cultivation of this popular
Unlike the stem rust fungus, the leaf rust pathogen does not have
an alternate sexual host in Canada, which makes the origin of
"new" races a puzzle. AC Barrie appears to be on the way out, but
there are other cultivars that are highly resistant, and new ones are
continually being developed. But AC Barrie will likely retain its
popularity in the non-rust areas of the Prairies for some time yet. It
is in the rust area that it will be replaced the soonest. - Mod.DH]
Center for Integrated Ag Systems, UW-Madison
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