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With wheat, better safe than sorry

By Ed Haag

In the summer of 2002, Kim
Kidwell, Washington State University (WSU) wheat breeder, was feeling confident
about Zak, the newly released variety from her spring wheat breeding program.
She believed it offered Washington grain growers a level of stripe rust
resistance that could withstand even the most adverse set of circumstances.

That was before her
telephone rang. “A farmer reached me on a Sunday, and I went out to check the
field that same day,” she recalls. “When I saw it, I knew we were in trouble.”

A quick visual assessment
indicated the stripe rust infection level on the new release had exceeded its
economic viability threshold. It required immediate spraying with a fungicide
in order to avert further crop damage.

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Kidwell and others viewing
the rust damage harbored no illusions that the infection was a localized event.
They knew from past experience that with a rust outbreak of this magnitude, it was highly likely that the problem extended well beyond that particular farm.
An Extension crop emergency was declared and through a telephone tree, the
other growers of Zak were immediately contacted.

“At the time, there were
about 150,000 acres of Zak in production,” says Kidwell. “Most had to be
sprayed.”

The stripe rust damage
wasn’t just confined to Zak. Other wheat germplasm sources had new releases
that were also seriously impacted by the rust that year. The newly released
soft white spring wheats were particularly hard hit.

“In 2002, the best new
releases from every program in the region were overcome by stripe rust,” says
Kidwell. “It was an event that literally rocked the foundations of the Pacific
Northwest grain farming community, and no one saw it coming.”

Besides the $15- to
$20-per-acre spraying cost, many growers who had planted spring wheat crops
faced serious yield reductions attributed directly to a stripe rust infection.
“The costs ran into the millions of dollars,” says Kidwell. “We had to figure
out a way to prevent such an event from happening in the future.”

Resistance Is Relative

Working directly with
Xianming Chen, USDA Agricultural Research Service research plant pathologist,
Kidwell and her fellow breeders at WSU analyzed the circumstances surrounding
the stripe rust epidemic. Two major factors emerged. 

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“In 2002, we had a cool, wet
spring, which was very conducive to stripe rust,” says Kidwell. “Then we had a
pathogen shift and the arrival of a new race of rust for which our new
varieties had no resistance.” 

Kidwell and her colleagues
concluded that while it was impossible to control future weather events, a
breeder could look at improving the resistance in future releases. “We decided
the only way to prevent the same thing from happening again was to completely revamp how
we selected for stripe rust resistance,” she says.

Chen says wheat breeders
since the 1960s have opted to select for one or more of two different types of
rust resistance. “One type is race-specific, all-stage resistance; it’s also
known as seedling resistance,” he says. The other is HTAP (nonspecific
high-temperature adult plant resistance).

Chen says the functional
difference between the two types of resistance lays in the fact that the
race-specific all-stage resistance does not protect the plant from a new race
of rust – as occurred in 2002 – and HTAP does.

He and Kidwell concluded
that the inclusion of nonspecific resistance would help protect future wheat releases from the new races of stripe rust that were regularly appearing in the
Pacific Northwest.

“Because HTAP is
nonspecific, it’s more durable than race-specific all-stage resistance in
wheats like Zak,” says Chen.

It was decided that in their
revamped breeding program, all future releases would include at least HTAP
resistance with an overall goal of having both all-stage and HTAP resistance in
as many releases as possible. “We wanted that dual-action protection both when
the plants were young and when they were older,” recalls Kidwell.

All wheat lines in current
production that didn’t have HTAP resistance would be replaced as soon as
possible by ones that did.

A Major Achievement

Arron Carter, WSU’s winter
wheat breeder who worked on Kidwell’s breeding team as a graduate student,
recalls that this was no small task. Unlike other growing regions in the
country, the Pacific Northwest is not blessed with climatic consistency.  In Washington State alone, geographical
variation runs from rainforest to desert. It includes seven distinctly
different agriclimatic zones, four of which are represented in wheat production
areas.

The goal the WSU breeders
had set for themselves would not involve replacing one but several spring
wheats.

In spite of these
challenges, Kidwell and her colleagues in WSU’s spring wheat breeding program
are pleased with their results.

“Since the 2002 meltdown, we
have released a cache of individual wheats that have better resistance and more
diversity between them so we don’t put so much selection pressure on the stripe
rust pathogen,” says Kidwell. Each wheat corresponds to a specific niche in
Washington’s overall grain-production system, she adds. 

In 2006, Kidwell’s group
released Louise, the common white spring wheat replacement for Zak in the
intermediate- to high-rainfall regions of the state. In 2009, farmers planted
178,900 acres (62%) of the total common white spring wheat crop in Washington,
to the new stripe-resistant wheat.

This was followed in 2008 by
two more releases: Whit, another soft white common spring wheat with excellent
stripe rust- and Hessian fly-resistance suitable for high-rainfall areas, and
Kelse, a high-protein hard red spring wheat for intermediate- to high-rainfall
zones.

“Kelse is the first hard red
spring wheat variety developed by our program with excellent nonrace-specific
high-temperature, adult plant resistance to stripe rust,” says Kidwell.

In 2009 there were two more
spring soft white variety releases: JD, a new soft white, spring club variety,
and Babe, a soft white, common spring wheat.

“Our goal was to bring a new
package of soft white common wheats out into production to give growers some
options, because so many good soft white spring wheats were lost in 2002,” says
Kidwell. “I think we have accomplished that.”

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