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Better safe than sorry
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.
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.
“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.”