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).