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Stripe rust find puts resistance gene in question

Matthew Weaver
Researchers have found stripe rust on Kaseberg, an Oregon State University soft white wheat variety that has a stripe rust resistance gene. Further research will show whether it's a new strain of stripe rust or the resistance gene is breaking down, OSU Extension cereal specialist Mike Flowers says.

Oregon State University researchers are investigating why stripe rust has appeared in a wheat variety that’s supposed to be resistant to the disease.

Stripe rust first appeared last year in a commercial field of the OSU soft white wheat Kaseberg, a variety bred to be resistant to the disease. Then rust appeared in another commercial field of Kaseberg early this year, prompting researchers to look for a reason.

To solve the mystery, researchers are focusing on YR5 and YR15, the two major genes that provide stripe rust resistance to wheat in the region, said Mike Flowers, OSU Extension cereals specialist.

Kaseberg carries YR5, but not YR15, Flowers said. When Kaseberg was released, it did not show any stripe rust, even during heavy stripe rust years.

Researchers checked the variety for the YR5 gene and found it to be present.

“Either YR5 wasn’t giving the complete resistance we thought, which is possible, or we’ve got a new race of rust that can overcome YR5 as well,” Flowers said.

It’s still not clear whether YR5 has been overcome, since Flowers tested the best molecular markers for the gene, but noted those markers are not 100 percent certain.

Flowers and OSU wheat breeder Bob Zemetra are sending samples to USDA Agricultural Research Service plant pathologist Xianming Chen in Pullman, Wash., to determine whether it’s a new race of stripe rust or confirm the YR5 gene in the variety.

Flowers felt it important to bring to breeders’ attention because of the reliance on the two resistance genes. Many breeding programs incorporate the genes into their germplasm base for newly released varieties.

Stripe rust has not been found in varieties with YR15, Flowers said.

Many spring and winter wheat varieties in breeding programs have both genes.

“It’s disappointing to hear and see, but it’s not a surprise,” Flowers said. “Almost all major (resistance) genes have been broken down over time. That’s why we only have two that we know of that are still providing complete resistance.”

Zemetra said new stripe rust races evolve almost every year.

“It’s almost like an arms race — we deploy new genes for resistance and the rust mutates to overcome the resistance,” he said.

Breeders are constantly looking for new resistance. Breeders could still use the YR5 gene in conjunction with other resistance genes. Using several genes provides more resistance than a single gene, Flowers said.

“If you do have varieties with that single gene, it’s not going to give you complete resistance, it’s going to be more at risk for stripe rust,” he said.

If it’s no longer as resistant, farmers would have to treat Kaseberg like other varieties, Flowers said, keeping careful watch and using a fungicide when the rust shows up.

“It’s disappointing that the variety is not providing the same resistance it was, but there are management steps they can do to take care of the disease,” he said. “They’re doing that successfully in other varieties. This would be no different.”



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