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USDA-ARS researchers based out of St. Paul, MN, Raleigh, NC and Manhattan, KS are all playing key roles in the development of new resistance genes. But staying ahead of Ug99 and other wheat rust races is difficult because of its ability to mutate and overcome resistance genes.

AN EVER-CHANGING ENEMY

One of the most challenging aspects of combating Ug99 is the fact that it is constantly mutating and possibly even undergoing sexual recombination, resulting in several derivatives of the original race.

Dr. John Fellers, research molecular biologist at USDA-ARS in Manhattan, KS, says that although this pathogen is more aggressive than what’s been documented in the past, it is definitely not unexpected that a pathogen would mutate over time. “This will always be an issue, but as a community we know that rust changes and farmers know a new variety is not good forever,” he says. “So we constantly have to be working on it and making new varieties, getting new genes so the farmers have something to work with.”

Because of the pathogen’s ability to evolve so quickly, some genes that had worked against Ug99 are no longer working. “We made some calculations to see what percentage of varieties in the United States were vulnerable to Ug99 and although that figure wasn’t promising, we did have some resistant varieties,” says Bowden. “In the meantime, however, the pathogen has defeated some of the varieties we thought were good, so we had to lower our estimate of how many resistant varieties we have if it were to come here.”

Researchers are currently pursuing two solutions using genetic research to create new rust-resistant varieties that can stand up to Ug99 for an extended period of time.

DUAL STRATEGIES

One approach researchers at K-State are working on is dubbed the major gene approach. The major gene fights wheat rust using a sort of pathogen recognition system. When the plant is attacked by a pathogen that the major gene recognizes as dangerous, it rejects the pathogen and effectively beats the wheat rust.

But Bowden says the major gene has its flaws. “Using the major gene makes the plant very resistant, but not very durable, meaning that it can work effectively one year, but be defeated by the pathogen the next year,” he says. “The pathogen can eventually mutate to camouflage itself and not be recognized, and that’s how the major gene is defeated,” he says.

However, researchers have not abandoned the idea altogether. Bowden says there are theoretical reasons to believe that instead of using one major gene, using multiple major genes in combination should create a long-lasting variety. This combination is referred to as a gene stack or pyramid.