A 'time bomb' for world wheat crop

"All the seed needs to change in the next few years," said Ronnie Coffman, a plant breeder who heads the Durable Rust Resistance in Wheat project. "It's really an enormous undertaking."

Ancient adversary

Farmers have been battling stem rust for as long as they have grown wheat. The fungus' ancestors infected wild grasses for millions of years before people began cultivating them for food, said Jorge Dubcovsky, professor of genetics and plant breeding at UC Davis.

"The pathogen keeps mutating and evolving," he said. "It's one of our biblical pests. This is not a small enemy."

When a spore lands on a green wheat plant, it forms a pustule that invades the outer layers of the stalk. The pustule hijacks the plant's water and nutrients and diverts them to produce new rust spores instead of grain. Within two weeks of an initial attack, there can be millions of pustules in a 2.5-acre patch of land.

Wheat plants that can recognize a specific chemical produced by stem rust can mount a defense against the fungus. But the rust is able to mutate, evade the plant's immune system and resume its spread.

Stem rust destroyed more than 20% of U.S. wheat crops several times between 1917 and 1935, and losses reached nearly 9% twice in the 1950s. The last major outbreak, in 1962, destroyed 5.2% of the U.S. crop, according to Peterson, who chairs the National Wheat Improvement Committee.

The fungus was kept at bay for years by breeders who slowly and methodically incorporated different combinations of six major stem rust resistance genes into various varieties of wheat. The breeders thought it unlikely that the rust could overcome clusters of those genes at the same time.

After several outbreak-free decades, it seemed that stem rust had been defeated for good. Scientists switched to other topics, and the hunt for new resistance genes practically slowed to a crawl.

A new strain of stem rust was identified on a wheat farm in Uganda in 1999.

"It didn't draw a lot of attention, frankly," said Marty Carson, research leader at the Cereal Disease Laboratory, part of the U.S. Department of Agriculture. "There's very little wheat grown in Uganda."

East Africa is a natural hot spot for stem rust. Weather conditions allow farmers to grow wheat year-round, so rust spores can always find a susceptible host. Some of the wheat is grown as high as 7,000 feet above sea level, where intense solar radiation helps the fungus mutate.