Working inside a bio-secure greenhouse outfitted with motion detectors and surveillance cameras, government scientists at the Cereal Disease Laboratory in St. Paul, Minn., suspended the fungal spores in a light mineral oil and sprayed them onto thousands of healthy wheat plants. After two weeks, the stalks were covered with deadly reddish blisters characteristic of the scourge known as Ug99.

Nearly all the plants were goners.

"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."