The spores arrived from Kenya on dried, infected leaves ensconced in multiple layers of envelopes.

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

Nearly all of the plants were goners.

Crop scientists fear the Ug99 fungus could wipe out more than 80 percent of worldwide wheat crops as it spreads from its home base in eastern Africa. It has jumped the Red Sea and traveled as far as Iran. Experts say it is poised to enter the breadbasket of northern India and Pakistan, and the wind inevitably will carry it to Russia, China and even North America - if it doesn't hitch a ride with people first.

"It's a time bomb," said Jim Peterson, a professor of wheat breeding and genetics at Oregon State University in Corvallis. "It moves in the air. It can move in clothing on an airplane. We know it's going to be here. It's a matter of how long it's going to take."

Although most Americans have never heard of it, Ug99 - a type of fungus called stem rust because it produces reddish-brown flakes on plant stalks - is the No. 1 threat to the world's most widely grown crop.

World price spikes

The International Maize and Wheat Improvement Center in Mexico estimates that 19 percent of the world's wheat - which provides food for 1 billion people in Asia and Africa - is in imminent danger. American breeders say $10 billion worth of wheat would be destroyed if the fungus suddenly made its way to U.S. fields.

Fear that the fungus will cause widespread damage has caused prices to spike on world wheat markets. Famine has been averted thus far, but breeders say it's only a matter of time.

"A significant humanitarian crisis is inevitable," said Rick Ward, the coordinator of the Durable Rust Resistance in Wheat project at Cornell University. "We are at a stage where the equivalent of Hurricane Katrina is offshore and we are shoring up the dikes as fast as we can."

The solution is to develop new wheat varieties that are immune to Ug99. That's much easier said than done.

Race against time

After several years of feverish work, scientists have identified a mere half-dozen genes that are immediately useful for protecting wheat from Ug99. Incorporating them into crops using conventional breeding techniques is a nine-to-12-year process that has only just begun. And that process will have to be repeated for each of the thousands of wheat varieties specially adapted to a particular region and climate.

It's a race against time that pits Ug99 against the coordinated efforts of agricultural scientists from around the world.

More than 500 million acres of wheat are planted around the world, according to the U.N.'s Food and Agriculture Organization. "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."

Stem rust destroyed more than 20 percent of U.S. wheat crops several times between 1917 and 1935, and losses reached nearly 9 percent twice in the 1950s. The last major outbreak, in 1962, destroyed 5.2 percent 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.

As an added defense, a key rust-resistance gene - known as Sr31 - was imported from a rye plant and was therefore completely unfamiliar to the invading fungus.

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

A new race, or strain, of stem rust was discovered on a wheat farm in Uganda in 1999. Scientists published a report on the fungus the following year in the journal Plant Disease.

"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."

Researchers in Carson's lab confirmed that Ug99 - named for the country and year it was identified - had found a way to defeat Sr31. But scientists still saw little cause for concern.

Within a few years, the Ugandan stem rust had devastated farms in neighboring Kenya, where wheat is one of the biggest crops. Then it moved north to Ethiopia, Sudan and Yemen.

"There was a great deal of concern," Carson said. "Rust fungi are notorious for dispersing over very far distances in the wind."

Fungus is mutating

To make matters worse, the fungus is becoming more virulent as it spreads. At least one version of Ug99 can defeat all six stem-rust-resistance genes, and several variants can defeat groups of them.

Now, the pressure is on to develop wheat varieties that are impervious to Ug99. The first step is to identify Ug99-resistance genes by finding wheat plants that can withstand the deadly fungus.

Roughly 16,000 wheat varieties and other plants have been tested in the cereal disease lab over the last four years. The tests have been conducted between Dec. 1 and the end of February, when the Minnesota weather is so frigid that escaping spores would perish quickly, Carson said.

These and similar efforts at a research station in Kenya have turned up only a handful of promising resistance genes, which crop breeders are trying to import into vulnerable strains of wheat.