The Ug99 fungus, called stem rust, could wipe out more than 80% of the world's wheat as it spreads from Africa, scientists fear. The race is on to breed resistant plants before it reaches the U.S. The spores arrived from Kenya on dried, infected leaves ensconced in 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, 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...