A laborious task
Now the pressure is on to develop new wheat varieties that are impervious to Ug99. Hundreds of varieties will need to be upgraded in the U.S. alone.
"You can't just breed it into one or two major varieties and expect to solve the problem," Peterson said. "You have to reinvent this wheel at almost a local level."
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 were conducted between Dec. 1 and the end of February, when the Minnesota weather is so frigid that escaping spores would quickly perish, 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 such as Brett Carver at Oklahoma State University in Stillwater are trying to import into vulnerable strains of wheat.
Each year, Carver crosses hundreds of plants in a greenhouse to produce as many as 50,000 candidate strains. Over the next four years, those are winnowed down, and the most promising 2,000 are planted in the field.
Only the hardiest strains are replanted each year, until the 12-year process results in a single new variety with dozens of valuable traits, such as the ability to withstand drought and make fluffy bread.
The oldest of the plants Carver bred for Ug99 resistance are only 3 years old, but one of the strains has been planted in the field already in case the fungus hitches a quick ride to the U.S. on an airplane or in a shipping container.
"In the absence of stem rust, it would not be the highest-yielding wheat," he said. "In the presence of stem rust, it would be the only thing that would survive."
karen.kaplan@latimes.com