The highlands are also home to barberry bushes, the only plant on which stem rust is known to reproduce through sexual recombination. That genetic shuffling provides a golden opportunity for the fungus to evolve into a deadly strain.
Within a few years, Ug99 -- named for the country and year it was identified -- had devastated farms in neighboring Kenya, where much of the wheat is grown on large-scale farms that have so far been able to absorb the blow. Then it moved north to Ethiopia, Sudan and Yemen, putting more small farms at risk. Those that can afford it are trying to make do with fungicides, but that's too cumbersome and expensive to be a long-term solution, Ward said.
To make matters worse, the fungus is becoming more virulent as it spreads. Scientists discovered a Ug99 variant in 2006 that can defeat Sr24, a resistance gene that protects Great Plains wheat.
Last year, another variant was found with immunity to Sr36, a gene that safeguards Eastern wheat.
Should those variants make their way to U.S. fields any time soon, scientists would be hard-pressed to protect American wheat crops.
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."
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karen.kaplan@latimes.com
