Fighting a crop disease is a lot like fighting a human disease. Each year a new flu vaccine targets the most recent strain of the flu virus. Plant breeds try to target the latest mutation of a plant disease.
The difference with wheat is it takes 10 years to develop a new resistant plant variety.
“You get something like UG-99 and everybody is interested, so that's where they pour the money. But that's not good policy.”Alan Roelfs
Farmers in Africa were confident they were growing a kind of wheat resistant to stem rust. But then the rust fungus mutated, and everything changed.
"Essentially the one gene was protecting them and when a culture came along that attacked that one gene, that's all it took," says Alan Roelfs. Roelfs says the same thing could happen in the United States.
Few people know more about stem rust than Alan Roelfs. The retired USDA scientist who's considered an international expert on wheat rust started his research in 1965 at the University of Minnesota, and is now a consultant on the UG-99 epidemic.
Roelfs says most of the wheat now growing in Minnesota and the Dakotas is vulnerable to stem rust.
For the past decade or more researchers have focused on breeding wheat plants resistant to fusarium head blight, or scab, a disease that cost wheat farmers millions of dollars in the 1990s. "In the case of scab resistance, a lot of it has come from China, and those are super susceptible to stem rust. So when you cross with them your progeny it is much more susceptible than anything we grew in Minnesota over the last 50 or 60 years," explains Roelfs.
That doesn't mean an epidemic of wheat stem rust is inevitable, but it increases the odds.
Alan Roelfs says epidemics are hard to predict until they've started, and an epidemic requires a lot of things to happen at the right time.
If a new variation of wheat rust emerges, and if conditions are just right, and if the fungus catches favorable winds, it can spread quickly.
"So there's a lot of ifs you see," says Roelfs. "You can go 10 or 20 years even with a susceptible variety and get away with it sometimes. And then it hits you, and then you remember. But not many people here remember '53 or '54."
1953 and 1954 were the last big wheat stem rust epidemics in the Midwest. Those years, the disease wiped out half of the wheat crop in Minnesota and North Dakota.
Roelfs says those epidemics happened at a time when many people thought wheat rust was defeated.
After widespread epidemics devastated wheat crops in the early 1900s, there was a major eradication program for the barberry bush which harbors the rust fungus. Several new wheat varieties were bred to be resistant to stem rust.
But as the epidemics faded from memory, so did the focus on stem rust. Fewer new varieties of resistant wheat were developed.
"What happens is, you get a good one and everybody wants to cross with it. Every state makes selections with it and you get the same genotype strung out over a large area," says Roelfs.
To some extent, that's the situation now, according to Roelfs. The barberry eradication program ended in the 1980s.
Most Midwestern wheat varieties have the same gene for stem rust resistance. If a mutation gets around that one-gene defense, the wheat crop's in trouble.
At the Northern Crop Science Laboratory in Fargo, geneticist Steven Xu is looking for new wheat varieties resistant to the UG-99 fungus.
He's part of an international effort funded by the Bill and Melinda Gates Foundation and he recently got some encouraging news from the USDA Cereal Disease Research Lab in St. Paul, the only lab in the United States allowed to inoculate plants with UG-99.
"I tested a whole bunch of the lines, or collections. (They are) ninety percent resistant. Highly resistant to all the North American races, and UG-99 and the new variant of UG-99," says Xu.
In the greenhouse on the North Dakota State University campus where Xu does much of his work, the new genetic lines are each represented by a few stalks growing in a small flower pot. Most look more like a wild grass than a domestic wheat plant, and that's the challenge.
These plants are resistant to stem rust, but they're still mostly a grass with a few small seeds, far from the long plump heads of wheat harvested by farmers.
It will be years, probably a decade, and dozens of cross pollinations before the offspring of any of these plants make it into a wheat field in Africa, or Minnesota.
Stem rust expert Alan Roelfs says protection against crop disease epidemics requires a focus in two areas.
Careful attention every year by scientists and farmers to spot new variants of the disease and constant breeding of new crop varieties to keep the gene pool from getting stale.
But Roelfs says without enough money for research, science tends to focus on the current disease crisis and that sometimes opens the door for an old disease to make a comeback.
"Plant pathologists are like everybody else. If the money is on scab, everybody wants to work on scab. Who wants to work where there's no money? So you get something like UG-99 and everybody is interested so that's where they pour the money. But that's not good policy," says Roelfs.