By Becky Beyers
For half a century, wheat-stem rust has been a low priority for the wheat community, thanks to CFANS-developed techniques for breeding rust-resistant wheat varieties.
But it turns out that stem rust—called “the shifty enemy” by pioneering University of Minnesota plant pathologist E.C. Stakman (’10–M.A., plant pathology)—is more persistent than almost anyone expected.
A new and particularly virulent strain—called UG99 because it was first noticed in Uganda in 1999—has scientists around the world and on the St. Paul campus racing to find the right combination of genes and traits that plant breeders can use to create new rust-resistant wheat varieties.
Seventy percent of the world’s wheat supply has no resistance to UG99, and the disease is spreading. If it follows the expected migration path, it soon will be into southwest Asia and could eventually migrate to North America. With wheat supplies already at historic lows, the threat to food supplies, especially in developing countries, is real.
UG99 worries pathologists and breeders so much because it has the unique ability to break through the “pyramid” of rust-resistant genes that has been used in wheat in North America and much of the wheat in developing countries—rather than a single resistant gene—that have protected wheat crops since the 1950s. Even worse, it’s already mutating again.
Stem rust destroys a wheat crop by damaging stem tissues so that nutrients can’t get from the leaves to the grain, which causes the grain head to shrivel and die. The telltale rust-colored spores are produced on the stem but spread via the wind and rain, often traveling long distances.
The disease has been around for centuries; the Romans even had a god—Rubigal—devoted to wheat rust; they celebrated the festival of Robigalia each April 25 to plead for a rust-free crop. In modern times, rust epidemics periodically ravaged wheat crops worldwide, in a boom-or-bust cycle of finding a resistant variety only to have the disease mutate and return to kill another harvest.
But after scientists discovered how to bundle the resistant genes into new varieties, the disease was under effective control. Many wheat growers, pathologists and breeders got complacent. Budgets were tightened, training programs reduced and research priorities shifted to other priorities.
“I saw what was happening, but was told that stem rust was no longer a problem,” says Norman Borlaug (’37–B.S., forestry; ’39–M.S., plant pathology; ’42–Ph.D., plant pathology), the Nobel Peace Prize laureate who helped pioneer the pyramiding techniques that brought disease resistance and higher yields to wheat crops around the world. “Sure enough, this tiger showed up in 1999.”
The “tiger” was first reported by a South African plant pathologist named Zack Pretorius—who coincidentally spent a sabbatical studying at the University of Minnesota in the early 1980s. At first, scientists assumed UG99 was simply a local threat in Africa, but as it spread, concern grew.
In 2004, Borlaug sounded the alarm, telling a group of scientists that UG99 posed the greatest threat to the world’s food supply in 50 years. That got the wheat community’s attention and led to the formation of the Global Rust Initiative, which partners with the U.S. Department of Agriculture’s Cereal Disease Lab on the St. Paul campus as well as with scientists in the CFANS agronomy and plant pathology departments.
Now, pathologists and plant breeders are working as fast as possible, but it’s a race against time. While some UG99 resistance has been documented in test plots, bringing a new variety onto the market takes several years.
Resistance can be different in seedlings and mature plants, so plants must be tested at varying ages, says Brian Steffenson, a professor in the Department of Plant Pathology and holder of the Lieberman-Okinow Endowed Chair of Cereal Disease Resistance. “We want to be able to give growers varieties that have adult resistance.”
Testing new wheat varieties and breeding lines for UG99 resistance consists of inoculating plants with the fungus and then assessing the damage. Then there’s the safety issue: Steffenson (’80–B.S., plant health technology) and Yue Jin, a plant pathologist who works for the USDA’s Agricultural Research Service in the Cereal Disease Lab, will travel this fall to Kenya, where at least 12,000 lines of accession from all over the world will be field-tested for resistance. Because the fungus is already in the field in Kenya, testing can be done in open fields without fear of spreading it.
Despite the dangers, lab tests can be conducted on the St. Paul campus because of the cold winter, when all potential plant hosts are dead or dormant. Even if the UG99 spores were to somehow escape the lab—a highly unlikely scenario—they wouldn’t find the living plant host they need to survive. The new Biosafety Level 3 lab on campus might someday host more UG99 research, but federal clearance would be required, Steffenson says.
Potential new varieties are being developed in part from plant samples Steffenson and other scientists have collected around the world. For example, one project involved collecting Sharon goatgrass and wild barley from the Middle East and testing them for resistance, then isolating the genes that could be transferred into cultivated wheat and barley. The results were promising, but a solution is still a long way off.
“There’s lot of work to be done; transferring genes is laborious, and in this case you have one species with very different habitats,” Steffenson says. Along with testing wheat, potential new varieties of barley—which also is highly susceptible to rust—must be developed, because it’s an important food crop in many of the countries where UG99 is likely to spread. Once resistant varieties are developed, building up enough seed stock for farmers can take several years.
International cooperation likely will play a role in the battle against UG99, and much of it likely will involve training.
Borlaug says neglect of training programs built in the 1950s and ’60s is partly to blame for the somewhat surprising rise of UG99. “We had reached a stage where scientists were trained to recognize [potential problems] and promising lines were made available to everyone, regardless of ideology. That came to an end when we didn’t continue training people.”
An international fellowship named after Borlaug aims to foster cooperation again. Kenyans Joseph Macharia and Godwin Macharia, a plant pathologist and plant breeder, respectively, spent six weeks in St. Paul this summer learning techniques from Steffenson, Jin and others as part of the Borlaug International Fellowships. Along with technical skills, they are building relationships with some of the top cereal disease scientists in the world, they said.
“If you’re working in cereal rust, you couldn’t come to a better place” to learn advanced techniques, Joseph Macharia says. “The net effect for us, along with what we’re learning, is that we’ll have much more confidence and credibility” with the research teams working back in their home country.
While so far UG99 has spread in eastern African countries, if it spreads to North America it poses a serious threat to the spring wheat region, including Minnesota and surrounding areas, Jin says. Most wheat grown here is spring wheat, which is among the least resistant to UG99, even though it’s highly resistant to other stem races. Because stem rust epidemics move south to north—and about 60 percent of wheat in the southern U.S. is also susceptible to UG99—stem rust inoculum could build up in the southern states, and threaten wheat production in the central and northern states.
Even more troubling: UG99 has a new mutation that can infect wheat lines that contain the gene sr24—one of the best sources for resistance to UG99 in current wheat cultivars in the United States and worldwide, Jin says. “If we do not use the right strategy in gene deployment for UG99 resistance, we could be headed into another boom/bust cycle.”
In Kenya, where that cycle has already returned and concern is rising about not having enough wheat, priorities already are shifting back toward saving one of the country’s most important cereal grains. “It was a big mistake to be complacent about this,” Joseph Macharia says. “If you turn your back on stem rust, it will rise up and stab you in the back.”