Recent media reports have depicted stem rust as posing a renewed and significant threat to world wheat production, causing some producers to wonder how much they should be concerned.
The problem began in 1999, when a race of stem rust fungus was discovered in Uganda that caused disease on wheat varieties that had been resistant to stem rust for many years. Designated as Ug99, this race spread to Kenya in 2001, Ethiopia in 2003, Yemen in 2007 and most recently was reported in Iran.
“Losses due to stem rust in these countries have been significant since Ug99 was reported, but the greater concern is that this race will spread to the major wheat producing areas of India, China, Europe and the United States,” said Bob Hunger, Oklahoma State University Cooperative Extension wheat pathologist.
Estimates have been made that up to 80 percent of the wheat varieties grown in these areas are susceptible to Ug99. It makes quite a change from the historical level of concern for Oklahoma and other U.S. wheat-producing areas.
“Stem rust has caused yield losses in Oklahoma in only 16 years since 1918,” Hunger said. “The greatest loss was 4 percent in 1918.”
By comparison, leaf rust has been the rust that has most consistently caused yield losses in Oklahoma since 1918, usually in the 1 percent to 6 percent range. Stripe rust has only rarely caused losses in Oklahoma, with losses in the range of 1 percent to 5 percent being reported since 2000.
“Losses caused by stem rust have been more numerous and more severe in Texas and in states to the north of Oklahoma,” Hunger said. “Losses were greatest in North Dakota and Minnesota, where yield reductions in excess of 50 percent have been reported some years, going all the way back to 1918, when records on loss due to leaf, stripe and stem rust began to be kept in the United States.”
The reason why Oklahoma has been able to avoid severe yield losses from stem rust as compared to other states is a twofold answer. First, the stem rust fungus can overwinter in Texas, but does not typically overwinter in Oklahoma. As a result, stem rust that occurs in Oklahoma is dependent on spores blown northward from Texas in the spring.
Hunger said the second part of the answer is that wheat breeders historically have selected wheat varieties that mature early, a desirable trait that allows wheat in Oklahoma to mature before the weather begins to turn hot and dry and stem rust spores are blown into the state.
This “second part of the answer” is demonstrated by historical notes found by Brad Tipton, Canadian County Extension director and agricultural educator. The notes describe results from field trials in the 1940s conducted by Joseph Danne, an Oklahoma wheat breeder who developed the Triumph variety.
Danne’s notes indicated the typical time range of wheat maturity from heading to harvesting back then ranged from May 7-17 for heading and June 19-29 for harvesting.
“Heading nowadays occurs by mid April,” Tipton said. “Our earliest maturing varieties are quite likely a month earlier than those varieties popular in the days of Joseph Danne.”
Tipton, who has served Canadian County for 21 years, said he cannot remember a year that at least some of the wheat was not cut before Memorial Day, and harvest in most years had a solid start by the last week of May.
Since the stem rust fungus can overwinter in Texas, the Lone Star State can more easily experience bouts of severe stem rust than Oklahoma.
“The spores are present and ready to infect as soon as temperatures warm in the spring,” Hunger said. “Wheat in states north of Oklahoma matures later than what we typically experience, so that wheat is susceptible to spores blowing northward from Texas.”
Fortunately, wheat yield loss from stem rust throughout the central and northern Great Plains states has been minimal since the 1950s.
“Highly effective genes providing resistance to stem rust have been incorporated into wheat varieties by plant breeders,” Hunger said. “Another contributing factor has been a program conducted across the northern Great Plains that had the objective of eradicating barberry, a key host in the stem rust life cycle.”
Still, fears remain that Ug99 may eventually spread to the United States.
“Of even more concern is the fact that a race with virulence greater than Ug99 has been discovered in parts of Africa,” Hunger said. “This may further compromise the genetic resistance incorporated into existing wheat varieties.”
Wheat researchers are not waiting for Ug99 to come to them, but rather have initiated steps to address Ug99. Steps include determining the reaction of hard winter wheat varieties grown in the United States to Ug99, identifying sources of effective resistance and incorporating that resistance into breeding programs.
“Monitoring for stem rust in the United States and identifying the races present also is being conducted, and Cooperative Extension programs are being planned to help inform growers about this potential threat,” Hunger said.
Plant pathologists and breeders from many land-grant universities throughout the United States are cooperating.
"This is an example of the land-grant system at work, where county, district, state, regional and national expertise can be brought to bear on solving concerns and issues of importance to local producers and communities," Hunger said.
An emergency program to breed Ug99-resistant wheat was funded by the Bill and Melinda Gates Foundation in April 2008.
“As a result of that funding, Ravi Singh, head scientist at the Center for Improvement of Corn and Wheat (CIMMYT) in Mexico, says they have found a complex of genes that should be harder for the fungus to evade,” Hunger said.
Still, it will take two to three years to generate enough seed for countries at risk, so it is now a race against time, wind and spores. Also, the CIMMYT wheat is a spring rather than winter variety.
“We in the division’s Oklahoma Cooperative Extension Service are going to be out and about the state, in the media and working through our county office system to keep wheat producers abreast of the stem rust issue,” Hunger said. “Right now, we’re OK, but we’re definitely keeping an eye on the situation.”