Asian soybean rust isn't the only rust to threaten U.S. crop fields. Wheat farmers face their own corrosive crop disease called stripe rust.
It has devastated high yielding, yet susceptible wheat varieties in recent years. According to the USDA-Agricultural Research Service, yield losses of 40 percent are common, and some fields are completely destroyed.
“You can tolerate leaf rust, but not stripe rust,” says University of Georgia Small Grains Breeder Jerry Johnson.
During the 2005-06 growing season, overall disease pressure on wheat was light with the exception of stripe rust, reports Georgia Extension agronomist Dewey Lee.
Johnson says stripe rust is a relatively new problem for growers in the Southeast. “Historically, stripe rust has been a problem in the Pacific Northwest and California,” he says.
“Eight to 10 years ago, it moved to Louisiana, east Tennessee and Arkansas. Then, two to three years ago, it moved here. This disease is just getting to the East Coast. There has been less stripe rust in the Carolinas than in Georgia and Arkansas.
“Many varieties are susceptible, including some of our highest yielding varieties,” adds Johnson.
In his own variety trials, those with resistance to the rust yielded 100 bushels or more per acre. By comparison, AGS 2000, a previously popular but susceptible high yielding wheat variety in the Southeast, yielded only 87 bushels per acre.
Johnson says the Roberts wheat variety, one of his most recent releases, is very susceptible to stripe rust. The rust has forced Johnson and other breeders to discard a number of promising yet susceptible breeding lines.
“You can control stripe rust with a fungicide,” says University of Florida Small Grain Breeder Ron Barnett, “but you have to spray early. AGS 2000, for example, will yield 100 bushels per acre if you use a fungicide.” He says fungicides such as Quadris, Tilt and Headline will control the rust.
Early and timely fungicide application is required because the rust desiccates plants quickly. “Fungicides are expensive on a marginal crop such as wheat,” adds Barnett. “So even if you have to pay more for seed, it makes more sense to use a resistant variety and save the $15 to $20 per acre fungicide cost.”
University of Georgia Plant Pathologist John D. Youmans says stripe rust is a challenge to control because every year is not severe for stripe rust infection. He notes that most of the new wheat lines released by the Mexico-based international wheat breeding program known as CIMMYT are showing good resistance to stripe rust.
Youmans says stripe rust blows into the United States from central mountains in Mexico. Once it gets into Texas, winds carry the disease north into Oklahoma and Kansas and east into Arkansas and the Southeast.
In Nebraska, for example, fungicides are recommended if stripe rust outbreaks are severe in Oklahoma and Kansas.
Nebraska Extension recommends fungicides for control if the yield potential of dryland wheat is at least 45 bushels per acre, and the projected irrigated yield is at least 75 bushels per acre.
“At first, stripe rust was a novelty,” says Youmans. “But starting in 2001, we had stripe rust every other year. Some 90 percent of the lines had stripe rust, and there are only a few genes for resistance.”
This past growing season, he saw the rust appear for the first time on the awns of wheat plants. He worries that some farms may get both stripe rust and leaf rust. “If that happens, growers will have, by law, only one chance to apply a fungicide,” he adds. “So selecting a resistant variety will be critical.”
As bad as stripe is, a new virulent strain of stem rust could be even worse if it ever gets into this country. It's called Ug99 because it was first discovered in Uganda in 1999 and is now centered in Kenya in east central Africa.
University of Georgia Small Grains Breeder Jerry Johnson says the new stem rust appears to be spreading to North Africa, Egypt and Asia.
“We have identified resistance to the new stem rust,” says Johnson, “but if it hits in the United States, it would be bad because most varieties here are susceptible.” In fact, according to the USDA-ARS, most of the world's wheat is vulnerable.
Johnson notes that resistance is being screened at a facility in Minnesota. Meanwhile, U.S. wheat breeders are scrambling to incorporate the resistance into new varieties before the new stem rust strain finds its way into this country. Field tests of U.S. wheat varieties and breeding lines, including those with identified resistance to Ug99, are now underway in Kenya.