Wheat Diseases: A Race Against Nature
By Dennis Brown, Marketing and News Services
Crop disease is a fact of life for eastern Washington wheat growers. Diseases with curious names such as Strawbreaker foot rot, Cephalosporium stripe and Barley yellow dwarf can cause a significant reduction in yields and turn a good year into a disaster for farmers.
Ten to 15 wheat diseases occur somewhat regularly in the state, according to Tim Murray, a WSU plant pathologist, but only a handful are of concern year-in and year-out.
Natural Conditions for Disease
Fungal diseases are the most common, he said. The primary factors influencing their development are temperature and moisture. Variations among the state’s climates dictate where diseases are most likely to occur, according to Murray.
Stripe rust |
“The Cascades and west is a cool, wet climate. Once you get east of the Cascades, the rain shadow effect takes over,” he said. “In the Yakima and Hanford areas, it is warm and very dry, and as you move east, you pick up an inch of rainfall about every 10 to 15 miles. Average daytime temperatures become cooler as you move east from the Columbia Basin. That will determine which diseases will occur and where.
“Like I tell my class,” he pointed out, “the only rule in biology is that there is an exception to every rule. Our climate is variable enough that you can find most of these diseases in most of the producing areas in any given year, if you look for them.
“We’re mostly dealing with soil-borne pathogens in this part of the country because our climate is so arid,” Murray said. “By the time our climate warms up enough for leaf rust or stem rust to be a problem, we’re too dry.”
Stripe rust, which is caused by the Puccinia striiformis fungus, is the exception. The disease shows up as fine lines of orange pimple-like pustules arranged along the veins of wheat leaves. Unchecked, the disease can reduce yields by 30 percent to 60 percent on susceptible varieties. Growing a resistant variety and timely applications of fungicides can stem the outbreak.
“In mild winters,” Murray said, the fungus “will overwinter on wheat in parts of Washington or Oregon where it is mild enough for the pathogen to survive. Then rust spores move east on prevailing winds. In the spring when the temperatures are cool or when there are a lot of dewy nights, there’s enough moisture for the pathogen to develop.”
Stem rust |
In February, an international team of researchers, including Xianming Chen, a USDA Agricultural Research Service plant pathologist stationed at WSU, announced discovery of a gene in a wild wheat found in Israel that provides partial resistance to stripe rust. When combined with other genes, it is expected to provide sufficient protection in commercial wheat.
Some diseases, like Common bunt, have been reduced to a minor concern, thanks to advances in science. The disease, also known as stinking smut for its rotten fish odor, once plagued the region.
“This part of the Palouse is the smut capital of the world because we have conditions that are very favorable for that disease,” Murray said. “It is caused by a soil-borne fungus that infects the seedlings. It works its way up through the inside of the plant and infects the ovaries…and the developing seeds. It turns them into a black dusty material.”
Pathogens’ Progress
Snow mold |
“There is a discussion of the evolution of plant pathogens within the world of plant pathology,” Murray said. “Are they opportunistic? Were they here and not causing much damage on wild plants and then jumped to crop plants? Strawbreaker foot rot was discovered in the region not long after pioneers broke sod to plant wheat, so we know that one has been around since the beginning of wheat production in the area.”
“One thing we know is that when people have migrated around the world, they took with them their favorite plants and animals as well as the pests those plants and animals carried.”
Some fungal organisms are so widespread, they are considered a natural component of the soil, Murray said. “We only become aware of them when we change conditions that favor them.”
Plant Health Care
Murray said the first half of the 20th century was consumed with efforts to control the yield-cutting disease. “A lot of things happened after Wold War II when agricultural chemicals came into use and scientists discovered that a combination of seed treatments and disease resistance bred into wheat varieties were effective in controlling Common blunt.”
The cheapest and most desirable defense against disease is bred-in genetic resistance. Seed treatments, rotations that break disease cycles, and fungicides are other disease management options used by farmers.
But scientists can’t rest on their laurels, because pathogens evolve. “We are directing the evolution of the plant through plant breeding programs and fungicide applications,” Murray said. “The pathogen is doing it (evolving) on its own. It is responding to what we put out there.
“In biology, the driving force is reproduction,” he said. “If you can’t reproduce your species, you go extinct. It’s very much a natural process.”
UG99: A New Global Threat to Wheat Production
Tim Murray, a WSU plant pathologist, is collaborating with scientists around the world to address UG99, the latest threat to world wheat production.
UG99 is a virulent new race of stem rust found in research plots in Uganda in 1999 that has been able to overcome resistance genes bred into wheat varieties commonly planted around the world. Some estimates say that 80 percent of all wheat varieties planted in Asia and Africa are susceptible.
Ravi Singh, a wheat breeder with Centro Internacional de Mejoramiento de Maiz y Trigo(CIMMYT), provides an overview of the center’s wheat breeding programs during a tour of research plots at Obregón, Mexico. |
Since 1999, the disease has spread to Kenya, Ethiopia and the Sudan as well as to Yemen and now Iran. Prevailing winds are expected to carry the spores of the fungal disease to Pakistan and India and, eventually, the United States.
“It’s a concern in the United States because a large percentage of our varieties are susceptible,” Murray said. “The last major epidemic of stem rust in the United States was in 1954.”
Throughout history, stem rust has caused major famines around the world and major losses in grain production in the United States in 1903, 1905 and from 1950 to 1954. UG99 is of concern especially in the Midwest where the climate favors the disease. Stem rust is not a major concern in Washington because it requires summer rainfall, warm days and warm nights to thrive.
Murray is chairing a committee of land-grant university and government scientists devising a recovery plan, should UG99 be introduced to the United States. His committee is preparing a brief on UG99 for policymakers and is making plans to conduct surveillance for the disease. Surveillance will include planting and monitoring trap plots of vulnerable varieties around the United States. Based on the disease’s current locations, according to Murray, it should take about 10 years to reach the United States following natural pathways.
“The real challenge will be to know when it arrives, Murray said. “You can’t tell UG99 apart from any other stem rust by looking at it. They all look alike.”
Hundreds of scientists from around the world gathered at Obregón, Mexico, in March to discuss wheat health issues and the threat posed by UG99, a new strain of stem rust. |
Researchers already have identified sources of genetic resistance to UG99 and breeding lines with resistance are in development, but it will take several years of breeding to put seed with multi-gene resistance into the hands of farmers.
Also known as black rust, stem rust is caused by the fungus Puccinia striiformis, named for the rust-colored, pimple-like pustules that develop mainly on the stem of the plant and produce black spores. The disease can weaken stems, resulting in lodging (collapse of the plant) which causes significant losses in yield.
In 2005, Norman Borlaug, Nobel laureate and father of the Green Revolution, compared the dynamics of a rust epidemic to a forest fire.
“Once started, both are difficult to stop,” he said. “The prospect of a stem rust epidemic in wheat in Africa, Asia and the Americas is real and must be stopped before it causes untold damage and human suffering.”