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April 10, 2003

Pathologist: Airborne soybean fungus could take flight to U.S.

WEST LAFAYETTE, Ind. – A soybean disease native to Asia has crossed the Atlantic Ocean into South America, and only time will tell whether the fungus makes its way into the United States, said a Purdue University specialist.

Phakopsora pachyrhizi, commonly known as soybean rust, has decimated fields on the Asian and African continents, said Greg Shaner, a Purdue Extension plant pathologist. The disease is present in Argentina, Uruguay and now Brazil, and may be moving north.

Soybean rust is especially worrisome to plant researchers because the disease is caused by an airborne pathogen that can move hundreds of miles by wind; can be more devastating than soybean diseases common to the United States; and is difficult to control, Shaner said.

"Soybean rust has been in Asia for a long time," he said. "In the last few years it showed up in the southern portion of Africa, and two years ago it showed up in South America. Now that it's in the Western Hemisphere, we're concerned that it will find its way to the continental United States."

Plant pathologists aren't expecting soybean rust to appear in the United States this year, but based on its current track, the disease could spread to southern states within a few years. Researchers at the U.S. Department of Agriculture are screening soybean germplasm for genetic resistance in anticipation of the disease's arrival, Shaner said.

Researchers have yet to identify any highly resistant soybean germplasm, said Shaner, who has studied rust fungi resistance in wheat and oats.

Soybean rust attacks a soybean plant's foliage. Leaves drop early, harming pod setting and yield.

"The rust fungus produces small pustules," Shaner said. "Each little infection produces a mass of spores. The pathogen parasitizes the soybean plant, deriving nutrients for its growth from nutrients in the soybean plant. So nutrients that normally would go into seed production are diverted into the growth of the fungus."

Indiana and other Midwest soybean farmers familiar with deadly plant diseases and pests have experienced nothing like soybean rust, Shaner said.

"Soybean cyst nematode, the sudden death syndrome fungus, the Phytophthora root rot fungus – all those are soilborne," Shaner said. "Soybean rust has the potential of causing a much more widespread problem than those when conditions are favorable. If soybean rust becomes established it will infect fields over vast areas and could have a much greater impact because so many more fields might be affected in a given year.

"In field studies in Asia, Africa and South America, yield reductions of anywhere from 10 to 80 percent have been measured. The damage depends on how early in the growth of the soybean plant the infection occurs and how favorable the weather conditions are."

There are many parallels between soybean rust and foot-and-mouth disease, a fatal livestock ailment, Shaner said. Rust spores are thick-walled and pigmented, which protects them from natural controls, such as ultraviolet light. Spores are viable for weeks, even without a host plant. Also, spores can travel great distances with the help of wind.

"Soybean rust might also get into the U.S., inadvertently, through activities of people," Shaner said. "Someone who's in a soybean field in South America could come back here and bring it on their clothing. The one thing that somewhat reduces the risk of that happening is that our soybean seasons are opposite of South America's. The soybean crop is maturing now in South America, and we haven't planted ours yet."

The disease threatens not only soybeans but also numerous other leguminous plants, Shaner said.

Researchers hope to develop soybean varieties resistant to soybean rust so that farmers are prepared if and when the disease enters the United States. Currently, costly fungicide treatments represent the only option for containing soybean rust.

Additional information about soybean rust is available on the USDA's Animal and Plant Health Inspection Service soybean rust Web page.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Greg Shaner, (765) 494-4651, shaner@btny.purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web sites:
Purdue Department of Botany and Plant Pathology
Purdue Agronomy Extension Soybean Management


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