In just two days last fall, Asian soybean rust traveled 500 miles. It was the second time that it's traveled in such an explosive fashion, says Don Hershman, University of Kentucky plant pathologist. “This means that northern growers may not have much notice before rust arrives there.”
The number of counties with rust doubled from 2005 to 2006, many of them in the Ohio and Mississippi River valleys. Losses remained low due to its arrival after harvest, timely scouting and spraying.
“Last November rust spores traveled from Louisiana to Indiana,” says Greg Shaner, professor of botany and plant pathology and Extension specialist at Purdue University. “There had been no evidence that it was creeping north, and then in two days it hit all over the Mississippi River Valley.”
Rust was verified on Iowa soybean tissue recovered from a bin of 2006 Iowa soybeans. The finding drives home that rust spores did indeed land in Iowa, but does not mean that Iowa will have Asian soybean rust in the 2007 soybean crop. “This single case will have no impact at all on the 2007 growing season,” says Palle Pedersen, Iowa State University (ISU) Extension soybean agronomist.
Since Asian soybean rust first struck the U.S. in November 2004, plant pathologists have attacked this exotic disease with a vengeance, monitoring 753 sentinel plots and overwintering sites.
The United Soybean Board (USB) and soybean checkoff have partnered with the North Central Soybean Research Program, as well as government agencies, to again fund soybean sentinel plots across the country in 2007.
“I tell my audiences that the sentinel plot program is their biggest return on their checkoff investment,” says Anne Dorrance, associate professor of plant pathology, Ohio State University.
“The rapid spread of rust spores these last two years taught us that we particularly need to watch the Mississippi River Valley as a conduit to the Midwest,” Shaner says. “And we learned that it may not take a whole lot of rust in the South to provide sufficient inoculum to cause disease in the North. Of great concern to us this spring will be how early rust is found in Mississippi, Louisiana and Texas. “These areas are the most likely source of spores for the soybean belt.
“We've had so little experience with rust that we don't have a handle on what our risk is, one year out of five, 10 or whatever,” Shaner adds.
“We've been better prepared for rust than any other major plant disease, and the Internet gets the information out. Because rust in the Corn Belt does not survive the winter, every year is totally new, depending upon what's going on in the South and what happens locally,” Shaner says.
Unfortunately, rust does survive the winter in the South, much of it on kudzu, a wild legume. The percentage of kudzu with rust has more than tripled from 11% in 2005 to 40% in 2006, says Jim Marois, University of Florida professor of plant pathology. “Rust almost always overwinters south of Tampa,” he says.
Scientists suspect that up to 11 additional legumes might also be vulnerable to rust and therefore spread the disease, says Glen Hartman, plant pathologist at the University of Illinois and the USDA-ARS National Soybean Rust Research Center. We call it Asian soybean rust, but it's actually a legume rust.
As of late February, “Kudzu in south- central Florida was telling us it's been darned cold,” says Carrie Lapaire Harmon, University of Florida Extension plant pathologist and assistant director of the Southern Plant Diagnostic Network. The prospect of fewer spores overwintering is good news, but can also provide a false sense of security, given the disease's power to knock yields by up to 50%, as demonstrated in South America.
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Rust Readiness Plan
Soybean rust experts recommend the following plan of action to be ready for rust this year:
Stay informed. Know how much rust inoculum survives the winter in the South. The most likely sources of rust spores for the Midwest are Mississippi, Louisiana, Texas and Mexico, due to weather patterns. The official source of current rust outlooks and updates is the USDA site, www.sbrusa.net. “This is where state rust specialists post their commentaries, so it's a great site to get a lot of information in a short time,” says Don Hershman, University of Kentucky plant pathologist. It also carries results from the sentinel plot system, a vital monitor of rust inoculum dispersion. Your state Extension office is the best source of local rust advice.
If there is a storm that could carry spores into your area, scout the lower portion of your plants' canopy with a 20X lens and know what you are looking for. “The trouble is, it's pretty tricky to spot at a low level, so inexperienced scouters may see rust and not know it,” says Greg Shaner, professor of botany and plant pathology and Extension specialist at Purdue University. “For that reason, sentinel plots are very important; they are scouted by knowledgeable people who get the word out in time.”
Maintain a solid relationship with your local fungicide dealer/applicator. “Keep up on the latest information on management of soybean rust with fungicides and watch the Web sites to see where soybean rust is developing,” advises Dean Malvick, assistant professor of plant pathology and Extension specialist, University of Minnesota.
“Remember that rust may not slowly creep toward you, providing ample warning,” Hershman says.
“Have your ear to the ground and have a plan, don't wait until rust is in your field,” says Carrie Lapaire Harmon, University of Florida Extension plant pathologist. “You don't need to purchase new equipment or buy fungicides right now, but do have a plan. That applies to anything in life.”
A New Theory
Last year, Asian soybean rust spores moved a great distance, but the disease did not. The fungus itself may have something to do with it. Rust spores were detected many times in each state of the northern soybean-growing region. Why then, was the disease not found there?
A University of Illinois study found that rust spores cannot survive more than two consecutive days of sunlight. The theory is that if spores were exposed to more than two days of a cloudless sky, they would be too weakened to cause disease, reports X.B. Yang, Iowa State University professor of plant pathology. A preliminary study suggests that rust spores may be sensitive to light, a factor previously unnoticed. Shaded rust spores resulted in a 95% rust infection, compared to only a 25% infection rate in unshaded spores.
This might explain why in South America the disease has always been found in lower portions of the canopy, despite the upper leaves being more vulnerable due to more dew and greater spore deposition.
Perhaps the cloud cover of rainy weather is what creates the connection between rust outbreaks and rainy weather. “If future research confirms these findings, the Midwest's past two summers' dry weather may have prevented the broad dispersal of spores from turning into a dangerous outbreak.
Last year, growers had six fungicides registered for use against rust, and another 11 had provisional Section 18 approvals in selected states.
This year's final list of approved fungicides are available on the EPA Web site later this spring: www.epa.gov/oppfead1/cb/csb_page/updates/soybean_rust.htm. Additional state Section 18 labels are available on state departments of agriculture Web sites.
One new rust fungicide in 2007 is Topguard, from Cheminova. With a triazole called flutriafol as its active ingredient, “it showed long residual activity in our University of Florida trials,” says Jim Marois, University of Florida professor of plant pathology. Topguard has Section 18 approvals in South Dakota and Minnesota.