<b>By DOUG SCHMITZ<br>
Iowa Correspondent</b> </p><p>

AMES, Iowa — According to a leading Iowa State University (ISU) plant pathologist, overwintering may be the key to knowing whether or not the infectious plant disease would pose a major threat to Iowa’s soybean crops this year.<br>
“Because soybean rust has to overwinter on living tissue, it will not overwinter in Iowa and the pathogen will need to retrace its steps to get back to Iowa next year,” said Daren Mueller, an ISU plant pathologist, who leads ISU’s soybean rust extension efforts, including the Soybean Rust Fast Track System, and the Corn and Soybean Initiative.<br>
Although 336 confirmed cases of Asian soybean rust were reported in 19 states in the U.S. last year, Mueller said the Hawkeye State wouldn’t be any more susceptible to soybean rust in 2008 than it was with the 2007 discoveries.<br>
“We will continue to look at the potential overwintering sites in Central America and how this affects infection in Louisiana and Texas each spring,” he added. “This may be the key for increased risk of rust getting to Iowa – but we will need more years of data to see how this plays out.”<br>
Iowa’s first discovery of soybean rust in the state’s history was on March 13 of last year in a Mahaska County leaf bin, which came from the 2006 soybean crop. While ISU and USDA experts said one leaf in the bin from a low-producing field confirmed rust pustules and spores, no other evidence of rust was found anywhere in the area.<br>
But the state’s first confirmed case of soybean rust was found in Dallas County on Sept. 28, 2007.<br>
“Soybean rust is very difficult in the field when at low incidence and severity, which is what the level was in most of the finds in Iowa,” Mueller said. “Positive identification only was confirmed after incubating the samples for at least 24 hours and looking for pustules with a microscope.<br>
“This is different than other foliar diseases and our original thinking, when we thought that we could see the pustules in the field if we used a hand lens,” he added. “If the disease is at levels high enough to spot in the field, it may be too late to get maximum efficacy from fungicide applications.”<br>
Currently, Mueller said, ISU’s XB Yang is one of only a few U.S. epidemiologists conducting research to predict the risk of soybean rust nationwide, posting the results on a secure USDA site for extension specialists to help determine the risk for their respective states.<br>
“XB Yang is one of the contributing epidemiologists for the national effort and continues to specifically look at the risk of rust getting to Iowa,” he said. “In addition, we continue to work closely with plant pathologists across the U.S. in scouting sentinel plots and posting the data on the USDA website (www.sbrusa.net).”<br>
In a recent meeting, Mueller said he and his staff were reconsidering how exactly they’ll find the early infections in Iowa, which will include gathering more soybean rust samples.<br>
“We will probably propose that growers and agribusiness personnel will need to collect leaf samples (minimum of 100 leaves) from suspect areas when the risk of rust is “medium” to “high” and use their First Detectors (from the Iowa Soybean Rust Team) to get the samples to us as soon as possible,” he said.<br>
“The risk of rust was “medium” near the middle of August and was not “high” until the last week of August 2007,” he added. “I am curious to see how this conversation turns out.”<br>
Virgil Schmitt, ISU field agronomist in Muscatine, said Iowa’s vulnerability in 2008 now depends on where soybean rust survives during the winter.<br>
“It must have green tissue from a host plant to survive, so for all practical purposes, it cannot survive north of the frost line this winter,” he said. “So the question is, “Where is it surviving south of the frost line?”<br>
Schmitt said if soybean rust is surviving only in Florida, Iowa is no more at risk than in the past; however, if it is surviving and building up someplace in Texas or Mexico, it will be much better positioned for a move into the Midwest than it would be moving from Florida.<br>
As of last month, soybean rust was still present in Hidalgo County, Texas; Jefferson and Leon counties in Florida; Mobile County in Alabama; Bolivar County in Mississippi; and Liberty and Polk counties in Texas, according to www.stopsoybeanrust.com<br>
Schmitt said, “In January, it was found in Mexico, Texas, Mississippi and far eastern Alabama. This is worrisome.”