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Corn+Soybean Digest

They Found Soybean Rust In His Field

After decades of growing cotton and soybeans in Warren County, MS, nothing surprises Newell Simrall anymore. That may explain his nonchalant reaction to being told that Asian soybean rust had found its way into one of his soybean fields.

“Asian rust may be a blessing in disguise because we probably should have been using a fungicide on soybeans all along,” says Simrall.

Until November 2004, the U.S. was the only major soybean-producing region of the world that was free of Asian soybean rust. USDA's Animal Plant Health Inspection Service has since confirmed the aggressive fungal disease in several Southern and Southeastern states.

Simrall had long since finished harvesting his 2004 soybean crop in late November when a trio of Mississippi State University researchers — Alan Blaine, Billy Moore and Jim Hamer — made a stop at his farm as part of a fact-finding mission they were conducting across the state.

They were inspecting any and all soybean plants they could find in search of evidence that soybean rust had made its way to the area.

It turns out that two rows of soybean plants were still growing along a ditch bank on Simrall's farm. The soybeans, which were green and left uncut during harvest, had been seeded accidentally when the tractor driver turned around at the field's edge during planting.

“We received 19.3 in. of rainfall in June 2004, and the river was up substantially. Flood waters delayed planting in some fields, and in this field were some of the last Group V soybeans we planted on June 1,” he says.

Blaine, Moore and Hamer examined the soybeans and, after further testing, positively confirmed the presence of Asian soybean rust.

“I was hoping we wouldn't have rust in the U.S.,” Simrall says. “It's kind of scary, but I believe it's something we can handle with the fungicides that are available.”

His plan of action for the 2005 crop year is still uncertain, but he's leaning toward taking a preventative approach.

“We wouldn't necessarily have to spray if we could learn how to identify the disease,” says Simrall. “Unfortunately, rust is similar in appearance to other more common diseases in soybeans. I believe it will be difficult for farmers to detect whether or not the disease is present in their fields.”

Mississippi soybean specialist Alan Blaine agrees that rust is easy to confuse with other diseases, including bacterial blight. In addition, he says the disease has the potential under the right environmental conditions to completely defoliate a crop within two weeks.

The fast-moving consequences of the disease, Blaine says, may convince some soybean growers to make a preventative fungicide application in 2005.

Simrall sees another advantage to the preventative approach. “In all of the on-farm, large-scale research tests conducted by Mississippi State University scientists since 1997, yields have been increased by at least 5.9 bu./acre with one fungicide application. If you can get a 6 bu./acre increase at $5/bu., and you spend $20 on fungicides, you're picking up $10/acre,” he says.

Interested in realizing that economic advantage and believing the disease will be found in more fields next year, Simrall is seriously considering making an automatic fungicide treatment 50 days after the emergence of his soybeans.

At grower meetings this winter, researchers stressed that production factors don't affect the presence of soybean rust. According to Gabe Sciumbato, plant pathologist at Delta Research and Extension Center in Stoneville, MS, row spacing and seeding rate have no effect on the airborne disease spores, which can survive up to 30 days and can travel several hundred miles.

However, planting date and maturity group could have some effect on the disease, plant pathologists say.

Asian soybean rust thrives when the temperature is between 69° and 89°, and the humidity level is 80% or greater. “If the temperature rises above 95° and humidity levels are low, the fungus will not thrive. Then again, the soybean plants also will not thrive,” says area Extension agent Tommy Baird, Indianola, MS.

“Early plantings of early maturing varieties would benefit because of the temperature range at first bloom,” he says. “To control the disease, plant pathologists recommend the soybean plant receive its first fungicide treatment at the R1 stage of development or at first bloom.

That stage of development will come quicker on the early maturing varieties, which could possibly mean you wouldn't have to treat a second time.”

Simrall isn't taking any chances. He believes his later-than-normal planting date was at least partly to blame for rust finding its way into his field in late November. He's also determined now to plant only Group IV maturity soybeans.

“We must plant Group IV soybeans in late March or early April to dodge this bullet,” he says. “I think a man is fooling himself if he plants late. A late Group V variety already will yield less. And now, with rust, you're really taking a chance.”

It's not known yet whether or not rust will overwinter in the U.S.

“It may be in our case the host plant survives the winter, allowing rust to overwinter in southern states, including Mississippi. We really don't know,” says Sciumbato. “If it were to be killed by the cold, there would still be the chance the disease would blow back into our region through the wind.”

Whether or not soybean growers find rust in their fields next year may depend both on the disease's ability to overwinter in the U.S., and the ability of researchers to track movement of the airborne disease spores.

Researchers will plant sentinel crops along the Gulf Coast to track movement of the disease, and will alert growers if the southern-most crops are infected with rust.

If the early planted soybeans are able to make it past first bloom stage, and the disease hasn't yet been found on the sentinel crops in the Deep South, growers may be able to avoid any additional fungicide applications, plant pathologists say.

“Even if we get a hard freeze on the coast and it kills all of the kudzu, which is a primary host, we may still have to worry about more cold-hardy host plants such as clover,” says Baird.

“A good hard freeze will kill kudzu host plants, but I don't believe it will get cold enough in places like Florida to kill it. And even if it did, the wind currents would bring the disease back here rather quickly,” Simrall says. “There always will be obstacles in farming, whether it's a new disease like Asian soybean rust or some new zoo-zoo bug. It's all part of farming.”

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