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Corn+Soybean Digest

More Spores?

One plant pathologist likens Asian soybean rust to other potentially devastating crop diseases and says the threat remains, despite recent history.

Until November 2004, the U.S. was the only major soybean-producing region of the world free of Asian soybean rust. USDA's Animal Plant Health Inspection Service has since confirmed the aggressive fungal disease across much of the country's soybean-producing region.

As of early December 2006, Asian soybean rust had been found infecting soybeans in 231 counties in 15 states, including Alabama, Arkansas, Florida, Georgia, Illinois, Kentucky, Louisiana, Missouri, Mississippi, North Carolina, South Carolina, Tennessee, Texas and Virginia. Including reports on kudzu, there were a total of 270 counties in 15 states with rust this past year.

Weather conditions in Mississippi this past summer were hot and dry, the exact opposite of conditions that are conducive to rust development, says Billy Moore with Mississippi State University. One case of rust on soybeans was found July 2006 in Jefferson County, in south Mississippi, he says.

“It was a 4-ft. by 4-ft. area in a large field, and we were lucky to find it. We put an application of fungicide on the next morning and four weeks later it hadn't progressed past that point.”

Moore doesn't expect a devastating rust situation but once every eight to 10 years. “Rust could cause a problem if everything comes together just right. In all my years I haven't seen everything come together just right to promote severe disease development except for one time, and that involved corn blight. That was in 1970, and about $1 billion was lost in U.S. corn yields,” he says.

Early plantings of early maturing soybean varieties may be less likely to be infected with Asian soybean rust, he says.

“The temperature at that time is very much in favor of rust development, but the lower level of inoculum blown in at that time reduces the chances of rust infection,” he says. “If one spore lands in a soybean field, you have nine to 10 days before it begins producing additional spores. You may go through four generations before you have rust spores capable of moving elsewhere and spreading the disease.”

Rust also was found at kudzu sites across Mississippi, says Moore. One such area was treated by the Highway Department, causing the leaves to drop and destroying the inoculum. Conditions were not favorable enough for the disease to move from the kudzu to soybean fields.

The susceptibility of kudzu to Asian soybean rust depends on the biotype of the kudzu. “There is a great deal of variability in our kudzu, so we're collecting kudzu seed to determine which biotypes are most susceptible to rust,” he says.

Asian soybean rust thrives when the temperature is between 69 and 89F, and the humidity level is 80% or greater. In overwintering sites in Florida, there was very little development of the disease in 2006, says Moore, and very little inoculum was produced that could spread spores. Rust is known to overwinter in south Florida and Mexico.

Last September was an active month for Asian soybean rust in Georgia, according to Extension specialists there. At the time, most of the soybean crop was in the R4 stage and moving to the R5 stage. If the crop has reached the R5 growth stage, it's still technically at risk to soybean rust. However, given the slow spread of the disease up to that point, specialists reasoned that it was unlikely soybean rust would severely affect the crop before it reached R6.

Georgia growers who were concerned and wanted maximum insurance against the disease were advised to follow previous spray recommendations. However, there is no general spray recommendation for soybeans that already have reached the R5 stage and where rust is not yet found.

Growers who did not expect to make at least 35-40 bu./acre were asked to carefully consider whether or not there was an economic benefit to applying fungicides to their crop at that point in the season.

Spore trap counts in 2005 documented the presence of rust in North Dakota and Minnesota. Then, in 2006, rust spores were found in Iowa and several other Midwestern states. However, the disease failed to infect any grower fields.

Daren Mueller, plant pathologist at Iowa State University, believes there are sufficient explanations about why rust hasn't arrived in the Midwest, including abnormally dry conditions in certain areas of the Southeast. But, he says, “We're not going to be able to rely on abnormally dry conditions every year to slow the disease. We are going to remain vigilant.”

According to Mueller, different studies in the U.S. and abroad have evaluated the viability of rust spores after they have traveled long distances. “While spores seem to be able to travel fairly long distances on wind currents, they won't be viable if they are exposed to too much light. That said, there always could be an exception where spores quickly move north in very heavy cloud cover and remain viable,” he says.

Mueller recommends that growers take the necessary time to remain current on the weather and to be aware of the disease tracking data available to them. “At minimum, check the Web sites available in the middle of March to see how well the pathogen overwintered. Then, sometime in spring, check again to see if the disease has spread and how far it has moved from those overwintering areas. That will give you an idea of how much inoculum is present to move into the Midwest,” he says.

Monitoring programs are in place to help growers deal with the threat of soybean rust, says Mississippi's Moore. A recommended spraying program for soybeans will be triggered if rust is identified in sentinel plots and in SMART program fields, he says.

The disease is being monitored throughout the soybean production areas of the U.S., he adds. “It's still a question of whether or not the spores are viable when they move all the way to the Midwest. Climatic conditions must favor the development of the disease in any areas north of Florida or north of Mexico. A very mild winter could present problems for the Midsouth and Southeast because inoculum could move closer to us, but that's already out of the picture this year because we've had freezing temperatures all the way to the Gulf Coast,” he says.

Technology helps to monitor those areas where inoculum is most likely to be blown in, based on weather, atmospheric data and air currents, says Moore. “We look all over, but we can best concentrate on those areas that are most likely to be infected. Sentinel plots along the Gulf Coast are absolutely essential to monitor what's going on with rust. We'll continue to plant sentinel crops along the Gulf Coast to track movement of the disease, and we'll alert growers if the Southern-most crops are found to be infected with Asian soybean rust,” he says.

Researchers can track the movement of the airborne disease spores with spore traps, says Moore. The problem, however, is that there are many other things that can produce spores similar to the soybean rust fungus.

“There are techniques and chemicals we can use to examine the genetics and positively identify rust, and private companies are working those tests to the field,” he says.

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