Cold weather may help delay ASR

The colder the weather in winter, the better Louisiana soybean farmers like it. That’s because freezing temperatures kill kudzu, a noxious plant that among its many faults harbors the fungus that causes a dreaded disease called Asian soybean rust. This globe-trotting disease, new to North America since 2004, has emerged as the major threat to the soybean industry — not only in Louisiana but worldwide — because it spreads quickly and is difficult to control.

Getting rid of kudzu as an over-the-winter host won’t make Asian soybean rust go away, but it helps hold it at bay.

“Asian soybean rust is here to stay,” said Clayton Hollier, LSU AgCenter plant pathologist. “We’re learning the factors that promote and inhibit its development. That’s a major research and extension focus for the AgCenter.”

In 2006, the first discovery of Asian soybean rust in Louisiana was on kudzu. Agricultural consultant Blaine Viator of Plattenville, La., found the disease in a shaded kudzu patch just south of Lafayette, La., on June 30.

Viator is one of a team of consultants, farmers and AgCenter agents and scientists who systematically scout designated areas of the state for the disease.

But even when the rust spread to soybeans later in the season, it had little effect on yields in Louisiana.

“We were lucky in 2006,” said David Lanclos, LSU AgCenter soybean specialist based at the Dean Lee Research Station in Alexandria, La. “Conditions in the spring were too dry for the disease to take hold.”

Asian soybean rust likes warm, humid conditions — not too dry or hot, said Ray Schneider, LSU AgCenter plant pathologist and the one who found the disease for the first time in North America in a soybean production field at the LSU AgCenter’s research farm near Baton Rouge, La.

By the end of July 2006, however, the drought was over. And the disease was found for the first time on soybeans on July 26 in a sentinel plot at the Dean Lee Station.

“Rainfall accelerates the disease,” Schneider said. “It came late but it hit like a hammer. I’ve never seen anything like it.”

LSU AgCenter scientists had planted 15 sentinel plots early in the season in strategic areas to catch the disease first — when it happened.

Fortunately, most of Louisiana’s soybean plants were mature and pods were set before they caught the disease. Eventually, the disease was found in 26 parishes in 2006, which is most of the soybean production area in the state.

“The only area where there was a threat was southwest Louisiana, where the soybeans had been planted late because of unfavorable conditions,” Lanclos said.

The solution was immediate treatment with fungicides. LSU AgCenter specialists worked with farmers on when, where and what to spray.

“There’s a very narrow window for applying fungicides for maximum benefit,” Schneider said.

For 2007, the strategy is again scouting, planting sentinel plots and continuing with research to help learn more about controlling the disease and develop varieties resistant to the disease.

“It helps to kill kudzu,” Lanclos said. “One strategy that growers will try to use this year is planting earlier-maturing beans. I applaud this effort but always caution that proper planting date is still extremely important in maximizing yields.”

In 2006, the disease spread as far north as southern Illinois. But because of good scouting and timely fungicide application, the effect on yields was negligible. In fact, soybean yields set a new record of 35 bushels per acre on average nationwide in 2006.

The impact on yield was minimal in 2005, where the disease was confined to the southeastern United States. Effects were negligible in Louisiana with weather hot and dry much of the 2005 growing season.

Research projects under way at the LSU AgCenter to help keep Louisiana soybeans safe from Asian soybean rust include:

• Testing the various fungicides with different maturity groups of soybeans.

• Determining the effectiveness of spore traps. These traps catch the wind, and if any spores are aloft, they get caught on the petroleum jelly on a slide inside the foot-long tube-like device.

The only hope for eventual elimination of the disease is the development of resistant varieties. To that end, Zhi-Yuan Chen, LSU AgCenter plant pathologist, is looking at soybean proteins of plants infected with Asian soybean rust and comparing them to proteins of uninfected plants.

“We also want to identify fungal genes important for infection and find ways to inhibit it,” Chen said.

Svetlana Oard, a researcher in the AgCenter Biotechnology Laboratory, is working with tiny proteins called peptides that provide resistance to infections from bacteria and fungi. Her goal is to transfer this resistance to Asian soybean rust in a soybean plant.

“Resistant varieties are still several years away,” Hollier said. “Meanwhile, our producers will have to be vigilant and adjust their management practices to the reality of Asian soybean rust.”

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