It could have been a soybean farmer’s worst nightmare. The e-mail rust alerts were coming in hot and heavy, and the map on the soybean rust monitoring Web site (www.sbrusa.net), which had been mostly green, suddenly lit up like a Christmas tree.

Between Oct. 11 and Oct. 24, the www.sbrusa.net monitoring system issued 17 soybean rust alerts for counties in the Mid-South states and Kentucky. Nationwide, the monitoring program said the latest findings had brought the 2006 total to 161 counties in 15 states ranging from Texas to Indiana to Virginia.

Fortunately for farmers, most of the nation’s soybean crop had already been harvested and, in the fields that remained, the soybeans were way beyond the stage where Asian soybean rust could impact yields.

But why, after being confined to the Deep South states for most of the 2006 growing season, did Asian soybean rust seemingly explode into the Mid-South and the eastern part of the Corn Belt in mid-October?

It was due to cooler temperatures and more rainfall, according to X.B. Yang, professor of plant pathology at Iowa State University and one of the country’s leading experts on Asian soybean rust.

Yang, who was interviewed in his office in Ames, Iowa, has been thinking quite a bit about why soybean rust hasn’t been the scourge of U.S. soybeans that many scientists thought it would be after it arrived in the United States in November 2004.

“The areas of Brazil where soybean rust has been most severe are mainly on a plateau that is 2,000 feet and higher in elevation,” he said. “It’s cooler there in the summer nights — and it typically rains 15 days out of the month — so rust has been very bad most years since it came to Brazil.”

For each 100-meter increase in elevation, says Yang, there’s a 0.7-degree Celsius decrease in the average mean temperature. “So temperatures generally are cooler in Mato Grosso and Goias and other soybean-producing states in central Brazil than in the same timeframe in the southern United States.

“Understanding the situation in Brazil helps us understand what’s happening in the United States,” says Yang.

The situation was similar when Asian soybean rust first surfaced in China, according to Yang, who received his master’s degree from China Agricultural University and his Ph.D. from Louisiana State University.

As it was expected to do in the United States, Asian soybean rust can over-winter in the South and move north into the major soybean producing areas of China. But the degree of severity of rust infections has proven to be highly dependent on rainfall and the number of rainy or cloudy days.

In a season of two months with 30 or more rainfall days and 8 inches or more of rain, Chinese soybean farmers have experienced heavy losses (40 percent to 50 percent) from soybean rust. With 30 or less days of rainfall and 8 inches or less of rain, the losses have been more moderate (20 percent to 30 percent). With less than 10 rainy days and 5 inches of rain, disease pressure has been low.

“Soybean rust was relatively light in Brazil in the 2005-06 growing season,” said Yang. “But it was also drier and warmer than normal in the Mato Grosso and Goias states of Brazil this past season.”

In most years, however, the weather in the summer in the soybean growing areas of Brazil is much wetter and cloudier than that in the summer in the soybean growing areas of the United States.

Take Ames, Iowa, where Yang is located, for example. Ames averages seven or eight days of rainy weather and slightly more than 4 inches of rain per month in June, July or August. Cuiaba, the capitol city of Mato Grosso state in Brazil averages from 18 to 20 days of rainy weather and 8 to 10 inches of rain each in January, February and March, the months that correspond with summer in the United States.

Mato Grosso ranges from 1,640 to 3,280 feet in elevation. At 1,000 meters (3,300 feet), the average temperature will be 7 degrees C (12.6 degrees F) cooler than at sea level. “In Brazil, soybean rust has been much worse for those who farm in Mato Grosso than in Parana (a state in southern Brazil where the average elevation is lower.)”

The temperatures for the Cerrados area in the neighboring state of Goias are about the same as for Des Moines in the different summers for the two locations. But the former receive an average of 66 inches of rainfall annually, while Des Moines’ average is 32 inches.

Yang says one of the latest studies of Asian soybean rust shows sunlight can have a significant effect on spore development in Asian soybean rust.

“In Mato Grosso in the central part of Brazil, the day-length is relatively short because of its proximity to the Equator,” he said. “The longer day-length in the southern states affects the survival of the spores in the United States.”

Another study conducted by researchers at the University of Illinois indicates that if spores are exposed to sunlight for 48 hours, they will not germinate on host plants such as soybeans.

“That tells us that if spores move from south to north, they almost have to survive in a cloud,” he said. “In bright sunlight, only a small amount of spores may survive. It’s been sunny and dry in the South the last two years, and rust spores couldn’t germinate.”

In the fall of 2004, when rust spores first moved into the United States from Columbia, they traveled in a “big cloud,” courtesy of Hurricane Ivan. (Monte Miles, plant pathologist with USDA’s Agricultural Research Service, has shown slides of Ivan covering an area from Columbia in South America into the southeastern United States in September 2004.)

The bottom line? A soybean production area may need at least 12 days of rain a month to get a significant level of disease, says Yang.

For Florida and Texas, the two states where researchers believe soybean rust are more likely to overwinter in the United States, Florida can receive significant amounts of rainfall, especially during the hurricane season.

“Florida can get 15 to 20 days of rainfall, but the rainfall has been very light in the last two years, especially in the spring when the disease would normally be building,” says Yang.

“For the disease to become established, it has to build very quickly,” he said. “In Florida, Asian soybean rust can produce spores in kudzu, but what we’ve found is that it prefers to sporulate on kudzu in the shade. And kudzu does not like to grow in the shade; it prefers to be out in the open.”

Wind patterns are not always conducive to moving soybean rusts spores into the Mid-South and the Midwest. Researchers believe soybean rust can also overwinter along Interstate 10 along the Gulf Coast in Alabama and Louisiana.

“In April, the prevailing winds can blow from the Southeast up into the Mid-South,” says Yang. “After April, however, they tend to blow out into the Gulf of Mexico. For rust to become established, it has to move from that region before May.

“The spores would have to develop in June in the Mid-South to be able to move north into the Midwest for the same reasons,” he said. “After that, it often is too hot and dry for the spores to germinate. That’s what happened the last two years.”

That doesn’t mean soybean rust cannot develop and become a threat to soybeans in the Midwest, he says, as has happened with other diseases such as Southern corn rust.

“If the disease builds up in the south, the spores can curve over in a cloud and move to the Mid-South and then into the Midwest,” he said. “With the cloudy, cool conditions that we see in the Mid-South some years, the disease could become a serious threat.”

For that reason, Yang believes USDA should continue funding at least a portion of the system of sentinel plots that he helped organize in the soybean-producing states. The plots, manned by Extension personnel, help provide an early warning to growers of the presence of soybean rust.

“It will depend on funding, but I think they should at least continue the plots in the southern states,” he said. “If we don’t have information about what’s happening in the South, people there and in the rest of the country can’t make informed decisions.”

Yang said the recent findings on the importance of temperatures and rainfall could help explain much about the lack of soybean rust development in the United States in the last two seasons.

“In the past, researchers said dew was sufficient to provide the moisture needed for soybean rust to develop,” he said. “Now we’re seeing that rain has a definite impact. Just having a heavy dew is not sufficient.”

Some scientists are also saying — and USDA data seem to bear this out — that glyphosate has an impact on the development on Asian soybean rust. “In the United States, 90 percent of the soybeans planted are Roundup Ready,” he said. “In Brazil, the percentage is much lower.

“Beyond the random weather effects, this may also help explain why we’ve had less disease, which needs more data to support anyway.”

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