June 2, 2004

 

 

Soybean Rrust Not Likely To Impact US Farmers This Season

 

Iowa and other prime U.S. soybean-growing states face very little risk of a soybean rust epidemic this year, one of the nation's foremost soybean rust researchers believes.

 

Iowa State University (ISU) plant pathologist X.B. Yang, who has studied soybean rust since 1989, said it is impossible to predict the disease's arrival in the United States.

 

He said there must first be confirmed reports of rust north of the equator.

 

A report from University of Illinois researchers last week raised eyebrows by saying the United States was facing an increased threat of a soybean rust infestation this growing season.

 

However, the report was based on unconfirmed rumors of the fungus having already spread north of the equator in Venezuela and Brazil's Roraima state.

 

Yang said when the disease moves north of the equator, prevailing winds are more likely to carry rust spores over the land bridge from South America to North America.

 

"If the rumor is confirmed, the risk is higher. If it is still a rumor, the risk is very small," said Yang.

 

Soybean rust has garnered the attention of U.S. farmers because of its rapid spread in Brazil, where it caused an estimated $1 billion in damage to this year's soybean crop.

 

Yield losses can range from 5 percent to 80 percent under the right conditions.

 

The U.S. Department of Agriculture is planning to dispatch a research team to South America in June to scout for soybean rust in the areas where the most recent rumors originated, Yang reported.

 

However, even if rust is confirmed in those regions, the disease probably wouldn't spread into the Midwest in time to cause major damage this year, he said.
Any U.S. rust infestation is likely to show up first in the South, where it will have the greatest impact, Yang said. Rust spores generally travel only about 20 miles per day.

 

Rust poses the most risk to soybean plants around the flowering stage, and only under the right weather conditions, Yang explained. The pathogen flourishes at temperatures in the 70s and 80s with adequate moisture.

 

Potential yield damage is relatively insignificant after the seed-fill stage.

"If we find soybean rust in Iowa after the middle of August, I don't think it will cause significant damage," Yang said. "If it comes in late July or early August, we might have to think about advising farmers-depending on the weather¡ªwhether to spray or not."

 

Two fungicides, Bravo and Quadris, are currently labeled for soybean rust control. Several other treatments, including Eagle and Laredo, are expected to receive emergency exemptions from the Environmental Protection Agency if rust is confirmed in the continental United States.

 

There have been reports that some large farmers are stockpiling fungicides in anticipation of rust's arrival. That type of action is premature, according to Yang.
"Historically, it takes over a month for an airborne crop disease to spread from the southern (U.S.) coast to Iowa," he said. "Although unlikely, if rust enters the southern United States this season, Iowa farmers will have sufficient time to prepare fungicides for controlling rust."

 

The Iowa Soybean Association and Iowa Department of Agriculture and Land Stewardship are working with ISU to develop an action plan to identify soybean rust upon its eventual arrival in the state.

 

Identification will be critical, and difficult, because rust-infected leaves appear similar to other diseases, such as brown leaf spot or frogeye leaf spot.

 

ISU Extension personnel are receiving identification training. And in July, nearly 2,000 crop consultants across the state will be trained to distinguish the brown rust lesions.

 

Early suspect samples will be sent to the federal research laboratory in Fort Dietrick, Md., to confirm presence of the disease in Iowa. The Diagnostic Laboratory at ISU will then test other samples.

 

If the rust appears in Iowa late in the growing season, only one fungicide treatment may be required. The estimated cost is $15 to $20 per acre, says David Wright with the Iowa Soybean Association. If rust appears earlier, two to three treatments may be needed.

 

The spread of rust requires the presence of a large number of soybean plants or other hosts, such as kudzu, as well as weather-related factors, such as wind currents and moist conditions that can bring spores down to the ground.

 

The fungus cannot survive winter weather in the northern United States, but can survive on kudzu plants along U.S. southern coastal areas. Experts say the spores will then spread into the interior states during the growing season, but not to the same places every year.