A Fierce Fungal Fight

March 30, 2010 11:22 AM

Those who doubt the threat of soybean rust in the U.S. should have been in Arkansas this past fall. A long string of rainy, cloudy days coupled with late planting left nearly 1.1 million acres of soybeans vulnerable.

Scott Monfort, University of Arkansas Extension plant pathologist, says the disease had never moved so fast in previous years.

"Soybean fields that didn't receive timely fungicide sprays experienced an estimated 15% to 20% yield loss,” Monfort says. "The fact that we had so much rain prior to rust probably saved us because many growers were already spraying to protect against diseases like aerial blight and frogeye leaf spot.”

Few invasive threats have the notoriety in farm country of soybean rust, which is the name for the plant disease caused by the plant pathogen Phakopsora pachyrhizi. Although the disease was first described some 100 years ago in Japan, it wasn't until it blew across the Atlantic Ocean from Africa to South America that warnings that it could spread to North America went from
scientific prediction to genuine concern. The pathogen finally rode the winds of Hurricane Ivan and landed in the continental U.S. in the fall of 2004.

Since then, soybean rust has been the U.S. soybean grower's constant companion—more from the uncertainty of if and when it might hit than from actual infection. Carl Bradley, University of Illinois plant pathologist, says the nature of the disease makes it tricky to predict.

"The soybean rust pathogen must land on the soybean plant at the susceptible growth stage [R1 to R5] and during periods of extended leaf wetness,” Bradley explains. "Rust spores may be dispersed, but the disease may not develop if conditions are hot and dry.” Rust spores are also sensitive to solar radiation, so spores that travel in little to no cloud cover may be dead on arrival.

Southern growing regions will likely always be the most vulnerable because the soybean rust pathogen needs living tissue to survive and freezing temperatures kill the foliar tissue of susceptible plants. The favorite winter home for the pathogen is kudzu, the poster child of invasive plant species and the scourge of the South.

Still, soybean rust infection has been documented as far north as Canada (see map) and throughout the central soybean belt. David Wright, director of research for the North Central
Soybean Research Program, says outbreaks in northern producing states have come late enough in the season to avoid yield loss, yet he worries about growers becoming complacent.
"Rust is real and it continues to be a threat. The good news is we've learned a lot about the biology of the disease and have the methods to control it. However, growers should know the disease is being spotted earlier each year, spore loads appear to be building and the Midwest is not immune,” Wright says.

Symptoms. Soybean rust mimics other problems, such as spider mite damage and foliar diseases, such as bacterial pustule or Septoria brown spot, and therefore can be easy to overlook while scouting. Symptoms begin on the lower leaves deep in the canopy, where spore-producing bodies called uredinia appear as bumps within soybean rust lesions. These pimplelike pustules produce masses of spores that erupt from the lesions and spread by air currents. Soybean plants are more susceptible during and after flowering.

Since 2005, a network of scouts has combed small soybean plots scattered throughout the nation's soybean-producing areas to provide growers with an early warning system against the dreaded disease.

The 2010 season brings a change in this approach. This year, a scaled down, federally funded network of sentinel plots from Texas and Oklahoma to South Carolina will be monitored from April 1 through Oct. 15, instead of year-round.

Wright says the leaner program is an indication that researchers have made progress in fine-tuning models that help predict disease outbreaks and their economic costs. Work continues to develop resistant soybean germplasm, identify and track genetically distinct populations of the fungus, create diagnostic tools for field use and learn more about how environmental factors affect the disease.

"We've gone from fearing this disease to being confident we can manage it if it becomes a problem,” Wright explains. "While we certainly didn't wish for soybean rust, growers have benefited from what has been learned from it.

"Never has an invasive rallied such coordinated efforts. The agricultural, governmental and scientific communities came together to learn how to fight this disease before it became problematic. Soybean rust is the model for how we will address future threats,” Wright says.

To track soybean rust outbreaks, visit http://sbr.ipmpipe.org.


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