The U.S. soybean industry moved from waiting for the inevitable arrival of soybean rust to waging war against the disease in early November, when USDA confirmed the first Asian rust (Phakopsora pachyrihizi) spores in soybean fields at a Louisiana State University research farm near Baton Rogue. Soon thereafter, researchers confirmed rust infection in Mississippi and Florida.

The dreaded disease causes early leaf defoliation of soybean plants, resulting in fewer pods and seeds, and yield reductions as high as 50 to 80%. Officials believe this year's active hurricane season brought the wind-borne spores into the U.S.

Time to prepare

Since most U.S. soybeans have already been harvested, the impact to the 2004 crop should be minimal, and with several months before the next growing season, the industry has adequate time to prepare to combat the disease.

“We have the winter,” says Iowa State University plant pathologist Alison Robertson, which should give chemical companies time to manufacture fungicides and producers time to plan management strategies.

David Wright, director of production technologies for the Iowa Soybean Association, adds, “Producers should not panic. We do have the tools available to manage this disease.”

Those tools will include early disease detection and aggressive fungicide application programs. Rust-resistant soybean varieties aren't expected to be available for at least five years.

It pays to spray

Growers in Australia, Africa and South America who have dealt with soybean rust report fungicides are effective in managing the disease but say they must be applied quickly once it's detected.

In Brazil, where farmers spend $750 million to $1 billion annually on soybean rust control, Eloi Marchett, who manages a 63,000-acre operation in Mato Grosso, says, “We start spraying for Asian rust at the R4 stage right through R6.” He reports that growers who didn't spray lost 50 to 60% of their crop.

In South Africa, plant pathologist Neal McLaren has found crop rotation and planting wider rows won't stop rust's progress. Instead, he says, “in endemic areas where you know you'll have problems — spray.”

South Dakota State University plant pathologist Marty Draper estimates fungicide treatment costs will be $10 to $20/acre in the U.S., but adds, “costs will vary greatly by location.” Robertson says if rust infects the soybeans at the early reproductive stage, two or three applications could be necessary, causing those costs to climb.

Approved fungicides

Presently two active compounds are registered for soybean rust in the U.S. — azoxystrobin (Quadris) and chlorothalonil (Bravo WeatherStik and Echo 720) — found in products made by Syngenta and Sipcam Agro.

Five additional active ingredients for control against rust have received labeling approval under Section 18 permits from the EPA: myclobutanil (marketed as Laredo EC and Laredo EW), tebuconazole (Folicur F), propiconazole (Tilt EC, Bumper EC and PropiMax EC), boscalid (Pristine), and pyraclostrobin (Headline EC). Others are being considered, but each state will have to approve the use of these fungicides as well, Draper says.

Looking ahead, the impact of soybean rust on the 2005 U.S. soybean crop will depend on whether it will overwinter in the U.S.; how early the spores reemerge during next year's growing season; and how quickly prevailing weather conditions promote their dispersal north.

Draper says, “With the mild El Nino expected for the coming winter, there could be survival across a wider area in the gulf south than we would expect in other years,” meaning rust could pose a huge problem early on next summer.

The USDA is using climate and wind patterns to develop prediction maps showing the potential dispersal of soybean rust. You can view them at www.aphis.usda.gov/lpa/issues/sbr/sbr.html.