Unfortunately, no rust-resistant varieties have been discovered. “We've screened more than 1,000 commercial soybean varieties and found all of them are susceptible to rust,” report USDA-ARS researchers Glen Hartman and Monte Miles. Both are stationed at the Soybean/Maize Germplasm, Pathology and Genetics Research Laboratory at Urbana, IL, and work cooperatively with the University of Illinois.

This means, when soybean rust first arrives, U.S. producers will need to initially rely on fungicides and other management tactics to minimize potential yield losses. With time, researchers hope rust-resistant soybean varieties will be developed through germplasm research and breeding.

RESEARCH UNDER WAY

Through soybean checkoff-funded research, Hartman and Miles have been working with fellow USDA-ARS researcher Reid Frederick for the past 18 months. They have been screening existing varieties and exotic soybean germplasm for sources of rust resistance.

The process includes taking soybean germplasm housed at the University of Illinois and exposing it to the soybean rust pathogen at the USDA-ARS Biological Safety Level 3 Containment Facility in Fort Detrick, MD. That's where Frederick, who is coordinator of the checkoff-funded rust research, is stationed. Since this disease is not found in the continental U.S., this is the only place in the country where soybean rust research can be conducted.

In addition, international field trials with U.S. soybean varieties are being set up at rust “hot-spot” regions in countries where the disease occurs, such as China, Thailand, Zimbabwe, South Africa, Brazil and Paraguay.

Because Asian soybean rust does not yet exist in the U.S., Miles says, “We're not set up to evaluate how broad resistance is, thus we need to work collaboratively with international researchers.”

Frederick adds, “These international trials are pivotal because we want to look at the disease across as many different lines and environments as possible.”

A LONG PROCESS

Miles reports that there are more than 16,000 soybean germplasm lines in the research facility at the University of Illinois.

“From the first 6,000 accessions we've screened, there are less than 100 that are still of interest and might be useable — meaning they exhibit some degree of resistance to soybean rust,” he says. “The ultimate goal is to identify lines with either partial or single gene resistance.”

But Miles adds, “It's going to be relatively time-consuming. The virulence of the disease is complex and diverse. This problem is not going to be solved using a single gene; we will need to combine several single genes and have them in a background with some partial resistance to have a chance at limiting this disease.”

Miles and Hartman say that initial resistant varieties won't offer complete resistance to rust right away. Instead, traits that decrease lesion size and susceptibility will likely be identified first and incorporated into newly developed varieties with backcrossing.

Of these yet-to-be-developed varieties with partial resistance, Miles says that, in the short-term, fungicide applications will still be needed for combined control. “At best, we can hope for yield stability in presence of the disease.”

Meanwhile, these researchers continue to conduct the germplasm screens and hope to have preliminary data by summer 2004. They aren't able to put a timeline on when a genetically resistant variety to rust might be available. Frederick admits, however, that “It's a few years away.”