In This Issue

Exotic Germplasm Could Hold Key
To Controlling Emerging Diseases

In recent years, new soybean diseases such as sudden death syndrome (SDS) and white mold have emerged as major problems in Illinois and much of the Midwest. During a survey in 1998, SDS was observed in 90 percent of the counties in the state. In 1993, the disease was reported in 43 percent of the fields in some parts of east central Illinois, with yield losses ranging from 20 to 46 percent. White mold or sclerotinia stem rot also was widespread in the northern half of Illinois and throughout the upper Midwest during the 1990s.

"A big problem for growers is that most of the commercial soybean varieties are highly susceptible to the pathogens that cause those diseases," says plant pathologist Glen Hartman of the USDA Agricultural Research Service. "Because of the narrow genetic base of commercially grown soybeans, the outbreaks of SDS and white mold in most popular soybean varieties are likely to continue. It would be a huge advantage for growers to have commercial varieties available with resistance to these emerging diseases."

Hartman notes that only partial resistance to SDS and white mold has been reported in commercial varieties. All the varieties tested, including those with partial resistance, still have roots that are colonized by the fungus that causes the disease and exhibit at least some foliar symptoms.

"Many people really did not pay attention to this problem until recently because neither of those diseases were thought to be very important," Hartman says. "Now we are finding that the fungus that causes white mold is present in many more fields than we ever imagined. Likewise, it appears that the pathogen that causes SDS is in the soil in most fields and probably has been for a long time. When environmental conditions are right, both diseases can be quite severe."

In response to this emerging problem, Hartman has been collaborating on several research programs aimed at locating new sources of resistance. In one of the projects underway at the National Soybean Research Laboratory, he has been working with Randall Nelson, curator of the USDA Soybean Germplasm Collection at the University of Illinois, and U of I plant geneticist Theodore Hymowitz on screening accessions of the wild annual progenitor of the soybean, Glycine soja, for resistance to SDS, white mold, and other major diseases.

Primary funding for this research was provided by soybean growers through the Illinois Soybean Checkoff Board. Additional funding was provided by the Illinois Council on Agricultural Research (C-FAR) and the Agricultural Research Service.

"The wild annual soybeans, G. soja, have only been screened for a few economically important traits," Hartman says. "So far, there are only a few publicly released, specialty-use soybean cultivars containing known genetic material from the wild annual soybeans. Even though soybeans and the annual progenitor can be crossed rather easily, this unique collection of genetic material has been virtually overlooked."

To date, the research team has screened more than 1,100 accessions of G. soja for resistance to the pathogen that causes SDS. About 5 percent of the collection of wild annual relatives has been classified as resistant. The entire collection was also screened for resistance to phytophtphora root rot. Six accessions were rated as resistant to that disease, including two from maturity groups III and IV. Several of the most promising accessions already are being crossed with soybean for further field and genetic studies.

"From a soybean breeding point of view, the wild annual soybeans are excellent candidates for gene exchange with soybean," Hartman says. "The sources of resistance found in G. soja accessions potentially include new genes that can be introduced into commercial soybeans. The release of this new germplasm could impact most breeding programs and provide soybean growers with new resistant varieties that would reduce losses due to SDS."

As part of this overall effort to combat emerging diseases, Hartman also has been working with Hymowitz on screening the wild perennial relatives of the soybean from the subgenus Glycine for resistance to both SDS and white mold. The subgenus contains 16 wild perennial species. These plants are indigenous to Australia and have been collected from other diverse environmental areas in southeast Asia and the South Pacific.

Major funding for the screening efforts came from the Agricultural Research Service and from the U of I Experiment Station.

"The perennial Glycine species represent a virtually untapped resource of potentially novel genes for improving traits in soybeans, including disease resistance," Hartman says. "We have already screened more than 1,000 of the perennial accessions. There are numerous examples where wild relatives of a cultivated crop have provided unique traits, including disease resistance. That scenario could well be repeated for soybeans."

Initial evaluations of the perennial species for disease resistance have provided a number of promising resistant accessions that will be further tested in the field during the coming growing season. The researchers also are searching this collection for resistance to other important diseases, such as brown stem rot and charcoal rot. Future plans call for undertaking the complicated crosses with commercial soybeans for the most promising genetic material.

"These resistant accessions probably represent a totally different gene pool than is present in our commercial soybean varieties," Hartman says. "For the perennials, in particular, it is important to realize that this screening is just the first step. A lot of field testing will be needed even after we complete the difficult task of crossing them with soybeans. Nevertheless, this material could well represent what soybean growers will be using in their fields in the years to come."

According to Hartman, the genetic material from the wild annual relatives rather than the perennial species almost certainly will have a more immediate impact in breeding programs, although both hold major promise for the future. He further adds that this research project is only part of a larger effort that has just begun to unravel many of the mysteries of these emerging diseases.

"Often times people expect nothing less than a silver bullet from this kind of research," he says. "It is important to realize that we have had ongoing research on SDS for only a relatively short time—four or five years. For white mold, it has been even a shorter time. Nevertheless, what's encouraging is that we already have been able to discover a lot of valuable information on these diseases that will benefit soybean growers far into the future."

The NSRL Bulletin is published three times a year by the National Soybean Research Laboratory at the University of Illinois, 170 Environmental and Agricultural Sciences Building, 1101 W. Peabody Drive, Urbana, IL 61801; telephone (217)244-1706; e-mail nsrl@uiuc.edu; FAX (217)244-1707. Steven T. Sonka, director; Robert J. Wynstra, editor; David Riecks, photographer; Lynn Hawkinson Smith, graphic designer.

Unless otherwise stated, articles may be reproduced or quoted if credit is given to the NSRL Bulletin. The National Soybean Research Laboratory at the University of Illinois is an affirmative action and equal opportunity institution.