Extension Ag Update
September/October 2001
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Research Results

Wild Australian Soybean Relatives Hold Genes for Nematode Resistance

Linda McGraw, ARS News Service, USDA, 309-681-6530, mcgraw@ars.usda.gov

Future soybeans may have stronger resistance to a pest that has long plagued Midwestern farmers, thanks to the diligence of Agricultural Research Service (ARS) and University of Illinois scientists in Urbana, Ill. The pest, the soybean cyst nematode (SCN), was first found in North Carolina in 1954. It has consistently overcome the best genetic resistance available in soybeans, and its presence in most soybean-producing states can mean a 5 to 10 percent yield loss annually. In Illinois alone--where 10 million acres of soybeans are grown--this could mean millions of dollars in losses.

ARS plant pathologists Glen L. Hartman and Gregory R. Noel and University of Illinois scientist Ted Hymowitz screened 328 accessions of Glycine tomentella, the wild Australian relative of soybeans. They found 50 percent of these germplasm accessions had high levels of resistance to Race 3 nematodes, the most common type in the Midwest. There are nine known SCN races in the United States.

In the greenhouse, the researchers inoculated the wild G. tomentella with nematode eggs and cysts containing eggs. The outcome: no reproduction of the nematodes on resistant plants. Hymowitz has crossed G. tomentella and commercial soybeans without using biotechnology methods. ARS scientists are testing the populations of these crosses to determine if the resistance genes have been transferred. This work represents just a portion of the untapped resources in wild perennial relatives of soybeans that possess resistance to many other soybean pathogens, according to Hartman.

Hartman and Noel are based at ARS's Soybean/Maize Germplasm, Pathology and Genetics Research Unit in Urbana. (www.life.uiuc.edu/plantbio/ars/ppgru.html) The germplasm is part of the USDA Soybean Germplasm Collection. This work is being funded by ARS, the University of Illinois, the Illinois Council for Food and Agricultural Research (C-FAR) and the Illinois Soybean Promotion Operating Board (ISPOB).

Soybean Resistance to Phytophthora

Candace Pollock, Ohio State University, 330-202-3550, pollock.58@osu.edu
Scientist: Anne Dorrance, Ohio State University, 330-202-3560, dorrance.1@osu.edu

The Ohio State University researchers have identified new soybean varieties that exhibit both partial and complete resistance to Phytophthora sojae, a pathogen responsible for severe production losses. But farmers may have to wait several years for a commercial product if a resistant gene is found that could be introduced into Ohio varieties, said Anne Dorrance, a plant pathologist at the university's Ohio Agricultural Research and Development Center.

Dorrance and OSU plant pathologist Fritz Schmitthenner evaluated 1,015 soybean plant introductions (varieties found in other countries) and found that 32 of them exhibited complete resistance to Phytophthora and 130 exhibited high levels of partial resistance. All of the varieties originated from South Korea and were obtained from the U.S. Department of Agriculture Soybean Germplasm Collection in Urbana, Ill.

Phytophthora sojae causes soybean root rot and is a major problem in Midwest states that have heavy clay soils. Heavy rains saturate the soil producing areas with standing water, which provides an outlet for the pathogen to infect plant roots. The fungus grows in the roots and into the plant stem, eventually killing the plant.

"The timing right now is perfect to find a new gene and get it out in the market," said Dorrance, adding that Phytophthora is slowly adapting to current soybean resistant genes. "We are continuously putting pressure on those resistant genes. Eventually they will no longer hold up to the pathogen." Dorrance said the next step in research is to identify the gene in the South Korean soybean varieties that are showing resistance to Phytophthora and introduce it into current commercial varieties in the hopes those varieties will carry the resistant gene. "If we do find the gene, it may be five or six years before we can get out a completely resistant variety, and four or five years before we could release a partial-resistant variety," she said. OSU horticulturists Steve St. Martin and Ron Fioritto are conducting the genetic work.

Though a completely resistant variety is ideal, Dorrance said varieties that show partial resistance are also exciting. "Partial resistance basically means that the pathogen has little effect on the plant once it has grown up and out of the ground," she said. "Partial resistance varieties can be very effective, sometimes having a 30-percent difference in yields compared to soybean plants that have no resistance to Phytophthora at all, depending on the disease pressure."

Funding for the project is supported by the Ohio Soybean Council. Some of the results from the research conducted by Dorrance and Schmitthenner are available online at http://www.ars-grin.gov/npgs/. The report is also discussed in OARDC research bulletin 1193 available at the OARDC Department of Plant Pathology, 1680 Madison Ave., Wooster, OH 44691.

Red Clover Silage Boasts Benefits over Alfalfa Silage

Linda McGraw, ARS News Service, USDA, (309) 681-6530, mcgraw@ars.usda.gov

Alfalfa is still the most important forage fed to U.S. dairy cows, but red clover may be a superior alternative. Switching cows from alfalfa to red clover silage could help reduce manure nitrogen levels--a benefit to the environment, according to Glen Broderick, an Agricultural Research Service dairy scientist in Madison, Wis. That is because red clover has an enzyme called polyphenol oxidase that reduces protein breakdown in the silo. Typically, more than half the protein in alfalfa silage gets broken down to nonprotein nitrogen (NPN) that is used inefficiently by the cow. On average, red clover silage has only 60 percent of the NPN of alfalfa. If not used to make milk, un-used NPN is excreted by the animal.

Five feeding trials were conducted with cows at the ARS research farm at Madison. In the latest two studies, cows produced the same amount of milk on less feed: an average of 68 pounds of milk a day on 54 pounds of alfalfa dry matter, compared with cows producing 69 pounds of milk a day on 49 pounds of red clover dry matter. This means a 10 percent increase in feed efficiency and a 10 percent greater energy value for cows fed red clover silage, according to Broderick. Protein efficiency was 17 percent better on red clover than alfalfa in these last two trials. Even if this improvement applied only to the first half of lactation, when cows are fed the most protein, nitrogen excretion would be reduced by about 1.5 tons per year on a 100-cow dairy farm.

Red clover will now be easier for farmers to grow, thanks to new red clover varieties developed by ARS. In the summer of 2000, ARS released newer red clover varieties that should persist 12 to 15 months longer than older red clover varieties. Seed will be available to farmers in about two years. For information see: www.ars.usda.gov/is/AR/archive/jul01/clover0701.htm