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Corn+Soybean Digest

Why So Few SCN Varieties?

Each year, the seed industry trumpets its new soybean varieties. Recently, the biggest and loudest blast has come from, not surprisingly, Roundup Ready soybeans. Their popularity among farmers has caused supply to mushroom. But from a disease perspective, earlier-maturing varieties with soybean cyst nematode (SCN) resistance have seemingly grown at a snail's pace.

Consider these numbers. In Iowa State University yield trials for 2000, of 820 varieties tested, only about 100 were SCN-resistant. In Soybean Digest's 2002 new-variety listings, limited to four per company or public institution, only 49 out of 123 listed for Groups 00 through IV were SCN-resistant. A total of 110 were Roundup Ready selections. Also, consider that the first Roundup Ready varieties hit the market in '96. Breeders have been breeding for cyst resistance for more than a decade in the Midwest — and much longer in the South.

That's the situation, even though soybean breeders, nematologists and plant pathologists remind everyone that SCN is the hands-down No. 1 profit-stealing pest for U.S. soybean growers.

On the surface, at least, one might wonder if soybean breeding companies have been foot dragging in the SCN resistance breeding effort to focus on Roundup Ready technology — because of unprecedented farmer demand.

“The term ‘foot dragging’ may be a little harsh. I know many in the industry have been working hard on developing earlier-maturity soybeans with SCN resistance and good yield,” says Alan Walker. He's a veteran soybean breeder and director of soybean efforts at Monsanto.

Walker, other breeders and nematologists readily agree that there isn't enough SCN-resistant product in the earlier maturity groups.

Nematologists like Greg Tylka at Iowa State University, for example, estimate that SCN is now found in three of every four fields in Iowa and four of five fields in Illinois.

“Let's say every soybean grower who actually had cyst nematode woke up tomorrow and realized how important it is to manage. There wouldn't be enough SCN-resistant varieties and seed available,” Tylka says.

Adds Jim Orf, University of Minnesota soybean breeder, “In the Group 0 maturity area, we're now finding cyst nematode up to the Fargo-Moorhead area of Minnesota — and spreading.”

South Dakota was one of the last Corn Belt states to get SCN. Jim Smolik, South Dakota State University nematologist, says, “There's not a lot to choose from in the Group 0s and even Group Is with SCN resistance.”

However, these scientists all agree that there's a good explanation for the much lower number of cyst-resistant varieties available as opposed to Roundup Ready.

“The Roundup Ready gene is a much easier situation to achieve,” explains Walker. “It's a single gene trait and a dominant trait, so it's easy to move in. And you can spray the seedlings and select for it in one generation.

“With SCN, you need at least two genes and a modifying gene. One is dominant and one is recessive of the major genes. So it's a lot more difficult to pull these together. Also, you can't screen for it visually like with Roundup Ready.”

The breeding challenge with SCN resistance is much more complicated, concurs Orf and John Soper, Pioneer Hi-Bred International soybean breeder. In fact, Orf adds, a fair amount of evidence indicates that there is actually a yield-reducing gene closely linked to the SCN-resistance gene.

“That linkage seems to be fairly tight, so you have to work hard to get something that yields well,” he explains.

A three-year-old technology, called molecular marker screening, will markedly speed up development of new SCN varieties, the breeders agree. It enables far more lines to be screened than with the traditional greenhouse route.

“We have the technology now to select for SCN resistance more aggressively and efficiently than we've ever been able to do before,” Soper says. “And, we have a much larger percentage of our efforts devoted to cyst resistance than we've had in the past.”

That development, along with more farmer demand, is starting to show up in the number of new SCN-resistant varieties in the Group Is and IIs.

There is solid promise of more good-yielding SCN-resistant varieties coming soon. Now, the challenge will be to get farmers to not only want them, but demand them.

Nevertheless, SCN Damage Is Dropping

While SCN numbers may not be plentiful in earlier-maturing varieties, as indicated in the above story, positive disease-controling measures are taking place.

The North Central Soybean Research Program's (NCSRP) effort at SCN education and awareness is a good example. That campaign has helped reduce the amount of SCN damage in the north central region of the U.S.

Allen Wrather, a professor of plant pathology at the University of Missouri, collects SCN data. He says the disease caused a total loss of 270.4 million bu in the 12 north central Corn Belt states in 1998. By 2000, that estimated yield loss had dropped 51% to 133.6 million bu. Wrather credits producer awareness and efforts to control the disease to NCSRP.

“In my view, it's really good evidence of the success of the SCN Coalition program,” Wrather says.

The SCN Coalition, sponsored by NCSRP from 1998 to 2000, provided the campaign theme of “Take the Test. Beat the Pest.”

Funded by 12 state checkoff boards, NCSRP's goal is to find solutions to regional problems. Experts say that 75% of Midwestern soybean fields are infected with SCN, which can cause 30% yield losses without obvious plant symptoms.

SCN is detectable with a soil test and can be managed using resistant varieties and crop rotation.

So far, researchers do not know how to kill the pest. SCN can survive 10 years without food and reproduces quickly, according to Greg Tylka, an Iowa State University nematologist.

“In the Midwest, with our rich soils, you can have it for years and not see sick-looking soybeans,” he says. “It's hard to convince somebody to manage a problem that's not apparent.”

Damage from SCN can vary greatly, depending on the amount of rainfall, Tylka adds. The more the rain, the less the damage. In a dry year, SCN could cause yield losses of 50-60%.

For more on SCN, log on to the NCSRP Web site at

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