Jim Legvold figured he had soybean cyst nematode (SCN) under control. The north-central Iowa farmer has been planting SCN-resistant varieties since the mid-1990s. He was getting yields of 50-60 bu./acre on his heavy, black Des Moines Lobe soils. “But progressively, my yields kept dropping,” he says. “I was trying to understand what was going on.”

Soil testing told the story.

Legvold learned that the nematode populations in his fields were overcoming the leading source of SCN resistance, PI88788, which has protected soybean yields in this country for 30 years. The same thing is happening in most other major U.S. soybean-production areas, according to nematologists.

“We're hearing lots of complaints from farmers that SCN-resistant varieties aren't working as well,” says Terry Niblack, University of Illinois Extension nematologist.

The situation puts growers in a quandary, says Legvold, who serves on the United Soybean Board, because there are few alternative sources of SCN resistance. “You have to manage for yield and you have to manage for cyst counts, and they don't always go hand-in-hand.”

THROUGHOUT THE MIDWEST, it's now common to find SCN populations where more than one out of 10 nematodes can reproduce on PI88788 cultivars, says Greg Tylka, Iowa State University Extension nematologist. Ten percent reproduction is the threshold where plant resistance is considered breached.

What's behind this breakdown?

“We are using basically one source of resistance,” Niblack says. Nearly all the SCN-resistant cultivars sold in this country get their nematode armor from the same parent: PI88788. Plant resistance to SCN is never 100%, and nematodes readily adapt. So with repeated exposure to PI88788, the parasites are shifting to types that can reproduce at higher levels on formerly resistant cultivars. In terms of selection pressure, “this is the same as not planting an insect refuge with Bt corn,” Niblack says.

A recent study funded by the North Central Soybean Research Program shows how this happens. The study compared SCN population shifts in various regions, based on how long PI88788 cultivars have been widely grown there. In Tennessee, where they've been grown the longest, “almost all of the nematode populations break down the PI88788 resistance,” says Jamal Faghihi, a Purdue University nematologist who led the research.

BY CONTRAST, in Ontario, Canada, where SCN is a more recent arrival, “there's much less resistance breakdown,” Faghihi says. “And in Indiana and Illinois, we're somewhere between the two.” In Indiana, about one-fourth of SCN samples tested in 2005 were able to break through PI88788's defenses.

In Illinois, about two-thirds of SCN populations tested in 2005 had adapted to PI88788, according to a University of Illinois survey. This is a “complete reversal” from 1991, when more than 60% of nematode populations were unable to develop on any source of resistance, Niblack says.

There's been a similar shift in Missouri nematodes, says Melissa Mitchum, University of Missouri nematologist. Her 2005 statewide survey found that nearly 80% of SCN populations could reproduce on PI88788. That's up by more than one-third since 1998, she says.

The same transition has occurred in Kentucky. Every SCN population tested in 2006 and 2007 was able to reproduce on PI88788, at levels ranging from 15% to 80%, says Don Hershman, University of Kentucky plant pathologist. Nearly two-thirds of the nematode populations had reproduction rates greater than 30%, he says. The upshot: “In 10 years, we've seen a shift from easily controlled populations to harder-to-control populations.”

What's the effect on yields? “We don't know the impact of these changes,” Hershman says. In general, PI88788 varieties out-yield susceptible varieties in SCN-infested soils — even with elevated reproduction, says Tylka, the Iowa nematologist. Still, Hershman adds, “I'm constantly hearing growers complain that their soybean yields — unlike their corn yields — aren't up to snuff.”

Disappointing yields led Iowa soybean grower Jim Legvold to resume sampling his fields for nematodes a couple of years ago. SCN numbers averaged 4,000 eggs/100cc of soil (about half a cup), and ran as high as 10,000 eggs/100 cc of soil in hot spots, he says.

Because he had elevated egg counts on PI88788 varieties, Legvold also invested in HG-type tests — formerly called race tests — to find out exactly what he was dealing with. The greenhouse tests revealed that there had been a shift to SCN HG-type 2, which attacks PI88788.

In 2008, Legvold switched two-thirds of his soybean acres to cultivars with the Peking (PI548402) source of SCN resistance. But he had only four variety choices, and his defensive strategy cost him some yield. “But I hope I've driven the level of cysts down,” he says. He'll soil sample again before he plants soybeans in those fields in 2010.

Like Legvold, farmers should sample SCN-infested fields every few years “to see whether the numbers are going up or down in a particular field,” Niblack says. “If they are going up, then farmers need to change their approach.”

THE GOAL IS to maintain soybean yields while holding down SCN numbers. Use an integrated pest management strategy that includes regular SCN scouting and proper rotation, Tylka says. Growers should:

  • Know the numbers. Collect soil samples in the fall to see if SCN is present, Tylka says. Also sample to monitor the success of your SCN management program. Tylka suggests soil sampling every four to six years when you grow SCN-resistant soybeans in rotation with corn.

    Do an HG-type test if you are planting SCN-resistant varieties, yet your soybean yields are falling or your egg counts are rising, or if you see a lot of females on soybean roots in mid-season, Niblack advises. The HG-type test will help you choose the most effective source of resistance for your SCN population.

  • Rotate, rotate, rotate. Rotate soybeans with non-host crops to push down SCN numbers, and rotate the genetic source of resistance to discourage SCN adaptation.