Managing SCN More Effectively Use all the tools available to beat the pest
Soybean cyst nematodes (SCN) are the greatest robber of soybean yield, costing producers an estimated $1 billion per year. Nearly every soybean growing state in the country is infested with the nematode. In some states, 70 to 75 percent of the fields tested have been confirmed to have SCN. Nationally, it has been reported that 93 percent of the U.S. soybean growing area is infested with SCN.
Though millions of dollars are spent to study SCN and to develop resistant varieties, experts believe the greatest problem is that producers don’t understand SCN and underestimate its impact on yield.
"SCN can cause a 30 to 35 percent yield loss with no visual symptoms,” says NCSRP Director of Research David Wright. He notes that, in spite of campaigns to raise awareness of SCN, too many farmers don’t believe they have a SCN problem because they can’t see it.
Iowa State University Nematologist Greg Tylka agrees. “The effect of SCN is directly related to the number of nematodes,” says Tylka. Plants with poorly developed root systems cannot utilize nutrients and water effectively. This results in stunted plants with yellow foliage.
If you don’t think you have SCN because you don’t see stunted, yellow plants, you could be wrong. According to Tylka, SCN is often suspected only after all other possibilities have been eliminated. Soil testing is the only sure way to identify SCN.
Terry Niblack, professor at the University of Illinois, recently conducted a survey of farmers regarding SCN. She concluded that complacency is the chief problem, not awareness. According to Niblack, more than half of farmers say more than 75 percent of their fields are infested with SCN. “That shows a high level of awareness,” she says. “However, their estimate is still low because we know more than 80 percent of soybean fields are infested in Illinois.
“Nearly two-thirds of farmers plant SCN resistant varieties on more than 75 percent of their fields,” Niblack continues, “which means there is a high level of awareness that planting resistant varieties is a good way to combat yield losses due to SCN.” These responses from farmers correlate closely with reports from seed dealers about the percentage of resistant varieties they sell.
“We seem to be doing okay until we see that fewer than a fourth actually test for SCN,” she says. "Clearly, farmers think they’re taking care of their SCN problems just by growing a resistant variety. This nematode is causing major yield losses every year, but farmers think there’s no need to pay attention as long as they use a resistant variety.”
What We’re Learning About Resistant Varieties
SCN is not an easy target.
“Despite the common use of non-host rotation and host resistance to reduce the impact of SCN, population densities in Illinois remain at high levels capable of causing significant yield loss,” Terry Niblack cautions. “A survey in 2005 showed most SCN resistant cultivars in Illinois had resistance derived from a single source. Further studies showed 70 percent of the SCN had adapted to that source at some level.”
Tylka agrees. “Almost all SCN-resistant soybean varieties available to U.S. growers possess SCN resistance genes from the breeding line PI 88788, and it is no longer uncommon for SCN populations in the United States to have greater than 10 percent reproduction on PI 88788.” However, he adds, “SCN-resistant varieties with PI 88788 as the source of resistance continue to yield greater than susceptible soybean varieties and also continue to prevent increases in SCN population densities throughout the growing season. Growing resistant varieties is still an effective way to manage SCN.”
According to Tim Todd, nematologist at Kansas State University, by monitoring the SCN population density, a grower can get a feeling for the effectiveness of a given resistant variety with the particular population of SCN that infests a particular field. It is also important, he says, to vary the resistance used so the SCN doesn’t adapt to a particular variety.
“There’s new hope with a type of resistance becoming more available that is more resistant to more SCN populations,” Todd says. “It is referred to as the PI 437654-type or Hartwig-type resistance in public varieties. In private varieties, it is often called Cyst-X. If common resistant varieties don’t work, producers should ask about those.”
In addition to resistance from PI 437654, Niblack adds, “We also have some high-yielding varieties with resistance from Peking, which has resistance that acts a little differently from PI 88788 and is a good choice for rotation where available. In Minnesota, soybean breeder Jim Orf is also working on varieties with resistance from PI 209332, Iowa’s soybean breeder Silvia Cianzio is working on PI 438489B, and in Illinois Brian Diers is attempting to bring in resistance from a wild soybean relative, Glycine soja.”
As researchers do their part to find new sources and improve SCN resistance, farmers should not be satisfied just knowing they plant a “resistant” variety. Instead, they should be vigilant and strategic: varying the resistant varieties they use and testing repeatedly to monitor their progress.
“The main thing is that infested fields should be sampled every few years, not to see whether egg numbers are above or below some threshold, but to see whether the numbers are going up or down in a particular field,” Niblack emphasizes. “If they’re going up, then the farmer needs to change his approach. If they’re going down, then whatever he’s doing is working for him!”
