The soybean cyst nematode is a growing problem across Iowa, eastern South Dakota and the surrounding states, and with specialized seed varieties losing their resistance to the pest, agronomists say it will take an aggressive approach to keep SCN from biting into soybean yields.
The microscopic roundworm attacks the roots of soybean plants. Because it lives underground, the pest can go undetected, but its impact on yield can be significant.
SCN can reduce yields by as much as 30 percent without showing any visual symptoms above ground, according to Syngenta agronomist Randy Kool. It can usher in other diseases as well.
"Farmers don't see it, so farmers don't manage it as aggressively," he said. But the out-of-sight, out-of-mind approach is not the way to manage for SCN, he said.
In the soils around his area near Des Moines, Iowa, SCN is so prevalent that farmers plant corn on corn, skipping the soybean rotation in order to reduce the numbers of SCN.
The final remaining SCN-free county in Iowa was found to have the pest last year after a soil sampling project done by the Iowa State University Soybean Research Center and the Iowa Soybean Association. Low egg counts were confirmed in a field of Allamakee County in the far northeastern corner of the state.
In South Dakota, SCN has been found in 30 counties in the eastern third of the state.
Growers are encouraged to sample their soil for SCN. Many states offer a free soil test. South Dakota's is paid for through the state's Soybean Research and Promotion Council.
"If you find a cyst, it's time to start managing it," Kool said.
Sampling soil in the spring and again in the fall can give growers an SCN count that tells whether their management practices are working.
Farmers across the area typically plant soybean varieties that are bred to be resistant the cyst nematode. However, some cysts are now able to reproduce on the most popular resistant soybean variety, the PI 88788. The source of genetic resistance was released in the late 1980s, and more than 97 percent of the SCN-resistant varieties depend on that gene.
"Since most are using it, and it's been around so long, SCN are finding a way around it," Kool said.
Extension plant pathologist Emmanuel Byamukama has been studying SCN resistant genes at South Dakota State University in Brookings. His team collected 72 SCN populations and found 18 of them could reproduce on the resistant beans.
There are a few other options for resistant varieties, but most don't yield as well as the PI 88788. Byamukama said 15 varieties were released last year that offer the Peking source of resistance, and those seem to be yielding better.
There are other management tools producers can use in addition to using another type of resistant seed. Byamukama said those with high SCN populations 10,000 eggs per 100 cubic centimeters of soil should consider taking soybeans out of the crop rotation for at least two years.
"That is a high population that's going to take out yield," he said.
Nematicide seed treatments also help when populations are high. There are about four nematicides on the market for soybeans. Syngenta offers Clariva Complete Beans, which adds a nematicide to CruiserMaxx Vibrance Beans with their insecticide and fungicide seed treatment. It also combines with Syngenta's fungicide to protect against sudden death syndrome, which is linked to SCN damage.
Other management techniques haven't been proven, but Byamukama said some are finding that planting a cover crop particularly brassicas help reduce SCN numbers.
As this growing season starts, he will be looking at other ways that SCN survives in farm fields. Last year, Byamukama and other researchers studied common South Dakota weeds to determine whether they could be hosts for SCN. They found eight species with SCN living on their roots, including leafy spurge, horse weed and cocklebur. Further testing this year will help determine whether those weeds are truly hosts.
Byamukama said producers should add aggressive weed management to their list of strategies for battling the soybean cyst nematode.
"If a grower has SCN in his field, he may want to consider more active weed management to control the nematode host," he said.
Reach reporter Janelle Atyeo at 605-335-7300, email firstname.lastname@example.org, or follow on Twitter @JLNeighbor.
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