SCN-Resistant Varieties Prove Their Worth

Using resistant soybean varieties is an effective strategy for managing soybean cyst nematodes (SCN), and Iowa farmers have numerous SCN-resistant soybean varieties available. Each year, Iowa State University (ISU) researchers evaluate public and private SCN-resistant soybean varieties in SCN-infested fields throughout the state.  The recently published report, “Evaluation of Soybean Varieties Resistant to Soybean Cyst Nematode in Iowa in 2010,” describes research assessing the agronomic performance of those varieties and determining their effect on SCN numbers.

ISU Nematologist Greg Tylka says, “With direct soybean checkoff support for the first time, the 2010 program evaluated more SCN-resistant soybean varieties, more varieties with different sources of resistance and SCN-resistant varieties from more seed companies than ever before.”

With 26 seed companies represented, a total of 160 SCN-resistant varieties were evaluated, 60 percent more than 2009 and 24 more than the highest number ever previously tested in the program. Of the varieties evaluated, 82 percent were glyphosate-resistant; 18 percent were not.

The 2010 results were unusual in several ways, Tylka says. “SCN numbers decreased through the season on more SCN-resistant varieties than in the past few years. Surprisingly, SCN numbers also decreased on some SCN-susceptible soybean varieties, likely due to the wet growing conditions in 2010. For an unknown reason, SCN numbers do not increase in wet conditions as much as in average or dry soils.”

The 2010 trials were also unusual because a few experimental locations – at Thornton in north central Iowa, Nevada in central Iowa and Oskaloosa in south central Iowa – suffered high levels of sudden death syndrome (SDS).

“SDS likely had as much effect on yields in these locations as SCN did,” Tylka says. “But SCN causes SDS to develop earlier and to higher levels, so SCN resistance may have protected some yield in those locations.”

Tylka says, despite the effect of unusual weather, some observations can be made. “First, there were statistically significant differences in yield among the SCN-resistant soybean varieties (and susceptible varieties) evaluated in SCN-infested fields. There were also significant differences in SCN reproduction among SCN-resistant varieties. Among glyphosate-resistant soybean varieties, the highest-yielding variety at all nine locations was an SCN-resistant variety, and the SCN-resistant varieties also had lower end-of-season SCN numbers than susceptible varieties at every location.”

Tylka says, at a few locations, there was a large effect of SCN. At Moorhead, in west central Iowa, SCN-resistant soybean varieties averaged 64.1 bushels per acre while susceptible varieties yielded 52.3 bushels per acre, an 11.8 bushel difference.  At Fruitland, on sandy and well-drained soil in southeast Iowa, SCN-resistant soybean varieties averaged 48.2 bushels per acre and susceptible varieties yielded 39.0 bushels per acre, a 9.2 bushel difference.

“The results illustrate that SCN-resistant varieties can suppress SCN reproduction and provide increased soybean yields relative to susceptible varieties,” Tylka says. “The experiments demonstrate the benefits of utilizing SCN-resistant soybean varieties for management of this pest.”

Tylka notes there are currently three main genetic sources for SCN resistance in commercial soybean varieties, PI 88788, Peking, and PI 437654 (also known as Hartwig, PUSCN14 or CystX®).  Each contains several genes that confer SCN resistance. Therefore, soybean varieties developed from the various resistance sources may not all contain the same genes in the same combinations. All sources of SCN resistance allow limited SCN reproduction; resistant varieties must be part of an integrated management program.

The “Evaluation of Soybean Varieties Resistant to Soybean Cyst Nematode in Iowa in 2010” is available at http://iasoy.us/1wfN.

While data represents a limited number of locations and varieties’ performance will vary by location and year, the information can serve as a starting point to help farmers develop a SCN-management program specific to their own conditions.