Adaptation of soybean cyst nematode populations to the PI 88788 source of resistance from 2000 through 2015 in Iowa and the effects on soybean yields
M. MCCARVILLE (1), C. Marett (2), M. Mullaney (2), G. Gebhart (2), G. Tylka (2) (1) Bayer CropScience, U.S.A.; (2) Iowa State University, U.S.A.
Most soybean cultivars grown in Iowa with resistance to the soybean cyst nematode (SCN), Heterodera glycines, have SCN resistance genes from a breeding line named PI 88788. Each year, field experiments are conducted throughout Iowa to evaluate 60 or more SCN-resistant and three or more SCN-susceptible cultivars at up to nine locations. Data on SCN population density, SCN virulence (HG type), soybean yield, precipitation, and growing degree-days from the experiments conducted from 2000 through 2015 were compiled and analyzed to determine how these factors affected SCN reproduction and soybean yield. Reproduction of SCN was positively correlated with temperatures and negatively associated with precipitation. Reproduction of SCN on resistant soybean cultivars with resistance from either PI 88788 or another source, Peking, was positively correlated with increasing virulence on these resistance sources. Both virulence on PI 88788 and reproduction on PI 88788-derived cultivars increased from 2000 to 2015, but not so on Peking. Yields of susceptible, PI 88788-derived, and Peking-derived SCN-resistant soybean cultivars were negatively correlated with SCN reproduction. Results indicate that virulence to PI 88788 is increasing in SCN populations in Iowa, resulting in greater SCN reproduction and yield loss. This trend is problematic because almost all SCN-resistant soybean cultivars available for Iowa farmers have resistance genes from the PI 88788 breeding line.