Soybean Cyst Nematode Virulence Evolution, Population Dynamics, and Yield Impacts Under Long-Term Monoculture and Rotation of Resistant Cultivars.

Soybean cyst nematode (SCN) is a major pathogen of soybean, causing significant yield losses worldwide. This study investigated the long-term effects of three SCN-resistant soybean cultivars, R1, R2, and R3, derived from resistance sources PI 88788, Peking (PI 548402), and PI 437654, respectively, along with susceptible cultivar (S) on SCN population density, virulence and soybean yield over 14 growing seasons in a field initially infested with SCN HG Type 2.5.7. Results showed that all resistant cultivars reduced SCN population densities compared to S, with R3 maintaining the lowest SCN populations throughout the study. Continuous monoculture of R1 increased SCN virulence, measured as Female Index (FI), on PI 88788 from 23.4 to 53, while Peking-derived cultivar (R2) exerted the strongest selection pressure, increasing FI on Peking from 2.35 to 65.8 over 12 years. Interestingly, R3 did not select for virulence to its own resistance source but increased virulence to both PI 88788 and Peking. The susceptible soybean had a negative effect on virulence to Peking, indicating a fitness cost to the Peking resistance. Planting S reduced SCN reproduction factor (Rf) over time, suggesting potential soil suppressiveness or environmental effects. Soybean yield varied significantly among cultivars, with R1 consistently yielding the highest, while the S suffered the greatest yield loss due to SCN. However, relative yield R3 was highest, R1 and R2 intermediate and S lowest. This study reinforces the importance of rotating resistance sources to manage SCN virulence and population density, with PI 88788 and Peking showing distinct resistance mechanisms suitable for rotation.