Are We Losing the Resistance Battle Against Soybean Cyst Nematodes? Distribution and Virulence Phenotypes of the Nematode Populations in South Dakota

Soybean cyst nematode (Heterodera glycines Ichinohe; SCN) is the biggest economic constraint to soybean production in South Dakota. Since SCN was first confirmed in the state in 1995, it has spread to at least 46 counties as of 2024. Monitoring and understanding SCN virulence phenotypes is essential for adopting effective sources of resistance, a key management strategy of the pest. During 2023 and 2024, 320 random soil samples were collected from fields and processed for extraction of eggs and cysts to determine the distribution of HG types in South Dakota. About 45% of the samples tested positive for SCN with egg counts ranging from 250-56,000 eggs per 100 cm³ of soil representing 40 counties across the state. 37 SCN populations from infested fields were characterized on three indicator lines: PI 548402 (Peking), PI 88788, PI 437654, and a susceptible check, Williams 82. Nearly 95% of the populations were virulent on indicator line PI 88788, the major source of resistance for soybean cultivars. More than half of the populations (56%) were also able to reproduce on indicator line PI 548402 (Peking), the second major resistance source. Our results also show about 16% of the populations were virulent on PI 437654, suggesting the SCN populations are overcoming this source of genetic resistance. The results highlight the need for exploring novel resistance sources for managing SCN in South Dakota. Our field surveys also demonstrated the importance of continued monitoring for new soybean growing areas in the state as SCN appears to establish quickly in these areas.