Strategic Rotation of Resistance Genes to Manage Soybean Cyst Nematode Population Density and Virulence

The soybean cyst nematode (SCN), Heterodera glycines, is a significant yield-limiting threat to soybean production in the United States. Although growing resistant soybean cultivars is the most effective management strategy, prolonged use of PI 88788-derived-resistant cultivars has led to a decrease in effectiveness of this mode of genetic resistance (the rhg1-b allele). The hypothesis we tested was whether soybean cultivars with resistance genes/quantitative trait loci (QTL) combinations other than rhg1-b could suppress population densities and prevent or delay virulence shifts. In four independent field experiments, the effects of soybean genotypes with different SCN resistance gene combinations, grown continuously and in annual rotations, on SCN egg population densities and virulence profiles were investigated. Field experiments were established in Illinois, Missouri, and Central and Northern Iowa in 2019 and conducted for four successive growing seasons. We utilized different combinations of resistance genes and QTL, including rhg1-a/rhg1-b, rhg2, Rhg4, GmSNAP02-ins, GmSNAP14-ins, cqSCN-006, cqSCN-007, and cqSCN10. Microplots inclusive of 12 treatments with three replications were infested with SCN populations with known virulence profiles. Our results showed that the population density and Heterodera glycines (HG) types were influenced primarily by the rotation schemes and the virulence profile of SCN inoculum used. Continuous use of any single genotype increased virulence on resistance sources that shared common resistance genes, whereas deploying resistance genes in rotation slowed the increase in nematode virulence and reduced overall population densities. However, some rotation schemes accelerated the development of virulence on 'Peking' and PI 90763. Our results suggest that resistance genes should be deployed strategically in rotation schemes for more durable SCN management.