Soybean cyst nematode (SCN) is a destructive disease for soybean production. This disease has now spread to at least 24 soybean-producing counties in North Dakota. Variety resistance is the primary nematode management tactic. SCN is known to be genetically diverse populations and can develop new virulent forms over time due to continuous use of the same sources of resistance. Thus, it is necessary to determine the HG types, monitor the virulence changing of SCN populations, and assess their occurrence frequency in soybean fields in the state. Host resistance is among the best approach for managing SCN. Molecular markers to select rhg1 resistance allele and identify PI 88788-type resistance have been used for high-throughput marker-assisted selection. Every year, new crosses and breeding lines are developed, hence, evaluating new soybean breeding lines and their copy number variation at Rhg1 is essential for understanding their resistance levels to SCN and recommending a list of resistant lines to the NDSU breeding program for developing new varieties with improved resistance to SCN. Sudden death syndrome (SDS) is an important fungal disease of soybean. In 2024, SDS was observed in many soybean fields in the state. SDS combined with SCN can cause the most yield losses of soybean. However, the presence of SCN in SDS-infested fields in ND is unknown.

• Sample soybean fields known with SDS for the presence of SCN in the same fields.
• Determine the virulent types of SCN in additional 20 fields to monitor the virulence changing of SCN populations.
• Evaluate 50 NDSU soybean breeding lines for their resistance levels to two common SCN populations detected in North Dakota.
• Assess the copy number variation at Rhg1 locus in the breeding lines against SCN.

• Information on co-existence of SCN and SDS in fields will be shared with growers.
• Virulent types (HG types) of SCN in fields will be disclosed and shared with growers.
• Resistance reactions of 50 new soybean breeding lines to SCN and their copy number variation will be disclosed and shared with the NDSU soybean breeder.

Both SCN and SDS are important diseases in soybean. SDS is most likely to occur in areas where SCN is a problem and SDS combined with SCN cause the most yield losses of soybean. Detection of SCN in SDS-infested fields is necessary for making an effective disease management plan. The resistant breeding lines will be provided to the NDSU soybean breeding program for transferring the resistance to locally adapted susceptible varieties to develop and release new varieties with improved genetic resistance to SCN. Assessment of the copy number in soybean lines will help improve resistance selection and breeding accuracy and efficiency. Knowledge of virulence types of SCN should greatly influence the choice of populations and HG types in the resistance assay of new soybean varieties and breeding lines. The proposed research findings will be useful to navigate the use of different resistance sources for growers and the new resistance sources that should be introduced to North Dakota for the soybean breeding program.