Recently, we chatted with Nathan Schoeder, University of Illinois nematologist, about potential ISA on-farm trial research projects. We also let him know that we planned to start a continuous soybean demonstration plot on the ISA farm and that there could be an opportunity to sample for soybean cyst nematode (SCN) each year to monitor egg counts. Dr. Schoeder then told us about one of the few long-duration, field-scale experiments in the Upper Midwest that directly tracked how continuous soybean affected SCN population density and virulence over time.

This 12-year field experiment was conducted at the University of Minnesota Southern Research and Outreach Center (SROC) in Waseca, MN. Researchers specifically evaluated SCN dynamics under:

• Continuous soybean (soybean-on-soybean)
• Soybean rotations with different SCN resistance sources
• Comparisons to rotated cropping systems

Under continuous soybean, the following variety types were compared:

• SCN‑susceptible soybean
• PI 88788–derived SCN resistance (the dominant resistance source in Minnesota)
• Peking‑derived resistance
• Additional resistance sources in some years

Some treatments kept the same resistance source every year, while others rotated resistance sources, allowing researchers to measure how quickly SCN adapted under repeated exposure. Here is what they learned:

Rapid SCN population increases

• Continuous soybean consistently increased SCN egg counts, even when resistant varieties were planted.
• SCN population growth was fastest in soybean-on-soybean compared to rotated systems.
• Susceptible soybean caused the largest and fastest SCN population increases, but resistant varieties still allowed reproduction over time.

Breakdown of PI 88788 resistance

• In continuous soybean planted to PI 88788 varieties, SCN populations adapted within a few seasons, showing:
  • Higher Female Index (FI) values
  • Increased reproduction on PI 88788
• This demonstrated that soybean-on-soybean strongly selects for virulent SCN populations, accelerating resistance erosion compared with rotated systems.

Peking resistance remained more effective (but not permanent)

• Peking‑derived resistance suppressed SCN reproduction more effectively than PI 88788 in continuous soybean.
• However, even Peking resistance showed gradual declines in effectiveness when used continuously, highlighting that no resistance source is rotation‑proof under soybean‑on‑soybean production.

Yield implications

• Yield losses in continuous soybean occurred before obvious symptoms were visible.
• In fields with rising SCN pressure, yield penalties of 5% to 10% or more were observed.

Based on these soybean‑on‑soybean studies, UMN researchers concluded:

1. Soybean‑on‑soybean is the highest‑risk system for SCN
2. SCN resistance alone is not sufficient under continuous soybean
3. Resistance sources must be rotated, not just varieties.
4. Non‑host crop rotation (corn, small grains) is the most reliable way to:
   • Reduce SCN populations
   • Restore effectiveness of resistance genes
5. Continuous soybean should only be considered when:
   • SCN egg counts are very low
   • Peking (or alternative) resistance is used
   • SCN is aggressively monitored with soil testing – For more information about Free SCN testing, go to Free SCN testing | An Illinois Soybean Association sponsored program | Illinois.

This Waseca, MN long‑term experiment is frequently cited because it predicted the resistance breakdown problems many growers are now experiencing, especially in fields with frequent or continuous soybean. Many current SCN management recommendations (resistance rotation, soil testing thresholds, cautious approach to beans‑on‑beans) trace directly back to these findings.

Resource:   Variety rotation as a strategy for managing soybean cyst nematode | UMN Extension