If you were asked what the most economically destructive soybean disease in the United States is, what comes to mind? White mold? Sudden death syndrome (SDS)? Frogeye leaf spot?
Did soybean cyst nematode (SCN) cross your mind? Soybean cyst nematode remains the most destructive soybean pathogen in the US, racking up annual yield losses estimated over $1 billion and is a looming threat to Canadian soybean producers. Soybean yield loss may reach 30% before any visual aboveground symptoms develop.
Root examination during the summer can reveal the presence or absence of SCN and the relative degree of infection. The cyst, which holds the SCN eggs, can be seen on soybean roots. Cysts are much smaller in size compared to soybean root nodules, are lightly attached to soybean roots, about the size of a grain of sand or smaller and can be whitish to tan colored (see image below). To learn the actual SCN egg count, collect a SCN soil sample at soybean maturity in late summer or fall (before tillage).
Soybean cyst nematode undergoes 3 to 6 generations per growing season, can thrive in stressful growing conditions including dry/sandy/droughty soils and cause problems in high or low pH situations. Cysts from SCN can last over 10-years in the soil; once SCN infests a field, it is highly unlikely that it will ever be eliminated in that field. After detection, SCN control starts with integrated pest management strategies to minimize yield loss and reduce SCN populations.
The following practices are recommended to manage SCN:
- Plant SCN-resistant soybean varieties.
- Rotate SCN-resistance sources (i.e. use PI88788-sourced resistance during one soybean rotation; use Peking sourced the next time soybeans are planted in that field).
- Use non-host crop choices in crop rotations (e.g. corn, small grains, sorghum, alfalfa).
- Utilize seed treatments with proven efficacy controlling SCN.
For the past 20+ years, the most effective option has been SCN-resistant soybean varieties. The majority of soybean varieties available contain SCN-resistance traits, and nearly all these soybean varieties (>95%) use the same resistance trait, PI88788. By relying on only one genetic trait, can we expect the similar resistance problems like we have experienced with herbicide-resistant weeds or insecticide-resistant insects? You bet. Whatever the selection pressure is, nature always finds a way and evolves.
Several years ago, Iowa State University and University of Minnesota plant pathologists began documenting SCN populations overwhelming the PI88788 resistance trait. The problem has become more widespread, and we have observed higher and higher SCN egg counts in regions where SCN was previously well controlled. In 2019, AGVISE Laboratories started a project investigating SCN resistance in west central and southwest Minnesota with cooperating agronomists. We collected soil samples from four soybean fields and had the SCN population “HG type” determined. All SCN populations had varying resistance to the PI88788 resistance trait.
In 2020, we expanded the project to track the SCN populations (i.e. SCN egg count) with at-planting and at-maturity SCN soil samples. If the SCN population builds during the growing season, it is a simple indicator that the SCN-resistance trait is failing. If you observe SCN populations continuing to increase, even though you use SCN-resistant soybean varieties, you may need to find new resistance traits like Peking. This simple soil sampling protocol, at-planting and at-maturity, is a quick and effective way to check for SCN resistance in fields. We will report on the SCN resistance project at the AGVISE Soil Fertility Seminars in January 2021.
Suggested resources on SCN diagnosis, management and resistance