Our most costly pathogen threats to soybean are typically found in the soil. Soybean cyst nematode (SCN) is one such threat and can result in estimated yield losses of 30% without soybean having any aboveground symptoms.

Spring is a good time to sample fields for SCN. Comparisons between SCN egg counts done at or near planting to SCN egg counts at harvest can provide information on effectiveness of the SCN management strategy and the extent to which yield losses may be occurring in a given field.

Over the last two years we have found a range of SCN densities in Missouri soybean fields from minimal detection to levels associated with 30% yield losses.

This spring consider collecting soil samples and submitting them to the University of Missouri SCN Diagnostics Clinic for free SCN egg counts (Figure 1). Farmers can submit up to 4 free samples this spring and again at harvest because of support from the Missouri Soybean Merchandising Council. The sampling form and instructions can be found on the SCN Diagnostics website. Click on the green banner "Free SCN Egg Counts for Farmers"; e-mail the SCN Diagnostics Clinic; or contact your regional MU Extension Field Agronomy Specialists with questions.

Figure 1 MU student assistant Jacob Heitman uses the elutriator to separate SCN cysts from soil samples.

Another in-season option to monitor for SCN is to count female SCN on soybean roots approximately 5 to 6 weeks after planting. The females are oval to lemon-shaped structures and are much smaller than nodules (Figure 2). These females can easily fall off the roots so plants must be dug up (not pulled). Each female will swell as she produces approximately 200 to 250 eggs, and as the female dies, her body will darken and harden into a cyst to that protects the eggs.

Figure 2 SCN females are attached to soybean roots and are visible approximately 5 to 6 weeks after soybean planting. The red circle encompasses a group of visible females on the root. Image by Jeff Barizon.

Soybean genetics and SCN

In 2023, we began comparing PI88788 to another type of resistance known as Peking-type resistance. We evaluated SCN levels at three locations in Missouri: two small plot trials and one farmer field. The SCN populations at all locations were moderately low at the time of planting with average SCN counts ranging from 100 to 175 SCN eggs per 100 cc soil. Over the course of the season, the number of SCN eggs decreased where Peking soybean were planted. SCN egg counts were reduced by an average of 100 eggs per 100 cc soil at harvest when compared to SCN levels at planting. Meanwhile SCN egg counts increased by approximately 260 eggs per 100 cc soil where PI88788 soybean were planted. The Peking varieties yielded similarly to PI88788 when averaged across locations. We plan to continue comparing Peking and PI88788 in 2024.

The ability to include Peking soybean in a rotation with corn and PI88788 soybean would be beneficial in both high SCN fields and those with low levels of SCN, because such a rotation could help better maintain those lower levels.

New soybean genetics tools for SCN management are being developed. However, they are unlikely to be commercially available for 4 to 5 more years. BASF is working on an SCN-transgene, and MU breeders in collaboration with University of Georgia recently discovered a modified Peking type of resistance that is likely to be bred into commercial varieties. These future tools will not be silver bullets. Stewardship of our current tools and wise incorporation of new technologies is important for managing this soil-born pest.

For more information on SCN contact the University of Missouri SCN Diagnostics Clinic or visit the Crop Protection Network overview and the SCN Coalition website.