By Greg Tylka
Looking for adult females of the soybean cyst nematode (SCN) on roots of young soybean plants is an easy and effective way to check fields for this yield-reducing pest. The moment a soybean field is planted, the clock starts ticking on when the first new egg-filled adult SCN females of the year will appear on the surface of roots. Generally, it takes about four weeks for the nematodes to infect and develop into adult females, but SCN females have been seen on roots in Iowa as soon as 26 days after planting (see ICM News article here). Because soybean planting in Iowa has taken so long to complete in 2024, the first SCN females of the season will begin appearing over many weeks in June and perhaps early July.
How to look?
Looking for SCN females on soybean roots is simple. Dig roots from the top 12 inches of soil with a spade or shovel, do not pull them out of the soil, and gently shake the soil from the roots. If soil adheres to the roots, crumble the soil away with your fingers.
SCN females look like small, white or light yellow, round objects on the roots (see figure below). On average, the females are about the size of a period in a printed sentence, and most people can see them with an unaided eye. They are much smaller and lighter in color than nitrogen-fixing nodules that appear on healthy soybean roots.
All SCN females do not emerge at the same time, they slowly appear over many days. Females can be seen on roots dug from soil through late summer because a new generation of SCN occurs every 28 days.
Where to dig roots?
SCN was found in 70% to 75% of sampled fields in three separate two-year random surveys of Iowa conducted by Iowa State in the 1990s, the 2000s, and the 2010s. Unfortunately, SCN-infested fields often don’t show obvious symptoms of damage. That is, plants often are not stunted or yellow, especially in the first two months of the growing season, even though the nematode can greatly reduce soybean yields. Despite the lack of aboveground symptoms, the SCN females will be apparent on roots of plants growing in those fields.
One approach to checking for SCN females is to look in areas of fields where SCN often is introduced, such as near field entryways where soil from other fields may have been brought in on farm machinery and along fence lines where windblown soil accumulates. A great place to look for SCN females is in areas of fields that have had lower-than-expected soybean yields with no apparent reason. The figure below illustrates these and other areas that you may target to check for SCN females on roots.
What next if SCN females are observed on roots?
There is nothing that can be done during the season to manage SCN. But mid-season is a good time to learn more about the hundreds of SCN-resistant soybean varieties and the several nematode-protectant seed treatments that can be used to help manage SCN in future years. There is a wealth of information available online about SCN management from Iowa State University and from the checkoff-funded SCN Coalition.
Whether or not you dig roots and see SCN females in fields this summer, plans should be made to collect soil samples to test for SCN in the fall if that hasn’t been done in the past five years (or at all). Sampling a soybean field after harvest will provide the best assessment of the SCN situation and may help explain yields in the fields. Fall sampling of corn fields where soybeans will be grown in 2025 also is a good idea. Results of these fall samples will allow for time to plan management efforts for the next season if SCN is found.
More info on fall SCN soil sampling will be provided in an article here later in the season.
Source : iastate.edu