Soil sampling for the soybean cyst nematode (SCN) has increased dramatically in the last couple of years. Several private soil fertility laboratories have begun to offer SCN soil analysis as a service. As growers continue to increase their sampling for SCN and begin to compare results from various fields and various laboratories, there are six questions that may be asked to help them interpret their soil sample results.
How many bushels per acre soybean yield loss can I expect for a given SCN egg population density (egg count)? In other words, what is the relationship between soybean yield loss in relation to the number of eggs per 100 cc of soil?
It is impossible to provide such precise information about the potential for yield loss because the amount of damage caused by SCN is very dependent on numerous interrelated factors including:
(from Dr. Terry Niblack , University of Illinois at Urbana-Champaign & Dr. Pat Donald, University of Missouri)
Soybean cyst nematodes (SCN) are not evenly distributed in a field. For example, in a single field with an average number of eggs of 50,000, the actual number of eggs in any one section of that field might be anywhere from 0 to 100,000. Due to this natural variability in the distribution of SCN, it is very important to take a sample. Some of the questions we've received about this variability in SCN egg counts are given below, along with the answers.
If a grower simply wants to sample a field to check for the presence of SCN, I'd recommend sampling the field after soybean harvest. The chances of finding SCN after a soybean crop are greater than sampling after a corn crop because soybeans are a host for SCN. Of course, if the grower discovers SCN in a soil sample taken from a field in which soybeans were just harvested and corn will be grown the next year, he or she also should collect another sample after the corn harvest in those fields if soybeans will be grown again following the corn crop. Knowledge of the SCN population densities (numbers) in a field is important in making management decision. Detailed information on the proper way to collect good soil samples can be found on the back of the ISU Plant Nematode Sample Submission Form (publication PD-32).
Under what conditions would an Iowa grower need to get an SCN race test done and how expensive is it?
I do not advocate having race tests done when an SCN infestation is first discovered. It is not necessary to determine the race that is present to decide what resistant variety to grow. In general, any SCN-resistant variety available to an Iowa grower will suppress almost any race of SCN commonly found in Iowa and will yield better than an SCN-susceptible variety. You will get maximum suppression of SCN if you match up the race of the nematode you have with the type of resistance you grow, but unfortunately, most soybean seed catalogs and sales literature do not give complete information about the SCN races to which the soybean varieties are resistant.
A good starting point is the Soybean Cyst Nematode Management Guide published by the SCN Coalition in 1999. There are many excellent SCN-related resources available on the web. Many are accessable through our links section, or are available as the following ISU Extension Nematology Publications.
SCN Biology and Management
Scouting for Soybean Cyst Nematode
Disease-resistant soybean varieties for Iowa
Interpreting SCN soil sample results