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.
Virtually all SCN-resistant soybean varieties available in maturity groups appropriate for Iowa soybean growers have obtained their resistance from one of two sources, soybean breeding lines called "Peking" or "PI 88788" (the PI in PI 88788 stands for Plant Introduction). PI 88788 is the more common type of resistance available in soybean varieties adapted for Iowa and the upper Midwest. Peking and PI 88788 are resistant to eight SCN races each; Peking is resistant to races 1, 3, 5, 6, 7, 8, 10, and 15 whereas PI 88788 is resistant to races 3, 6, 8, 9, 10, 12, 13, and 14. Soybean varieties with SCN resistance derived from Peking commonly are advertised in soybean seed sales literature as resistant to race 3 or races 1 and 3. In contrast, soybean varieties with SCN resistance from PI 88788 usually are advertised as resistant to races 3 and 14 (sometimes erroneously listed as race 4).
It is unfortunate that the SCN-resistant soybean varieties are advertised as resistant to only a few SCN races, but there are at least two reasons for this. First, SCN-resistant soybean varieties typically are evaluated for resistance to only a few SCN races (1, 3, and, sometimes, 14) because keeping pure cultures of numerous SCN races is extremely difficult and testing for resistance to multiple nematode races is very labor-intensive. Race 3 is the most common SCN race in Iowa (and probably many other states, as well), so virtually all resistant soybean varieties are tested for resistance to this race. Consequently, varieties with Peking-derived resistance typically are advertised as resistant to race 3, or maybe races 1 and 3 if the seed company had the resources to test the variety against race 1 and 3 of SCN. Similarly, those soybean varieties with SCN resistance derived from PI 88788 usually are advertised as resistant to race 3, or races 3 and 14 if the seed company tested the variety against SCN races 3 and 14. You cannot assume that every SCN-resistant soybean variety that obtained SCN resistance from PI 88788 is resistant to all of the eight SCN races that PI 88788 is resistant to because the soybean varieties may have not received all of the SCN resistance genes from PI 88788 during the variety breeding process. The same holds true for varieties with SCN resistance derived from Peking.
The second reason that the SCN-resistant soybean varieties are advertised as resistant to only a few SCN races is that SCN resistance is very strictly defined as 90% or more suppression of SCN reproduction relative to reproduction on the standard SCN-susceptible soybean variety, Lee. So, for example, a soybean variety that consistently suppresses 80% on the reproduction of a particular SCN race, technically, is not considered resistant to that race of the nematode. Clearly, such a variety would be quite useful for a grower with a field infested with that particular SCN race. Until recently, there was no standard way to indicate that some soybean varieties were capable of suppressing reproduction of SCN by 70% or 80%. Within the past several years, though, it has become generally acceptable for soybean varieties with 60% to 90% suppression of SCN reproduction to be characterized as "moderately resistant" to a particular SCN race. Still, most seed companies today do not test soybean varieties for resistance to more than SCN races 1, 3, and 14.
Because of the complicated factors listed above, a grower who pays for a race test (which may cost from $25 to $100, depending on the laboratory) and is informed that she or he has SCN race 7, for instance, will not be able to find a race 7 SCN-resistant soybean variety advertised in any seed catalog or brochure. Unfortunately, that grower probably will not realize that a SCN-resistant soybean variety with PI 88788 or Peking resistance likely will suppress the race 7 SCN population enough to increase soybean yields and prevent a buildup of the nematode. The "bottom line" is: you don't need to have a perfect match of SCN race with soybean resistance to suppress the nematode and to obtain increased soybean yields in SCN-infested fields - it's not a simple "all or nothing" situation.
Having said all that concerning races, we (ISU) no longer offer SCN race testing as a service. Instead, we advocate using well-adapted SCN-resistant varieties in infested fields, taking care to rotate different sources of SCN resistance periodically, and closely monitoring SCN population densities in infested fields. If an Iowa grower really wants a race test done, they should contact me by telephone, and I can refer them to a qualified plant nematology laboratory at a university in an adjacent state where such a test can be performed.