Everyone who has soybean cyst nematode (SCN) knows that there are many good SCN-resistant varieties available. Everyone has also heard horror stories of resistant varieties that didn't work. Believe it or not, it is possible for a resistant variety to have good resistance that fails in certain fields.|
There are two main explanations for "resistance failure." First, if you grow the same resistant variety two or more times in a row (even with corn in between soybean seasons), you may have selected a nematode population that is adapted to that variety. Second, if you have extremely high numbers of SCN, they will cause yield loss in resistant varieties.
You can protect the useful life of resistant varieties by making sure you don't plant the same variety in the same field. With so many varieties to choose from (see, for example, http://www.cropsci.uiuc.edu/vt/), you shouldn't have much trouble finding alternatives. If you have extremely high numbers of SCN (say, more than 10,000 eggs/250 cm2 soil), the best thing to do is to plant a nonhost (such as corn).
If you don't have extremely high numbers of SCN and you haven't used the same resistant varieties in the same field, then you might have a population of SCN that is virulent (causes disease) on certain resistant varieties. Since 1970, we have identified virulent populations of SCN in terms of a "race test." Race 3 SCN was essentially avirulent (unable to cause disease) on resistant varieties, whereas the other common races in Illinois--races 1 and 5--were virulent on some resistant varieties.
The SCN race test will soon be replaced by a new test, called the HG-type test (HG is for Heterodera glycines, the scientific name for soybean cyst nematode), developed by agronomists, nematologists, plant pathologists, and soybean breeders.
An HG-type test is a greenhouse test performed on an SCN population isolated from a field to determine how well the SCN population can reproduce on various sources of resistance used in developing SCN-resistant soybean varieties. The test is similar to the old race test, except that the old test used three sources of resistance and the new test uses seven, giving us a lot more information about SCN populations. We call these seven soybean lines "indicator lines" because they indicate the virulence characteristics of the SCN population.
To determine the HG type of an SCN population, the nematode population from the field of interest is grown on the seven indicator lines under controlled conditions. After 30 days, enough time for a single generation of the nematode, the numbers of SCN females that form on the roots of each indicator line are counted and compared to the number of females that formed on a standard susceptible soybean variety. Finally, we note which indicator lines show elevated SCN numbers. Armed with this knowledge, we can recommend resistant soybean varieties that have effective resistance to the SCN population that is present in the field.
The HG-type test will become "legal" when it is published later this summer in a scientific journal. Until that time, the old race test will continue to be the standard for testing SCN virulence in most places. Seed companies will probably not incorporate information relating to HG types in their variety descriptions until after the test is published and the companies have time to determine how to deal with it. Expect to begin to see information on HG types later in 2002.
Do you need an HG-type test? Probably not. Most recommendations can be based on the number of SCN in the field and the field history. But if you think you need one or want more information, contact nematologist Terry Niblack, firstname.lastname@example.org, (217)244-5940.--Terry Niblack