Soybean cyst nematode (SCN, Heterodera glycines) is a microscopic worm that feeds on the roots of soybean plants.  This feeding can disrupt the normal function of the roots, causing poor plant growth and ultimately reducing yields.  Once established in a field, SCN can impact yields for years if not actively managed.  Therefore, preventing the establishment of SCN into a field is critical.  However, if SCN is already present in a field, how can we stop this nematode from worming into profits?

One key point about SCN is that some field populations of the nematode have developed ability to overcome certain SCN-resistant soybean varieties, while other populations cannot.  Put another way, SCN-resistant soybean varieties may still experience yield loss due to SCN depending on the biological profile of the unique SCN population in the field.

For example, say that two neighbors plant the same SCN-resistant soybean variety in separate fields with different cropping histories and distinct SCN populations.  The first neighbor notices below average performance due to SCN, while his fortunate neighbor notices no infection and excellent performance.  What happened?  The two SCN populations on the two farms reacted differently to the same soybean variety.  Is there a way we can know ahead of time how different SCN populations will react, so that we can select an SCN-resistant soybean variety that meets our needs?

In an effort to make sense of these differences, nematologists have developed a testing system for categorizing and describing SCN populations.  This system is called the “HG Type” test.  Let’s explore what the HG Type test is and it's potential to help manage SCN infestations.

On a related, but side note…

There is an old categorization system called the “SCN race” system.  This race system is accurate, but it has been superseded in utility by the HG Type test.  That is, the HG Type test provides more useful information and is easier to implement when making variety selection and nematode management decisions.

Also, when we talk about root-knot nematode (RKN, Meloidogyne species), some root-knot nematodes are classified into distinct races.  For example, the southern root-knot nematode (M. incognita) has four distinct races, and these are distinguished by which host crops they can reproduce on.  These RKN races are different than the SCN races, and the two should not be confused.

SCN-resistance in Soybean

When developing new soybean varieties, breeders may seek to include resistance to SCN in the new variety by including a parent soybean variety that is naturally resistant to SCN.  This “source of resistance” from the parent variety makes the new variety resistant to SCN, that is, not open to infection by the nematode.

There are several unique sources of resistance available to breeders that come from different SCN-resistant soybean parent varieties.  But there is a catch: soybean cyst nematodes are very dynamic and always changing.  Over time, a SCN population may be less affected and controlled by a particular source of resistance. Researchers call this “overcoming resistance”.  Making the best use of currently available sources of resistance and slowing the rate at which SCN overcomes resistance it is at the forefront of the minds of many farm managers and researchers.

To identify which sources of resistance a particular SCN population is insensitive to (that is, that the population can overcome), a HG Type test may be performed.  This gives us information on which sources of resistance are still effective in controlling the SCN population.

HG Type Testing

The “HG” in HG Type testing stands for Heterodera glycines, the scientific name for SCN.  In HG Type testing, a soil sample is collected from a field and submitted for evaluation.  During the test, several different parent varieties that each possess a different “source of resistance” are grown in the greenhouse as a test panel.  The SCN population from the soil sample is placed onto these soybean plants.  The plants grow for some time, then the roots are assessed for the number of new SCN cysts that were produced.  These are counted for each soybean variety in the test panel.  If the SCN population from the field can reproduce on one or more of these soybean varieties (soybean lines), that means the SCN population overcomes that particular source of resistance.  This is then indicated in the HG Type number for that population.  This also offers a basic quantitative measurement, meaning an SCN population may overcome one source of resistance to a greater degree than another.

Let’s take a look at an example.  The neighbor from our story above who suffered some yield loss due to SCN wants to be sure he is better prepared for next year. He submits a soil sample for a standard nematode assay to determine the relative population of SCN in the field.  This standard report indicates SCN is present at high counts.  The farmer then submits a soil sample for HG Type testing.  The table below displays his HG Type testing report.

In this example report table, the SCN population collected from the field was able to reproduce on lines #2 (PI 88788), #5 (PI 209332), and #7 (Cloud).  Our farmer notices how the nematode counts from these three lines are higher than the other lines (#1, #3, #4, and #6), but still less than Archer, than the susceptible check.  The average count on Line #1 (Peking) is negligible and below the 10% reproduction cutoff typically used in the test.  The SCN population is able to overcome the resistance found in lines #2, #5 and #7 in the test panel, and as such, this SCN population is assigned the HG type 2.5.7.

Based on the results of this report, soybean varieties that possess sources of resistance from PI 88788, PI 209332, or Cloud, will not adequately control the SCN population in his field.  Alternative sources of resistance such as Peking, PI 90763, PI 437654, PI 89772 should be considered to offer better control. Further, the farmer also notices that the population was much more aggressive on Line # 2 (PI 88788).  That is, it had a higher average count and risk index on Line #2 than it did on Line #5 (PI 209332) and Line #7 (Cloud).

After discussing the results of the test with his crop consultant, the farmer realizes another important consideration in report.  The report shows that the soybean Lines #2 (PI 88788), #5 (PI209332) and #7 (Cloud) still had a lower nematode count (less reproduction) than the susceptible control used in the test panel (variety Archer, at the very top of the list).  The crop consultant explains that in the field where this sample was taken from, our farmer could still plant a variety with SCN resistance derived from PI 88788, PI 209332, or Cloud, and still likely see better yields than if a SCN-susceptible variety was planted.  Yet, to keep this SCN population from becoming even more aggressive, it would be a good idea to rotate to a different source of resistance, such as from PI 90763, PI 437654, PI 89772 (all of which had a risk index of 0 in the HG Type test), and/or Peking (which had a very low average nematode count).

What does this mean for SCN management?    

While using the HG type identified resistance is a mechanism to select SCN-resistant soybean varieties, it is acknowledged that at current there is often limited HG-type reported resistance for sold soybean varieties in the state. This results in practical limitations for using this strategy to identify the optimally resistant varieties for your field. However, as this information becomes more readily available for sold varieties, this will be an important component of managing SNC.

To reduce plant pathogen pressure and risk of losses due to diseases, it is beneficial to implement crop rotation.  This is because not all plant pathogens (nematodes included) can infect the same crops.  In managing soybean cyst nematode, it is helpful to rotate SCN susceptible varieties, SCN resistant varieties and non-host crops to knock back the populations from year to year.  The table below outlines rotational plan for incorporating this SCN resistance.  Further, when selecting SCN-resistant varieties, it is beneficial for long-term SCN management to rotate used SCN sources of resistance.

A standard nematode assay is warranted and follow-up HG Type testing is helpful if continued problems with SCN are experienced and sufficient control in not being reached, particularly if losses are observed in SCN-resistant varieties.  Please check with the nematode laboratory performing the test for specific instructions for collecting a good quality soil sample for the test.