Soybean Cyst Nematode, Part 2: Interpreting Results, Management Considerations, & HG Type

Welcome back for the second part of our discussion on soybean cyst nematode. The vast majority of this information can be found be found on the Soybean Research & Information Network https://soybeanresearchinfo.com/ , the SCN Coalition https://www.thescncoalition.com , the Crop Protection Network https://cropprotectionnetwork.org/ , and Iowa State University https://scn.plantpath.iastate.edu/

Hopefully, refamiliarization with the basic phenology and lifecycle of the number one yield robbing pest in soybeans, appropriate scouting parameters, and why soil testing has importance in identifying what your SCN counts (cyst or egg) represent are met with an attitude of intrigue and commitment to a better understand this pest. All of this in an effort to raise the bar on management tactics and helping to recapture lost bushels for your farm. Estimates from the Crop Protection Network in 2022 show the five largest disease or pathogen losses from the 14 northern states mentioned below. SCN caused an estimated 81 million bushels of loss by an amazingly large margin compared to the other prevalent disorders experienced in the same year. We hope you see why SCN knowledge is a crucial part of your overall soybean management tactics moving forward.

Interpreting your SCN results from the lab requires understanding of what life stage was counted and reported in the sample. Most labs report cyst and/or egg counts within the sample while others may include juvenile stage numbers. Remember, it’s an estimation as each cyst contains between 10 and 250 eggs each. Additionally, when comparing results ensure that they are reported in similar amounts of soil processed for the sample. Results usually are reported per 100 cc (cubic centimeters or a little less than a half-cup) or 250 cc (about a cup) of soil. Expect wide variation within your samples as nematodes tend to be aggregated in a field as they do not move readily under their own power. The variability in SCN soil test results is especially pronounced in egg counts because eggs are clustered within cysts. It would not be uncommon for two soil samples collected from the same area (e.g., 20 acres) of a field to have cyst or egg counts that vary from 100 to 300 percent. For example, one sample may contain 8,000 eggs per 100 cc of soil and the other may have 24,000 eggs per 100 cc of soil. Similarly, it would not be uncommon to have results of two subsamples from the same bag of soil vary from 100 to 300 percent. The result of a soil test for SCN, therefore, is a rough estimate of the actual population density of the nematode in the field. Fortunately, a precise measure of the SCN population density usually is not necessary to implement sound management practices. The only difficulty arises when egg densities from samples collected from different parts of the same field vary among infestation levels that have different management recommendations. Even though variability cannot be eliminated, increasing the number of soil cores, and decreasing the area from which a sample is collected make the SCN soil sample results more meaningful.

The most effective and proven management tactic is genetic resistance to this pest. Currently, there are two effective options referred to as plant introductions (PI), as in PI 88788.

Identified in in 1962 and commercialized in 1978, PI 88788 has served us extremely well but as we all know, when a single site source of action is used repeatedly, we will constantly run into issues. Think of the herbicide resistance problems we have experienced. Peking type resistance is gaining notoriety. This PI type was discovered in 1957 and with recent advances in breeding and testing techniques, we are gaining more and more traction and providing different SCN race options in high yielding genetics.
With Peking type resistance there are multiple genes involved which can give it some advantages in certain areas and potentially provide greater durability and efficacy. Corteva soybean breeders are working diligently to incorporate even more of this type of resistance into the fold and are helping to reduce our reliance of PI 88788.

We must learn to use both and rotate sources with intent until portfolios begin to offer greater numbers of Peking lines or by adopting newer methods and tactics as they become available. Alternative management strategies include growing non-host crops (corn, wheat, other small grains, and alfalfa), use of proven SCN seed treatments such as ILeVO, as well as host weed management. As for host weed management, there are a litany of pesky weeds that can provide great housing for SCN populations. Common winter annuals like purple dead nettle and henbit are great for housing for this pest. Targeted timely weed control can be very beneficial for more than one reason.

Much has been learned in the past several decades about development of SCN on resistant soybean varieties. It is apparent from newer comprehension that a change in how we describe the abilities of a SCN population to reproduce on resistant soybean varieties is warranted. This new system, called the HG Type test (HG for Heterodera glycines, the scientific name for soybean cyst nematode) has been developed and adopted. Is this specific style of test necessary for improved management? Possibly. Testing your fields and knowing what your soil’s SCN pressure is a far more critical element to helping reduce losses. Working effectively, SCN resistant varieties marketed today suppress 90 percent or more of the development of most SCN populations, resulting in a significant increase in soybean yields in SCN-infested fields. An HG type test is a greenhouse test performed on a SCN population isolated from the field to determine how well the SCN population can develop on soybean lines used as sources of resistance. Growers with extremely poor soybean yields, regardless of their best efforts to improve them in SCN infested fields, may be good candidates for exploration of this type of test, but again knowing your overall SCN number is the first best step. This concept can be a bit tricky to understand here are some examples.

HG type 2 designation means the SCN population has at least 10 percent reproduction on PI 88788 (indicator line 2).
HG type 1.2 indicates that the SCN population has ≥10 percent reproduction on indicator lines 1 and 2, which are Peking and PI 88788, respectively.
HG type 0 SCN population has less than 10 percent reproduction on all indicator lines
Individual numbers in the HG type designation mean the SCN population has ≥10 percent reproduction on those indicators or breeding lines regardless of what other numbers are in the HG type designation.
- Example, the number 1 in an HG type designation means the SCN population will have at least 10 percent reproduction on Peking resistance whether the nematode population is an HG type 1, a type 1.3, a type 1.3.6, or a type 1.2.5.7
HG type test is similar to the previous SCN race test but uses all soybean breeding lines that are available as sources of resistance in SCN-resistant soybean varieties.
The table below shows how to convert the 16 SCN races to HG types. Note that multiple races have the same HG type designation.