Plant & Pest Diagnostics

Introduction

A soybean cyst nematode (SCN) type test is an assay that characterizes an SCN population. SCN type testing can aid growers in selection of SCN-resistant soybean varieties. The test measures nematode development on three sources of SCN resistance found in commercially available soybean varieties (this will become four sources beginning in 2021).  If a population of SCN develops well on one or more sources of resistance, those sources should be avoided if possible if managing an SCN population is a priority.

Beginning in 2014, the Michigan Soybean Promotion Committee (MSPC) agreed to pay for SCN type testing for growers. Over these seven years, 271 SCN populations were typed. Only samples where we recover greater than 2,500 SCN eggs and second-stage juveniles (J2s) from 100 cm³ soil qualify for type testing.  The SCN sampling program is also sponsored by the MSPC and has been on-going since 1996.

The SCN type testing results are broken out into three time periods. In 2014 and 2015, SCN type 2 populations were most common. From 2016 – 2018, type 1.2 and type 2 populations were detected equally. However, over the past two years, SCN type 1.2 populations are slightly more dominant. These types are explained in detail in the rest of this document.

Type Testing Results from 2014 and 2015

Thirty-three SCN type tests were performed in 2014 whereas; 26 tests were done in 2015. The results are shown in Table 1.

Table 1. SCN types identified in MI, 2014 & 2015.

 *Type 0 = none to very little SCN development on any resistant lines; Type 1 = development on Peking; Type 2 = development on PI 88788 and Type 1.2 = development on Peking and PI 88788

Of the 59 populations tested, it was determined that the PI 88788 line was no longer resistant in 55 (93%) of those cases. Since PI 88788 is the resistance source in about 97% of all commercially available SCN-resistant soybean varieties, these results were troubling. It is obvious a large percentage of our SCN populations have overcome 88788 resistance.

The quantitative information provided in a type test is also very important. Based on the amount of development observed, the resistant lines can be categorized as resistant, moderately resistant, slightly resistant, and susceptible to the SCN populations tested. In Table 2, the results of the 59 SCN plus the 5 HG type tests (more sources of resistance are used) done in 2014 and 2015 are broken down into these categories. We also included the number of tests where no SCN development was observed (these can be added to the resistant results).

Table 2. SCN types divided into categories based on resistance levels.

PI 88788 was only slightly resistant or susceptible to 34 (53%) of the SCN populations tested. In these situations, any varieties with 88788 resistance are probably going to experience yield losses from a few up to 20 or possibly more bushels/A. However, they will still, in most cases, yield better than SCN-susceptible varieties but they will not yield to their potentials.

Type Testing Results from 2016 and 2018

Twenty-seven SCN type tests were performed in both 2016 and 2017 and 71 tests done in 2018 bringing the total to 125 tests over this 3-year period. These results are shown in Table 3.

Table 3. SCN types identified in MI, 2016 – 2018.

 *Type 0 = none to very little SCN development on any resistant lines; Type 1 = development on Peking; Type 2 = development on PI 88788 and Type 1.2 = development on Peking and PI 88788

Of the 125 populations tested, it was determined that the PI 88788 line was no longer resistant in 120 (96%) of those cases. This figure represents only a slight increase over what was observed in 2014 – 15 (93%). It is still obvious that most of our SCN populations have overcome resistance to PI 88788. A troublesome trend that has emerged is that more of our populations (61 out of 125 or 49%) have also overcome resistance to Peking.

Table 4. SCN types divided into categories based on resistance levels for populations tested between 2016 – 2018.

This 3-year period suggested that we are beginning to observe more of our SCN populations overcoming resistance to Peking. In 2014 & 2015, we observed no development of SCN on Peking in 22% of the population tested but that decreased to 12% in 2016 – 2018. The Peking line was resistant to 69% (no development plus resistant) of our SCN populations tested in 2014 & 2015 but that fell to 48% in the 2016 – 2018 timeframe.

Type Testing Results for 2019 and 2020

Ninety-two SCN type tests were conducted over this 2-year period, 57 in 2019 and 35 in 2020 (see Table 5).

Table 5. SCN types identified in MI, 2019 – 2020.

 *Type 0 = none to very little SCN development on any resistant lines; Type 1 = development on Peking; Type 2 = development on PI 88788 and Type 1.2 = development on Peking and PI 88788

Of the 92 populations tested, it was determined that the PI 88788 line was no longer resistant to any of them. It is also obvious that more of our SCN populations have also overcome Peking resistance.

This was the first time period where no SCN type 0 populations were observed. Type 0 populations are preferred because all the lines tested were resistant. This makes variety selection quite simple. Those days appear to be history.

Table 6. SCN types divided into categories based on resistance levels for populations tested between 2019 & 2020.

Conclusions and Implications

These type testing results over the past 7 years indicate that virtually all our SCN populations have overcome resistance to PI 88788, the most common source of resistance against SCN. It is critical to recognize as soybean varieties with the PI 88788 source lose their resistance to type 2 SCN populations (these SCN populations become more virulent), these beans will lose yield and SCN numbers will increase. As SCN numbers increase, yields will be reduced even more. Growers need to take steps to prevent significant increases in SCN population densities (numbers). Type testing and using varieties with different sources of resistance will slow the process. Protecting resistant varieties with seed treatments will also be beneficial if these treatments are effective. Longer crop rotations will also be necessary.

Another alarming trend is the growing number of type 1.2 populations. In maturity groups 0-3, at least 90% of commercially available SCN-resistant varieties have PI 88788 resistance. Varieties with Peking resistance comprised about 80-90% of those remaining cultivars. So, as difficult as it can be to locate Peking varieties, it is nearly impossible to find ones without PI 88788 or Peking sources of resistance. For instance, varieties with PI 437654 resistance are the best choices for managing type 1.2 populations but even if you can locate a variety, they have not necessarily yielded well in MI (mostly anecdotal information). A new source of resistance (PI 89772) will be available to growers in limited supplies. In 2021, we will include this line in our SCN type testing for the first time. This should provide some baseline data prior to growers using this source in their fields.

A couple of caveats regarding this information need to be mentioned. First, although we conducted close to 300 SCN type tests over the past seven years, this still represents a fairly, small sample size when compared to the number of fields where soybeans are produced in MI. In addition, these tests were not the results of a random survey of soybean producers. These tests were done on samples with moderate to high numbers of SCN, so virtually of the growers who requested these tests undoubtedly have used SCN-resistant soybean varieties in the past and have experienced yield losses.

For assistance with selection of SCN-resistant varieties, it is recommended you consult Iowa State University’s SCN-Resistant Soybean Variety Trial results at https://www.plantpath.iastate.edu/tylkalab/iowa-state-university-scn-resistant-soybean-variety-trials. Dr. Greg Tylka does a great job making this information available to growers and is regarded as an SCN expert in the US.

If growers want to ensure high soybean yields in the future, managing the genetics of SCN populations is critical. We can observe genetic changes in a population by type testing. Not surprisingly, growers tend to focus most on yields but may not track population changes in the numbers and virulence of their SCN populations. These factors will greatly influence soybean yields and to minimize the risks of type shifts, growers should strive to keep SCN numbers as low as possible.  

If you would like a free SCN analysis, please be sure to complete the Michigan Soybean Committee soybean cyst nematode submittal form that states that the sampling program is funded by soybean checkoff dollars (one form per sample, please). This form is the identifier for this program and can be found on the Plant & Pest Diagnostic lab website at https://www.canr.msu.edu/pestid/forms, the Michigan Soybean Committee website at https://www.michigansoybean.org/sample-submission.html or by visiting your county office. To request SCN type testing, be sure to circle the yes option under the type test question.   It is imperative, if you desire a type test to submit at least one quart of soil since we need 30,000 SCN eggs to conduct the test (5 soybean lines, 3 replications per line with 2,000 eggs per plant). Pint-sized A & L bags are insufficient unless we recover extremely high numbers of SCN (>25,000 eggs) in 100 cm³ of soil (roughly 1/5 of a pint). The cost, in 2021, for an SCN type test (also referred to as a “mini” type test) is $75.00 (5 lines at $15.00 per line) and the fee is $120.00 for a HG type test (8 lines at $15.00 per line, a “full” type test). Please keep in mind, we need a minimum of 45 days to conduct type tests because the nematodes must develop from eggs to adults on the roots of the lines we screen.

If you have questions about sampling for SCN or type testing, please contact MSU Plant and Pest Diagnostics. Angie Tenney can be reached at 517-353-8563. Other options include Dr. Marisol Quintanilla on campus, Mike Staton in Allegan county, and Mark Seamon with MSC as they all have knowledge regarding SCN. We would like to thank the Michigan Soybean Committee for their support over the years. From 1996 – 2020, soybean checkoff dollars have paid for 24,678 SCN samples as well as the 271 SCN type tests, the results of which were shared here.