Not Just Hype: Know Your Type (Soybean Cyst Nematode Type Test)
Author: Fred Warner, Angela Tenney and G. W. Bird
Results of Soybean Cyst Nematode Type Testing 2014 and 2015
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. If a population of SCN develops well on one or more sources of resistance, those sources should be avoided in the near future 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 two years, 59 SCN populations were typed. The results of these tests are provided.
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*||Number of populations||% of total|
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 are troubling. It is obvious a large percentage of our SCN populations are over coming 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|
|Peking (1)||PI 88788 (2)||PI 437654 (4)|
|No Development||14 (22%)||0||63 (98%)|
|Resistant||30 (47%)||4 (6%)||1 (2%)|
|Moderately Resistant||18 (28)||26 (41%)||0|
|Slightly Resistant||2 (3%)||30 (47%)||0|
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.
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 more. Growers need to take steps to prevent this cycle from occurring. 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.
Another alarming trend is the number of type 1.2 populations. In maturity groups 0-3, there are about 800 commercially available SCN-resistant soybean varieties and roughly 775 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. 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 haven’t necessarily yielded well in MI.
For assistance with selection of SCN-resistant varieties, it is recommended you consult Iowa State University’s SCN-Resistant Soybean Variety Trials at http://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. The evidence seems pretty clear that in MI, many of our SCN populations are close to rendering varieties with PI 88788 resistance susceptible and this would be a very negative outcome since so many of the commercially available SCN-resistant soybean cultivars have this source of resistance. Couple this phenomenon with the additional observation we seem to have increasing numbers of type 1.2 populations, producing high yielding soybeans is going to become more challenging unless we develop a new technology(s) or discover new sources of resistance. Now is the time to take action to slow or stop these developing trends.