Say it ain’t so. Have we forgotten about soybean cyst nematodes?
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
When you’re employed in a diagnostic lab like I am, you sometimes judge the importance of a pathogen or pest on the volume of samples received by the lab. If I use sample volume as the sole criterion, I have come to the conclusion soybean growers in Michigan are losing focus on the soybean cyst nematode (SCN). From 2003-2006, we processed over 1,000 samples per year submitted under the SCN sampling program sponsored by the Michigan Soybean Promotion Committee (MSPC). The peak year was 2003 with 1,258 samples, and we averaged 1,149 samples over that four-year period. By comparison, in 2008 and 2009, we averaged 688 samples representing a 40 percent decline from the average of our peak sampling period. The MSPC has sponsored the SCN-sampling program since 1996, so what has changed? Have many soybean growers just acknowledged the presence of SCN, simply grown resistant varieties and decided sampling is no longer going to provide any useful information?
I suspect very strongly this is exactly what is occurring in Michigan. During our four-year period of 2003-2006, 65 percent of all soybean samples submitted to MSU Diagnostic Services were positive for SCN. However, in 2009, only 53 percent of the samples contained SCN (49 percent in 2007). Have we successfully eliminated the nematode from some of our fields? I assure you, this is not the case. Once introduced to a field, it is virtually impossible to eradicate SCN. But, I think this provides additional data to suggest growers with documented SCN infestations are not sampling as regularly as in the past.
This concerns me, big time. Have we forgotten the early impacts of SCN in Michigan? With new infestations, yield losses are minor and symptoms are often not even evident until the nematode has time to build in numbers to damaging levels. Then, yield losses become significant and symptoms severe until growers rotate to SCN–resistant varieties. It took two to six crops of soybeans for SCN to increase to high enough numbers to cause major yield losses when growing susceptible soybean varieties. So, you don’t think the same phenomenon is occurring because now you’re using SCN-resistant cultivars? Don’t let good yields fool you into a false sense of security.
SCN is a microscopic worm of very limited mobility. For survival, it must feed on soybeans along with a few other agronomic crops. SCN-resistant varieties retard the development and egg production of SCN females, but they don’t eliminate these things from occurring. Even under the best resistant varieties, SCN is persevering.
It is important from time to time to monitor SCN population densities even if you already know the nematode exists in a given field. Over time, if the same source of SCN resistance is used often, the nematode will increase in numbers to damaging levels just like we saw two decades ago in Michigan when we were feeling its initial impact while growing susceptible varieties. All that is required are a host (soybeans) and time. When you lose the benefits of growing SCN-varieties with the PI 88788 source of resistance, your yields will suffer. Monitoring SCN numbers through sampling is one way of monitoring change and to prevent the loss of 88788 yield advantages from occurring.
If growers are experiencing lower than expected yields when using SCN-resistant varieties, requesting an HG Type Test (replaces the old Race Test) is advised. For an HG Type Test, collection of two gallons of SCN-infested soil is recommended. Diagnostic Services can complete HG Type Tests, and they will aid in variety selection and possible diagnoses of problem yields. When conducting an HG Type Test, pure lines of the resistant sources are utilized, so if they develop successfully on these lines during a type test, you can rest assured they also will on your varieties grown in the field.
Being that my primary role as nematode diagnostician is to reduce the risk to growers of crop losses due to important nematode pathogens, the trajectory I’m observing has me nervous. Please help me abate this tension. Just because you already know you have SCN infesting your farm, please just don’t ignore it. Because, if you do, there is probably very little help I can provide you in the future.
For questions about SCN, HG Type Tests or other nematodes, don’t hesitate to call me at 517-432-1333, Angela Tenney at 517-353-8563 or Dr. George Bird at 517-353-3890. Feel free to consult our web site at www.pestid.msu.edu.