Soybean Cyst Nematode: The Greatest Threat to NY Soybean Production is Here to Stay. Now What?

J. Cummings, K. Wise, and M. Zuefle, NYS Integrated Pest Management Program;
E. Smith, M. Hunter, M. Stanyard, A. Gabriel, K. Ganoe, J. Degni, J.L. Putman, K. O’Neil, J.A. Putman, J. Miller, and M. Lund; Cornell Cooperative Extension
M. Dorgan, NYS Department of Agriculture and Markets

The soybean cyst nematode (SCN) is the number one pest of concern in U.S. soybean crops, causing an estimated $1.5 billion in annual losses. Considered a ‘silent’ yield-robber, SCN can cause 10-30% yield loss without any obvious, above-ground symptoms. SCN hadn’t been considered a pest of concern for NY soybean growers before it was first confirmed in Cayuga County in 2016. Even then, it wasn’t a priority consideration. However, based on recent findings, NY soybean (and dry bean) growers can no longer afford to ignore this threat.

The NY State Integrated Pest Management Program, in collaboration with Cornell Cooperative Extension field crops specialists, and funded under a grant from USDA-APHIS Plant Protection Act section 7721 administered by NY State Department of Agriculture and Markets, coordinated a statewide SCN survey in 2019 as part of a soybean commodity survey to test 25 fields. This testing revealed an additional six counties with fields positive for SCN. Those results prompted a continuation and expansion of a SCN survey in 2020 with additional funding from the NY Soybean Checkoff dollars to provide testing for 100 fields statewide. The 2020 survey identified an additional 22 counties with fields positive for SCN. These surveys, along with shared observations from individual testing efforts have now confirmed SCN in a total of 30 counties across NYS, and it is safe to assume that SCN will likely be identified in all soybean producing counties in NYS with continued testing in future years.

NYS maps — Figure 1. The progression of confirmation of the soybean cyst nematode throughout NY State. Counties shaded in green had fields tested with negative results, and counties shaded in red have at least one field confirmed positive for soybean cyst nematode. (All testing was conducted by the SCN Diagnostics Laboratory at the University of Missouri in 2019 and 2020)

Now that we know that SCN is here, and is widespread across NYS, what’s next? Unfortunately, eradication is not an option, but reduction and maintenance of low populations is. Management strategies depend on SCN population levels, which can vary significantly from field to field. Regular testing for this nematode will help you determine your best plan of action for management. Fortunately, most of our positive SCN detections have been in the “low” category, but we found four fields with “moderate” levels and one field with “high” levels of SCN. For reference, based on test results (according to University of Missouri SCN Diagnostics Laboratory), “An egg count of <500 eggs is considered low. An egg count of 500-10,000 is considered moderate. An egg count >10,000 is considered high”. Those egg counts are based on what they find in one cup of soil. Finding a field in NYS with an egg count of 20,000 was quite surprising this year, and it translated to measurable yield loss for the grower. This means we can’t afford to ignore this pest, and we need to start actively managing SCN before our “low” results all become “high” results.

photos of soybesn cysts — Figure 2. Female SCN nematodes produce ‘cysts’ on soybean roots, which contain the eggs (a). These cysts, when dislodged from the roots, are distributed within the soil (b). The cysts contain approximately 200 eggs each (c). (Images courtesy of G. Yan, S. Markel and E. McGawley, via the SCN Coalition)

It’s much easier to stay ahead of this pest than to try to manage high numbers. Fortunately, our number one management strategy is crop rotation. Once you know you have SCN in a field, the worst thing you can do is grow soybeans continuously. We are lucky to have a number of non-host crops available for rotation, including corn, small grains, clover, alfalfa, and forage grasses. Studies have shown that a one-year rotation to corn may result in up to a 50% reduction in SCN populations the following year. The next best option for managing SCN is by selecting and planting SCN-resistant soybean varieties, and rotating those varieties that you plant. More on that later. For dry beans, however, resistance is not an option, and rotation is even more critical. The third management option is the use of nematode-protectant seed treatments. There are a number of these products available, and most have shown promising results. However, those seed treatments will be most cost-effective in situations where there is high SCN pressure. So, for the vast majority of acreage in NY, based on our current survey results, the seed treatments can be an expensive option with limited benefits for many of our growers. But, that may change as SCN testing expands and we find more moderately to highly infested fields. Of course, an integrated pest management (IPM) approach will provide the best management results, by combining all available management tools.

It’s impossible to talk about SCN management without mentioning resistance. I said previously that you should consider selecting and planting SCN-resistant varieties, and that you should rotate those varieties. Unfortunately, SCN has been evolving and developing resistance to the traits most commonly available in commercial soybean varieties for decades. Slowly, SCN has developed different races that can overcome the resistant soybean varieties. This pest is highly adaptable. That’s why it’s important not to plant the same soybean variety, even if it’s labeled as ‘resistant’, in the same field repeatedly. Similar to chemical modes of action (like herbicides), it’s critical to rotate your tools to avoid, or minimize, resistance development. For more information on this topic, please visit the SCN Coalition website, where they have an abundance of resources available on this topic. Luckily, a number of major seed companies have soybean varieties in the pipeline with novel sources of SCN resistance, and we look forward to the new options.

soybean roots with cysts — Figure 3. SCN cysts are tiny, but can be seen on soybean roots with or without magnification. Much smaller than nodules, the cysts appear as whitish-yellow specks along the roots. (Images courtesy of G. Tylka via the SCN Coalition)

Moving forward, we hope to continue providing statewide SCN-testing services to growers through funded surveys. Please contact your local Cornell Cooperative Extension specialist if you suspect you might have SCN on your farm. Continued monitoring through testing will help us understand our populations of SCN, to help make the best management decisions. Let’s work together to maintain mostly low to moderate populations of this potentially devastating pest.

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