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Soybean cyst nematode (SCN) is known to cause considerable yield loss in soybeans. Often that loss is accompanied by undetectable crop symptoms and options to control it seem increasingly limited.

 

In an earlier blog SCN: The Soybean Nemesis, Dr. Nathan Kleczewski at the University of Illinois stated “You may not realize it, but soybean cyst nematode (SCN) remains the most important yield-robbing pathogen of soybeans in the United States. In 2015, over 109 million bushels of soybeans were estimated to have been lost because of this pesky roundworm. However, often these losses go unnoticed or are blamed on other issues.”

 

The standard recommendation for the last two decades has been to rotate soybeans with corn or other non-host crops and plant resistant varieties with the genetic resistance (mainly PI 88788). Today we can add in nematode-protectant seed treatments. In a recent webinar sponsored by Seed World, Dr. Greg Tylka said SCN ranks No. 1 in the U.S. and No. 2 in the world behind soybean rust as a threat to soybean yield. He added the root lesion nematode as a threat to soybeans in the Midwest, albeit much less than SCN. 

 

Tylka added “I consider it public enemy No. 1 and particularly when rainfall is limiting, and temperatures are high and in such extreme situations yield loss can be as much as 50%. And in situations with no field symptoms, yield loss can be 10% or more. And even resistant varieties have yield loss. That is how severe the threat is.”

 

Currently, there are three main sources for genetic resistance to SCN; PI 88788, PI 548402 (Peking) and PI 437654 (Hartwig and CystX). It is important to rotate resistance when you plant soybeans, particularly in fields at high risk or with large egg counts. Unfortunately, it’s easy to find varieties with PI 88788 resistance but difficult to find varieties with Peking or Hartwig/CystX® resistance. Rotating resistance is key to successful SCN management. Click here to read the Pioneer review of rotating resistance.

 

Is PI 88788 weakening? Tylka said resistance was defined as no more than 10% SCN reproduction on roots, but today it can be up to 50% reproduction. However, he emphasized that today’s varieties can endure higher infections while maintaining yield. “PI 88788 is widely available in today’s varieties and growers have to continue to use it. There are 30-40 varieties in the Northern Corn Belt with Peking resistant. While they don’t yield as well in fields without SCN, they have good yield potential in fields at risk. Scientists continue looking at new sources of genetic resistance, but those lines have to meet today’s high yield potential.”

 

With limited tools to control SCN, it is important to sample soils to understand the threat in the soil below. If egg counts are 10,000 or higher it probably behooves you to rotate away from soybeans for a few years. Click here to read about fall sampling. Use the following scale to assess risk to soybean yield.

• Low: 500 to 3,000

• Medium: 3,000 to 10,000

• High: >10,000

 

Today there are several novel seed treatments available. Tom Kroll, Technical Services Manager at Nufarm explained that these fall into three categories direct impact on SCN, stimulating a plant’s natural defenses and creating a barrier to infection. He emphasized three points. The active ingredient must be safe on the crop, safe for the product handler and performance must be consistent. He listed different commercial seed treatments that are efficacious against nematodes in general. For more information on seed treatments that control SCN click here to review the Iowa State University guide.

 

Kroll emphasized that seed treatments have their biggest impact in the beginning. but don’t provide season-long control. And that seed treatments are part of a package of controls that include resistant varieties and crop rotation.