This article was updated on November 28, 2016.
Unfortunately, soybean cyst nematode (SCN) isn’t being talked about much anymore. It is known to be the #1 threat to soybean yield around the world, and in the North Central U.S. we have been depending on the PI 88788 source of resistance in 95% of our seed market share. A smaller percentage source of soybean resistance against SCN comes from Peking, PI 437654, or combinations. What happens when we depend on a predominate source of resistance for an extended period of time? SCN populations can and do adapt to this source of resistance and the problem becomes more serious.
Resistance isn’t new, as this was also documented in 2007 and 2008 by Mitchum, et al. and Niblack, et al., respectively. But, as SCN within soil samples is screened at the university level, the problem appears to be getting worse. There has been an increase in the number of SCN populations that can survive on PI 88788, and within these populations there are a greater number of female nematodes, each of which can produce 200 to 500 eggs. Each nematode has different genes and those that are unaffected by plant resistance can pass those genes to their offspring (by reproducing sexually), thus increasing the number of SCN that are unaffected by P1 88788, read as “resistance to Race 3 and Race 14” on a bag of seed.
Figure 1. The experimental variety on the left consists of P1 88788 source of resistance and the experimental variety on the right has no P1 88788 resistance in a Burrus research plot located near Hillview, Illinois. We plan to test all of our Burrus soybean plots for soybean cyst nematode and its ability to resist SCN.
SCN can be difficult to diagnose because they are located within patches across a field, and symptoms may not be noticeable in favorable growing seasons with adequate rain. In cases where there are high populations, SCN cysts might be visible on roots and there could be poor nodulation, uneven soybean growth, stunted plants, or low number of pods or beans per pods. You can test for SCN by soil sampling and sending off a sample to the U of I Plant Clinic or a commercial soil laboratory. Once the SCN sample goes to the Plant Clinic or laboratory, they will sieve, process and do SCN egg counts.
Here is a chart of thresholds and management for each SCN Egg Count per 100cc of soil. This chart can be found here. If you have more than 10,000 eggs, per 250cc of soil, you shouldn’t plant soybeans and rotate out of the crop for several years till egg count declines. The U of I Plant Clinic test results or egg counts are based on 250 cc, so if you have results from a 100cc test, it needs to be multiplied by 2.5 to see if you are at the 10,000 egg threshold, so that you don’t significantly underestimate the SCN population.
Figure 2. Thanks to the University of Illinois for SCN populations, so that we could demonstrate P1 88788 resistance (back plants) as well as demonstrate various seed treatments such as PS SDS (ILeVO®) seed treatments at the Burrus Technology Day.
Another option is to have an HG (Hetorodera glycines) test on an SCN population within a field. The sample is tested against seven different soybean indicator lines that are given numbers 1 – 7. A female index is determined after a 30-day greenhouse test, and it is the average number of SCN females produced on the HG type indicator line relative to the number produced on a standard, susceptible soybean cultivar.
Recent research indicates that an HG type test is most meaningful if a soil sample of 100 or more cores is collected throughout a field, as the results of the HG type test can vary. Ultimately, the HG test can tell you if you have a SCN population in your field that can reproduce on a variety carrying the P1 88788 source of resistance.
The integrated management of SCN has not changed. Crop rotation to non-hosts such as corn can help, but this does not eliminate SCN populations, it just decreases them. SCN cysts can remain in the soil for several years and each cyst can contain more than 100 eggs. When they hatch, the juveniles will migrate to the plant root, set up feeding sites and steal nutrients, which can result in yield loss. SCN have also been deemed the “gateway for disease.” There are new seed treatments (chemical, biological and plant-health regulators) that offer early season SCN control, but this is not considered a “stand alone” treatment. We have come to realize that the use of a single source of resistance is not a viable long-term strategy for managing SCN, but there is hope after the recent discovery of genes underlying SCN resistance. You can learn more at Soybean Cyst Nematode, Heterodera glycines, an Update.
Stephanie Porter is a Sales Agronomist with Burrus® Hybrids with responsibilities that include educating growers and Burrus staff on all types of pests, weeds, diseases and other agronomic issues that affect corn, soybean and alfalfa production. Her territory encompasses Southern Wisconsin as well as Northern, Eastern and Southern Illinois.