Planting Soybean Resistant To Soybean Cyst Nematode is a Widespread Practice: Should Producers Still Send in Soil Samples?

Over the course of the past 10 years there has been a steady increase in the use of soybean cyst nematode (SCN) resistant cultivars by Missouri soybean producers to manage SCN. The availability and use of SCN resistant lines has led many soybean producers to believe that SCN is no longer a major cause of yield loss. As a result, this has led to a significant decline in the number of farmers who regularly monitor their fields for SCN. SCN, however, continues to be the number one pathogen on soybean and contributes to nearly $1 billion in soybean yield losses in the US annually. The reality is that over 90% of the SCN resistant soybean lines grown in Missouri derive their resistance from one source, Plant Introduction (PI) 88788. Continuous cropping of soybean lines with SCN resistance derived from the same genetic source has the potential for losing effectiveness when combined with genetically variable SCN field populations. This type of management practice decreases the durability of current sources of resistance by selecting for nematode populations that increase well on resistant soybean lines. To maintain the durability of existing sources of resistance for effective long-term management of SCN, an integrated approach incorporating crop rotation (through the use of resistant soybean and non-host crops) and regular monitoring of SCN field populations by producers is absolutely necessary.

In this study, collaborating MU Extension Agronomists located across Missouri soybean producing regions requested soil samples from soybean producers that met one or more of the following criteria: 1) producers who are not using SCN resistant varieties 2) producers who are continuously cropping SCN resistant soybean 3) producers who when asked, do not believe that they have a SCN problem in their field 4) producers who have noticed a decline in yield over the past few years 5) producers who have never submitted a soil sample for SCN testing. A total of 122 soil samples were collected from 47 soybean producing counties in Missouri. Producers submitting samples were also asked to answer 3 short survey questions: 1) When was the last time you submitted soil samples for SCN testing? Never; Less than 5 years ago; More than 5 years ago 2) How many years have you been growing SCN resistant soybean lines? Never; 0-5 years; 6-10 years; over 10 years. 3) Do you think that you have suffered yield loss as a result of SCN during the last three years? No; Yes: 3-5 b/a, 5-10 b/a, more than 10 b/a. All 122 soil samples were processed for an SCN egg count. Twenty samples were chosen with >5000 eggs/cup for HG Type tests (which includes the old race designation).

From the 122 samples collected and processed for an egg count: 26% had "0" eggs/cup soil, 13% had a low rating (1-500 eggs/cup soil), 42% had a medium rating (501-10,000 eggs/cup soil), 19% had a high rating (>10,000 eggs/cup soil)

(Figure 1). When producers were surveyed about whether they thought they had any yield loss over the last three years because of SCN: 62% said no loss of yield, 16% said 3-5 bushels/acre, 12% said 5-10 bushels/acre, 2% said more than 10 bushels/acre, 8% said they don’t know (Figure 2). Astonishingly, 61% of the producers had egg counts above the damage threshold, yet 62% of them did not believe they had any yield loss due to SCN. The survey indicated that only 8% of soybean producers have never grown SCN-resistant soybeans, and 7% were not sure. This means that between 85% - 92% of the farmers are currently planting SCN-resistant soybeans. Most of these producers have been planting SCN-resistant soybeans from five to 15 years (Figure 3). In fact, some bootheel farmers have been growing SCN-resistant lines for 20 to 30 years. Based on the survey responses and results of the egg count assays, it appears that soybean producers are assuming that any yield loss suffered is due to factors other than SCN because they are planting SCN-resistant soybean cultivars.

When soybean producers were asked when they last sent in a soil sample to monitor for SCN, 64% indicated that they have never sent in a sample. Of the 36% who had sent in soil samples to test for SCN, only 6% of the producers had sent in a sample for SCN within the last 5 years, however, 25% had sent in samples 5 or more years ago (Figure 4). This seems to indicate that SCN was considered a problem in the past, but is not seen as a "yield-reducer" today. This is likely because many producers today are under the false assumption that SCN-resistant soybean lines have eliminated the SCN problem.

The data from the 20 HG Type tests conducted in this project also seemed to confirm the fact that 90% of the resistant lines grown in Missouri derive from PI 88788 (complete test results at http://soilplantlab.missouri.edu/nematode/scnreport05.htm). Our study indicates that this source of resistance is not holding up well against existing SCN field populations. Of the 20 tests, 85% showed an increase of greater than 10% on PI 88788. Sixty percent of the tests showed an increase greater than 25% on PI 88788. The highest test increased on PI 88788 by 80% (the lowest 4%). The average of all 20 tests showed an increase of 33% on PI 88788.

Although one must be careful in generalizing from only 20 tests, this study clearly indicates that it would be wise for soybean producers in Missouri to monitor their fields for SCN by sending in a soil sample to the MU Extension Nematology Lab (instructions at http://soilplantlab.missouri.edu/nematode/) for an egg count every 2 to 3 years. If the egg count is above the damage threshold of 500 eggs/cup, but below 10,000 eggs/cup, consider planting and rotating SCN-resistant soybeans. When the egg count exceeds 10,000 eggs/cup, producers should consider rotation with a non-host crop (e.g. corn) and/or have an HG Type (Race Test) run on a soil sample to determine which HG Type or race of SCN exists in their field. The results of this test will help the producer determine what sources of resistance would be good to manage the SCN population in their field. Resistant soybean lines should be sought that have varied sources of resistance. This is not an easy task because most varieties sold in Missouri derive resistance from PI 88788. Soybean producers should be aware of the source of SCN resistance in the soybean they are planting. Soybean producers can consult the following website, http://www.ag.uiuc.edu/~wardt/cover.htm for a good listing of the "source of resistance" for most commercial soybean lines. Soil samples may be sent to the Extension Nematology Lab, Mumford Hall, Rm 23, Columbia, MO 65211. For questions, contact Bob Heinz at 573-884-9118.