Soil Testing and Plant Diagnostic Services


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Why sample: results of the latest study

Soybean Cyst Nematode (SCN) Awareness Project, January to September 2005

  • (PI) Robert Heinz, coordinator, Plant Nematology Lab
  • (Co-PI) Laura Sweets, soybean extension pathologist
  • (Co-PI) Melissa G. Mitchum, nematologist
  • (Co-PIs) Missouri regional extension agronomists in soybean producing areas
    • Sam Angima*
    • Bruce Beck
    • Karla Campbell
    • Alix Carpenter
    • Tate Castillo
    • Wayne Crook
    • Wayne Flanary
    • David Guethle
    • Matthew Herring
    • Gary Hoette
    • Jeff House
    • Don Huckla**
    • Todd Lorenz
    • Rick Mammem
    • Erin Marlow
    • Leon McIntyre
    • Pat Miller*
    • Anthony Ohmes
    • David Reinbot
*Some of the data from 2004 PPP grant in the West Central Region was randomly chosen and used for this study.
**A private consultant who helped find producers and acquire samples in open counties.

Results

One hundred twenty two soil samples were collected from 47 soybean producing counties in Missouri (representative of all soybean regions). Samples were not chosen in a totally random manner, rather, the regional agronomists sought out soybean producers that they knew and who they felt would be willing and interested in participating in this study. The farmers were asked to choose a field for sampling that they were not totally satisfied with; one that they felt was not performing up to par.

The producers were also asked to answer 3 short survey questions:

  • When was the last time you submitted soil samples for SCN testing?
    • Never
      Less than 5 years ago
    • More than 5 years ago?
  • How many years have you been growing SCN resistant soybean lines?
    • Never
    • 0 to 5 years
    • 6 to 10 years;
    • Over 10 years.
  • Do you think that you have suffered yield loss as a result of SCN during the last three years?
    • No
    • Yes: 3 to 5 bushels per acre
    • Yes: 5 to 10 bushels per acre
    • Yes: more than 10 bushels per acre

All 122 soil samples were processed for an SCN egg count. Twenty samples representing all soybean producing regions in Missouri were chosen that had medium or high egg counts (5000 to 10,000 or greater eggs per cup) and an HG Type test (which includes the old race designation) was conducted on each of these samples. The results were sent out to both the producer and the regional agronomist.

Egg count results vs. producer's opinions 

SCN egg levelsFigure 1
Soybean cyst nematode egg levels in soybean counties of Missouri


From the 122 samples collected and processed for an egg count

  • 26 percent had "0" eggs per cup soil
  • 13 percent had a Low rating (1 to 500 eggs per cup soil)
  • 42 percent had a Medium rating (501 to 10,000 eggs per cup soil)
  • 19 percent had a High rating (over 10,000 eggs per cup soil)

Yield lossFigure 2
Yield loss over the last three years


When producers were surveyed about whether they thought they had any yield loss over the last three years because of SCN, they replied

  • 62 percent said no loss of yield
  • 16 percent said 3 to 5 bushels per acre
  • 12 percent said 5 to 10 bushels per acre
  • 2 percent said more than 10 bushels per acre
  • 8 percent said they don't know

Years resistant soybeans grownFigure 3
Number of years resistant soybeans have been grown


Astonishingly, 61 percent of the producers had medium or high egg counts above the damage threshold, yet 62 percent of them did not believe they had any yield loss due to SCN. (in actuality 30 percent of these producers had counts over 5,000 eggs per cup soil, which is the high end of Medium). 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. The survey indicated that only 8 percent of soybean producers have never grown SCN-resistant soybeans, and 7 percent were not sure. This means that between 85 percent to 92 percent of the farmers are currently planting SCN-resistant soybeans. Most of these producers have been planting SCN-resistant soybeans from five to 15 years. In fact, some bootheel farmers have been growing SCN-resistant lines for 20 to 30 years. Again, it appears that producers feel that SCN is no longer a problem because they are now growing SCN-resistant soybean lines. However, our egg count results suggest otherwise.

Last sample submissionFigure 4
Last soybean cyst nematode sample submission for egg count


The last survey question also confirms our observations. When soybean producers were asked when they last sent in a soil sample to monitor for SCN, 64 percent indicated that they have never sent in a sample. Of the 36 percent who had sent in soil samples to test for SCN, only 6 percent of the producers had sent in a sample for SCN within the last 5 years, however, 25 percent had sent in samples 5 or more years ago. 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. In actuality, 90 percent of the SCN-resistant soybean lines grown in Missouri derive their resistance from one source, Plant Introduction (PI) 88788.

Table 1
HG type and race tests by region

Southeast region

Butler County

  • 61,200 eggs
  • Race 2
  • HG 1.2.5.7

Mississippi County

  • 51,750 eggs
  • Race 2
  • HG 1.2.5.7

New Madrid County

  • 33,750 eggs
  • Race 2
  • HG 1.2.5.7

Scott County

  • 141,375 eggs
  • Race 5
  • HG 2.5.7

Stoddard County

  • 88,000 eggs
  • Race 2
  • HG 1.2.5.7

Northeast region

Knox County

  • 14,850 eggs
  • Race 1
  • HG 2.5.7

Ralls County

  • 17,600 eggs
  • Race 1
  • HG 2.5.7

Scotland County

  • 22,825 eggs
  • Race 6
  • HG 5.7

Shelby County

  • 33,000 eggs
  • Race 2
  • HG 1.2.5.7

Sullivan County

  • 15,125 eggs
  • Race 1
  • HG 2.5.7

Northwest region

Andrew County

  • 12,375 eggs
  • Race 1
  • HG 2.5.7

Holt County

  • 19,525 eggs
  • Race 1
  • HG 2.5.7

Nodaway County

  • 7,700 eggs
  • Race 1
  • HG 2.5.7

Central region

Cooper County

  • 7,425 eggs
  • Race 1
  • HG 2.5.7

Saline County

  • 15,950 eggs
  • Race 4
  • HG 1.2.5.6.7

West central region*

Cass County

  • 3,825 eggs
  • Race 1
  • HG 2.5.7

Johnson County

  • 4,050 eggs
  • Race 3
  • HG 7

La Fayette County

  • 7,425 eggs
  • Race 5
  • HG 2.5.7

Platte County

  • 23,725 eggs
  • Race 3
  • HG 7

Southwest region

Barton County

  • 36,375 eggs
  • Race 2
  • HG 1.2.5.7

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 practice decreases the durability of current sources of resistance by selecting for nematode populations that increase well on resistant soybean lines. Resistant soybean lines should be sought that have varied sources of resistance, rather than from just PI 88788, so that soybean producers are not selecting for nematodes that will thrive on resistant soybean cultivars. This is not an easy task because most companies and varieties sold in Missouri derive resistance from PI 88788. It must also be noted that this process of selecting a nematode population that grows on resistant lines is not going to be the same for every producer. Farmer A may grow resistant lines for 15 years and have a serious SCN problem, while Farmer B may have grown the same lines and have a "0" egg count. There are many genetic and environmental variables in the different field populations of SCN that are not completely understood. The best thing for a soybean producer to do is to monitor their fields for SCN by sending in a soil sample for an egg count every 2 to 3 years. If a high egg count is detected, then an HG Type (Race) Test may be desired to determine what sources of resistance would be good to manage the SCN population in a grower's field. The data from the 20 HG Type tests conducted in this project seem to confirm the fact that 90 percent of the resistant lines grown in Missouri derive from PI 88788 (see below; Tables 1 and 2).

HG type test results

Ninety percent of the resistant soybean cultivars grown in Missouri are derived from the PI 88788 "source of resistance". In the HG Type test, PI 88788 corresponds to indicator line #2. The second most prevalent "source of resistance" derives from Peking , which corresponds to indicator line #1 in the HG Type test. Our study indicates that these two sources of resistance are not holding up well against existing SCN field populations (Tables 1 and 2). Eighty five percent of the 20 tests showed an increase of greater than 10 percent on PI 88788. Sixty percent of the 20 tests showed an increase greater than 25 percent on PI 88788. The highest test increased on PI 88788 by 80 percent (the lowest 4 percent). The average of all 20 tests showed an increase of 33 percent on PI 88788.

Although one must be careful in generalizing from only 20 tests, it appears that Missouri soybean producers have a long history of growing primarily SCN-resistant soybean cultivars which likely derive resistance from PI 88788 or Peking . As a result this has selected for populations of SCN that can reproduce well on these sources of resistance. In fact, the average egg count for the 20 samples chosen for the HG Type and Race Test was 30,893 eggs per cup of soil. In the past when many producers were still growing susceptible soybean lines or just starting to utilize resistant soybeans, the race tests often came back as Race 3 (HG Type 0). This was called the "wild type" and it didn't reproduce well on any of the sources of resistance. After 10 to 30 years of growing resistant lines, we seldom see a Race 3 anymore.

This study clearly indicates that it would be wise for soybean producers in Missouri to periodically monitor their SCN egg counts by sending in soil samples. If the sample is medium to high (5000 to 10,000 or greater) 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. For example, if the HG Type comes back with a "2" designation this means their SCN population can reproduce on soybean cultivars deriving resistance from PI 88788 and they should rotate with soybean deriving resistance from another source. Soybean producers should be aware of the source of SCN resistance in the soybean they are planting. Soybean producers can consult the University of Illinois Web site for a good listing of the "source of resistance" for most commercial soybean lines.

Table 2
20 HG type and race tests from across Missouri: a closer look

Race tests HG type tests
40 percent Race 1 10 percent HG 7
30 percent Race 2 5 percent HG 5.7
10 percent Race 3 50 percent HG 2.5.7
5 percent Race 4 30 percent HG 1.2.5.7
10 percent Race 5 5 percent HG 1.2.5.6.7
5 percent Race 6  

Updated 3/1/11