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11/7/2011 - 11/13/2011

Fall Time to Sample for SCN – But Not for Nematodes That Feed on Corn

By Greg Tylka, Department of Plant Pathology and Microbiology

Plant-parasitic nematodes are microscopic worms that live in the soil and feed on plant roots. Nematodes that feed on corn occur in almost every field in Iowa, but most do not reduce corn yields measurably until they increase to high population densities (numbers). Fall is not a recommended time to check fields for damaging population densities of nematodes that feed on corn. The ideal sampling times and methods for nematodes that feed on corn were discussed in an earlier article in ICM News.

The soybean cyst nematode (SCN) is considered by many to be the most damaging pathogen on soybeans in Iowa, the Midwest and the United States. SCN has a very unique biology that allows it to cause great yield loss (greater than 50 percent), to reproduce very quickly and to survive 10 years or more in the absence of a host crop.

Fall is a great time to sample fields for the presence and population densities of SCN.  Specific reasons to sample fields this fall for SCN include to:

  1. Discover if SCN is present before growing soybeans in 2012
  2. Determine of SCN is responsible for lower-than-expected soybean yields in 2011
  3. Monitor SCN population densities after growing SCN-resistant soybean varieties

General guidelines for fall sampling for SCN

  • Collect samples from harvested cornfields to determine if SCN is present before growing soybeans in 2012 (figure 1).
  • Collect soil cores from under the old crop rows if soybeans were grown this season (figure 2). There is no need to do this if corn was grown.
  • If grid sampling, collect one or two extra soil cores from every grid cell sample and combine these extra cores from the number of cells that represent approximately 20 acres.
  • If sampling conventionally (not grid sampling), collect 15 to 20 soil cores in a zigzag pattern from no more than 20 acres (ideally). The 20-acre sampling areas do not need to be square or rectangular; samples can be collected from zones according to the agronomic features of the field (see figure 3).
  • In fields where SCN has not been discovered, high-risk areas where SCN may be first found include high pH spots, low spots, and near fence lines and other places where soil from other fields may have been introduced (figure 4).
  • Soil cores should be a total depth of 8 inches.
  • Do not sample if fields are frozen or wet and muddy.

Numerous private soil testing laboratories in Iowa offer SCN analysis of soil samples. Additionally, the Iowa State University Plant and Insect Diagnostic Clinic tests soil samples for SCN.  Mail samples to:

Plant and Insect Diagnostic Clinic
327 Bessey Hall
Department of Plant Pathology
and Microbiology
Iowa State University
Ames, IA 50011-1020 

The current fee for SCN analysis at the ISU Plant and Insect Diagnostic Clinic is $15 per sample for samples from Iowa. Samples sent to the Plant and Insect Diagnostic Clinic should be accompanied by a completed Plant Nematode Sample Submission Form.


Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.


Figure 1. Soil sampling in a harvested cornfield to check for SCN in advance of next year’s soybean crop.

Figure 2. Collecting soil core from within the root zone of a harvested soybean crop to check for SCN.


Figure 3. Sampling areas for SCN according to the agronomic features of the field.

Figure 4. Areas of a field where soybean cyst nematode is more likely to be found for the first time.

Management Considerations for Post Flooding Soils

Mahdi Al-Kaisi, Department of Agronomy

Farmland in western Iowa and eastern Nebraska affected by flooding early this year and not planted to any crop has potential economic and soil environmental consequences if the soils are left unattended. Long-term damage to soil in areas of significant flooding need to be considered when planning for next season’s crop.

Several changes that take place when soil is under saturated conditions for an extended period of time can be carried into the next season. One of these potential changes is the change in biological health of the soil, with the greatest concern being when soil is left unplanted to any crop or cover crop. The existence of growing plants in such areas will help build up the microbial community in the root zone, which is essential to nutrient cycling, especially phosphorous.


Biological, chemical and physical soil health

Flooded soil may experience what is called “post flood syndrome,” similar to the fallow syndrome, where the land is left unplanted to any crop for the entire season. Flooded soils will encounter problems caused by the reduction of soil arbuscular mycorrhizae (AM) fungi colonization rates next growing season.

The AM fungi are colonized around the root systems of crops in a mutually beneficial (symbiotic) relationship. The fungi benefits from the host plant roots, the crop benefits from the increased nutrient uptake zone developed by the fungal hyphae (threads that make up the mycelium of fungi). Unplanted flooded areas can potentially be affected next season due to the absence of a root system that is essential to maintaining this microbial community that contributes to nutrient cycling.

In addition to potential biological changes that will be caused by flooding and the absence of active root system, there are some other chemical and physical changes that can occur when soil is flooded and left without any growing crop. Most of the chemical changes will be induced by temporary changes in oxidation and reduction conditions. However, physical-chemical-biological changes in soil such as aggregate stability, soil structure, pH, etc., can be significant, especially if there is no growing crop. 


Measures to manage previously flooded soils

Research documents that growing plants such cover crops, row crops and even weeds can increase the AM recolonization and ultimately the availability of phosphorous, which is the most affected nutrient due to reduction in mycorrhizae population. The following are a few management aspects need to be considered:

Land Leveling and Sand Cleaning – Sand cleaning depends on the depth of accumulation.

  • Few inches (i.e. 2-4 inches) can be incorporated in soil using normal field operations.  Otherwise, minimum soil disturbance is advisable to promote even weed growth till next spring.
  • If sand is up to 6 inches deep, then moldboard plow to a depth twice the sand depth to incorporate.
  • Sand 8-24 inches, it is advisable to consider spreading to areas with less sand and incorporate with special deep tillage equipment. However, it is advisable not to move sand to fill lower or severally eroded areas in the field without proper top soil to cover the sand.
  • Sand above 24 inches deep, evaluate cost of removing sand or stockpile to decide whether to remove the sand.
  • In case of severe erosion and deep cuts, top soil from surrounding fields should be used to fill such areas.

Soil Testing

  • Soil testing should be conducted after any land leveling is done.
  • Soil samples should not be collected immediately after soils dry, and may need to be collected in the spring.
  • Need to allow time for P reactions after soils aerate.
  • Potassium (K) deficiency can occur due to soil compaction.
  • Soil tests could increase from sediment deposition.

Cover Crop

  • Use a cover crop immediately after soil dries to promote growth of microorganisms that are essential for nutrient cycling.
  • Planting conditions should provide good soil seed contact for cover crop success.
  • Consider overwintering cover crops to provide additional benefits of continuous growth in the spring prior to planting.
  • When planting soybean, as a precaution seed should be inoculated with Bradyrhizobium japonicum to ensure nodulation and N fixation.
  • AM fungi inoculation of soil is not feasible.
  • Once soils become aerobic, soil microflora will recover naturally.


  • Corn growing on flooded soils showed purple leaves that were disappeared in a week.
  • Flooded fields with weeds or without tillage showed less purpling than those tilled to control weeds.
  • Fields with high manure application history (i.e., feedlots) showed no adverse effect for flooded soils on crop.
  • Crops planted after a fallow/flood period grew poorly.
  • P deficiency symptoms in crops – for corn it is slow early growth and purple coloration.
  • Flooded soils may have normal P test level and low AM population.
  • To alleviate P deficiency, high banded P rates needed – twice or more than the normal recommended rate.

This article was published originally on 11/14/2011 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.