By Greg Tylka, Department of Plant Pathology and Microbiology
Most Iowa cornfields harbor one or more species of these microscopic, plant-parasitic worms. At least 28 nematode species have been found associated with corn in Iowa. Most are not thought to cause damage at low population densities (numbers). The estimated damage threshold for most nematodes that feed on corn in Iowa ranges from 100 to 1,000 worms per half-cup (100 cc) of soil (see August 2009 ICM News article). Only dagger, needle and sting nematodes are considered damaging at low numbers. The results of Iowa State University testing, from 2000 to 2010, for nematodes that feed on corn in the state were summarized in a recent ICM News article.
Sample timing is key
Knowing the types and numbers of nematodes present in a field is useful in gauging the possible benefits of nematode-protectant seed treatments or soil-applied nematicides. Unfortunately, most nematode numbers will be low in spring soil samples, even though they can develop to higher, damaging levels later in the season. Because of this, results of spring soil samples usually are not useful and collecting such samples in the spring is not recommended.
The only situation in which spring sampling for nematodes that feed on corn is warranted is in fields where the soil has at least 70 percent sand. These fields may be infested with the needle (figures 1 and 2) and sting nematodes, which are damaging at very low numbers. The needle and sting nematodes migrate to deeper depths in the soil in the middle of the growing season and can be missed in mid-season soil samples that would be collected for the other nematode species. Consequently, spring soil sampling is recommended for these two nematode species in sandy fields.
Collect and store the samples properly
Spring sampling of sandy soils for nematodes is simple:
- Use a soil probe and collect cores that are at least 12 inches long.
- Collect 20 or more soil cores to represent an area.
- Sample from areas of the field where corn has not grown well in previous years.
- Combine, but do not mix, the soil cores and store them in a sealed plastic bag that is labeled with permanent marker.
- Protect the samples from physical jarring and from high temperatures (above room temperature).
- Send a completed Plant Nematode Sample Submission Form along with the soil samples to the Iowa State University Plant and Insect Diagnostic Clinic at the address below, requesting a complete nematode analysis.
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 a complete nematode analysis (the test for nematodes that feed on corn) at the ISU clinic is $30 per sample for samples from Iowa. Call the ISU clinic at 515-294-0581 to check about processing of soil samples from outside Iowa. A list of other land-grant university facilities that process samples for nematodes that feed on corn is available online.
More information about nematodes that feed on corn
There have been numerous articles in ICM News in the past several years that discuss aspects of the biology, sampling and management of nematodes that feed on corn. To find these articles, search for "nematodes on corn" in the search box in the upper left corner of the ICM News home page.
Also, there is an ISU Extension bulletin titled Nematodes That Attack Corn in Iowa (PM 1027), that was written by Don Norton, former ISU research nematologist and expert on nematodes that feed on corn.
Figure 1. Corn roots damaged by needle nematode feeding.
Figure 2. Needle nematodes with damaged roots and sewing needle. Photo credit Purdue University IPM Program — watch video.
Greg Tylka is a professor with extension and research responsibilities in management of plant-parasitic nematode in the Department of Plant Pathology and Microbiology at Iowa State University. He can be reached at firstname.lastname@example.org or 515-294-3021.