By Dr. Greg Tylka
As Iowa’s corn crop gets slowly planted and established in 2019, it’s time to turn thoughts towards pests. Plant-parasitic nematodes that feed on corn can cause damage and yield loss. Some nematode species are damaging to corn at very low population densities (numbers). But most species are not harmful until population densities reach many hundreds or more per 100 cm3 (a little less than a half-cup) of soil. And then there are some nematode species that are not thought to be harmful to corn at all. It is very common for Iowa fields to have several different species of plant-parasitic nematodes present at low numbers. A
summary of results of testing for nematodes that feed on corn in Iowa from 2000 to 2010 is available online.
Sampling to check for damaging levels of nematodes must be done during the growing season - ideally when symptoms of damage are seen. Following are guidelines on how to collect samples for assessing the potential for damage and yield loss caused by nematodes that feed on corn.
What type of sample should be collected?
Up until corn growth stage V6: collect soil and root samples.
- Use a soil probe and collect cores that are at least 12 inches long.
- Collect 20 or more soil cores to represent an area.
- Collect soil cores from within the root zones of plants with symptoms of damage (see figure).
- Combine (but do not mix) the soil cores and place them in a sealed plastic bag labeled with a permanent marker.
- Also collect, with a shovel, the root mass from 4 to 6 plants with symptoms of damage. Take care not to strip off the smaller, seminal roots (see figure). The tops of the plants can be cut off and discarded. Place the roots in a sealed plastic bag labeled with a permanent marker.
- Protect the samples from physical jarring and keep the samples cool (room temperature or below).
Figure: collecting a soil core (left) and a young corn plant for nematode testing (right)
From corn growth stage V6 through R3 (milk): collect only soil samples.
- Use a soil probe and collect cores that are at least 12 inches long.
- Collect 20 or more soil cores to represent an area.
- Collect soil cores from within the root zones of plants showing symptoms of damage. Combine (but do not mix) the soil cores and place them in a sealed plastic bag labeled with a permanent marker.
- Protect the samples from physical jarring and keep the samples cool (room temperature or below).
From corn growth stage R4 (dough) to harvest: sampling is not recommended.
There is not a reliable relationship between damage or yield loss and the number of nematodes present in soil and roots once the corn crop reaches the R4 growth stage. Therefore, sampling is not recommended after this point in the growing season.
Where to send samples?
The Iowa State University facility's location and address are: Plant and Insect Diagnostic Clinic, Room 2445 Advanced Teaching and Research Building, 2213 Pammel Drive, Iowa State University, Ames, IA 50011-1101.
The test for nematodes that feed on corn from the ISU Plant and Insect Diagnostic Clinic is called the complete nematode count. Samples sent to the ISU Clinic should be accompanied by a
Nematode Sample Submission Form (ISU Extension Publication "PIDC 32") and a check for the $35 per sample processing fee ($45 per sample for out-of-state samples).
Management options if damaging levels of nematodes are found
If damaging population densities of nematodes are found, there is nothing that can be done during the season to limit the build-up of nematode numbers and lessen the yield loss. Management options for future corn crops include use of soil-applied Counter® 20G nematicide and/or seed treatments such as Aveo®, Avicta®, Escalate®, Nemastrike™, Trunemco™, and Votivo®. Use of these management options must be decided upon before the corn crop is planted.