Wondering if plant-parasitic nematodes are the cause of stunted, yellow corn?

Plant-parasitic nematodes that feed on corn are relatively common in Iowa fields. But the presence of most of these nematodes does not mean that they are causing damage. Fields must be sampled to determine which types of nematodes are present and at what numbers to determine if damage is occurring.

Symptoms of nematode damage to corn

Only corn fields showing symptoms of nematode damage need be sampled. Symptoms of damage are swollen roots, dead root tips and dead areas or lesions on the roots (see figure 1), stunted roots and/or shoots, and yellow foliage (see Figure 2).

How to sample depends on the growth stage of corn

Sampling at V6 corn growth stage or earlier:
Collect a soil and root sample as described below.

• Angle a soil probe underneath corn plants (from within the root zone) and collect at least 20 soil cores 12 inches long from an area exhibiting symptoms (see figure 3).
• Combine (but do not mix) the soil cores and place them in a sealed plastic bag labeled with permanent marker.
• With a shovel, collect the root mass from 4 to 6 plants showing symptoms of damage (see figure 4). Be careful not to strip off the smaller, seminal roots. The tops of plants can be cut off and discarded. Place the roots in a sealed plastic bag and label with permanent marker. Do not add water to the bag.
• Protect soil and root samples from physical jarring, and keep samples at or below room temperature.

Sampling from growth stage V6 through R3:
Collect soil samples as described above, but not roots for corn that has developed past V6 but has not reached the R4 or dough growth stage.

Sampling at and after growth stage R4:
Once corn reaches the R4 or dough stage, nematode population densities are not consistently related to the potential for yield loss. Therefore, sampling is not recommended

Where to send samples?

The Iowa State University Plant and Insect Diagnostic Clinic processes samples for nematodes that feed on corn. The test is called a complete nematode count. Samples sent to the ISU Clinic should be accompanied by a Nematode Sample Submission Form (ISU Extension Publication "PIDC 32", found online here). The submission form has instructions on where to submit the samples. There is a $40 per sample processing fee ($45 per sample for out-of-state samples).

Many other university plant disease clinics and nematode diagnostic labs also process samples for nematodes on corn. A list of the university laboratories and their contact information is available online here. And samples for nematode analysis can be sent to some private soil-test laboratories in Iowa and surrounding states. Check with the private labs to determine if they process nematode samples for corn before sending the samples.

Damage thresholds

The number of nematodes that are damaging to corn varies greatly among the nematode species (see table 1).  The likelihood of yield loss can be gauged only by determining the types and numbers of nematodes present in a field.

Table 1. Nematode population densities corresponding to three risk categories for damage and yield loss based on root and soil samples collected from planting through corn growth stage R3.

The range of nematode population densities for the risk levels are the consensus opinion of university agronomists, plant diagnosticians, and plant nematologists from Arkansas, Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, Ohio, and Wisconsin developed at a workshop held in 2011.

* No population density of these nematodes is considered not damaging. These nematodes occur only in very sandy soils.

**No population density of these nematodes is considered medium or high risk of causing yield loss; other environmental conditions usually must be present and providing added stress for yield loss to occur.

Managing nematode damage on corn

If damaging population densities of nematodes are found in a corn field, there is nothing that can be done during the season to slow the build-up of nematode numbers and lessen the yield loss. However, information on which nematodes are present in the field will be useful in planning to manage the pests in the future.

Management options for future corn crops include using soil-applied Counter® 20G nematicide and/or seed treatments such as Avicta®, Lumialza™, Nemasect™, Trunemco™, and Votivo®.