Sampling Corn Fields to Assess Plant-parasitic Nematodes

Plant-parasitic nematodes that feed on corn are relatively common in Iowa fields. One or more species of these nematodes were found in 92% of samples submitted to the Iowa State University Plant and Insect Diagnostic Clinic from 2000-2010 (ICM News summary article).

The presence of nematodes feeding on corn does not mean that damage and yield loss are occurring in a field. Of the 92% of samples from Iowa mentioned above with at least one species of plant-parasitic nematodes, only 15% had nematode population densities or numbers considered to be damaging to corn.

This article explains the “why” and “how” of sampling corn fields to determine if nematodes are likely to reduce corn yields.

Symptoms of nematode damage to corn

Only fields showing symptoms of damage need to be sampled. There is no need to sample fields that do not have any of the following symptoms:

• swollen roots
• death of root tips (Figure 1)
• dark areas (necrotic lesions) on roots (Figure 1)
• stunted roots and/or shoots (Figures 1 and 2)
• yellow foliage (Figure 2)
• mid-day wilting or curling of leaves
   

How and what to sample

The recommended procedure to sample for nematodes that feed on corn varies by the stage of the crop.

For corn up to growth stage V6, collect soil and root samples.

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

For corn from growth stage V6 through R3, collect only soil samples, no plant samples.

• Use a soil probe, angle underneath corn plants to collect 20 or more soil cores that are at least 12 inches long from an area exhibiting symptoms.
• Collect the soil cores from within the root zone of plants showing symptoms of damage (Figure 3).
• Combine, but do not mix, the soil cores and place them in a sealed plastic bag labeled with permanent marker.
• Protect the soil samples from physical jarring, and keep samples at or below room temperature.

For corn from growth stage R4 and beyond, sampling is not recommended.

• Nematode population densities are not consistently related to the potential for yield loss once the corn crop reached the R4 or dough stage. Consequently sampling is not recommended.

Where to send samples?

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. Many 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.

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 $35 per sample processing fee ($40 for out-of-state samples).

Damage thresholds

The number of nematodes necessary to damage corn varies greatly among the different nematode species. Some of these nematodes can damage corn and reduce yields at very low numbers whereas others are not damaging even at very high numbers (Table 1). The potential for yield loss can only be assessed 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 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 buildup 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 nematicide and/or nematode-protectant seed treatments. Contact an agronomist at your input and service provider for information about specific options.