Are Nematodes Causing Stunted, Yellow Corn?

Farmers and agronomists may be familiar with the threat of plant-parasitic nematodes that feed on corn. These microscopic, soil-dwelling worms often are present in Iowa fields. They usually do not occur in high enough numbers to damage corn. However, there are confirmed instances of damage to corn caused by plant-parasitic nematodes in the state every year.

There are more than 10 different types of nematodes that feed on corn in Iowa. They vary tremendously in size, biology, and ability to cause damage. Determining if nematodes are damaging corn requires proper sample collection and interpretation of results. Below are some guidelines for checking fields.

Where to sample

Samples should be collected only from near symptomatic plants in fields. Corn plants damaged by nematode feeding will be stunted and/or have yellow foliage (Figure 1). Also, roots often will be stunted, swollen, lacking fine roots, or have areas of brown or black dead tissue (Figures 2 and 3).

How to sample depends on corn growth stage

I. V6 corn growth stage or earlier - collect a soil and root sample as described below.

• Insert a soil-sampling probe underneath corn plants at an angle into the root zone 12 inches deep and collect at least 20 soil cores from an area exhibiting symptoms (Figure 4).
• Combine, but do not physically mix, the soil cores and place them in a sealed and labeled plastic bag.
• With a shovel, collect the root mass from 4 to 6 plants showing symptoms of damage (Figure 5). Take care not to strip off the smaller, seminal roots. The tops of plants can be cut off and discarded. Place the root masses 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.

II. V6 through R3 growth stage - collect soil samples as described above, but root samples are not needed.

III. After growth stage R4 - sampling is not recommended.

Nematode population densities are not consistently related to potential yield loss once plants reach the R4 stage. Therefore, sampling is not useful for assessing whether damage is occurring.

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. The form has instructions on where to submit the samples. There is a $40 per sample processing fee ($45 per sample for samples from states other than Iowa).

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. Samples for nematode analysis can be sent to some private soil-test laboratories in Iowa and surrounding states, too. Check with the labs to determine if they process nematode samples for corn before sending the samples.

Damage thresholds

The number of nematodes necessary to damage corn varies widely among the nematode species (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 yield loss based on root and soil samples collected through corn growth stage R3.

Compiled by university agronomists, plant diagnosticians, and nematologists from Arkansas, Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, Ohio, and Wisconsin at a workshop held in 2011.

* These nematodes occur only in very sandy soils and are damaging at any number.
**These nematodes have low risk of damage at any number; other stresses must occur for them to cause damage.

Management

During the growing season nothing can be done to reduce damage to corn caused by plant-parasitic nematodes.

Continuous cropping of corn seems to promote increases in the numbers of these harmful parasites, and rotating corn with soybeans annually may help prevent nematode numbers from increasing to damaging levels.

Counter 20G is a soil-applied insecticide/nematicide that can be applied before planting if recurring damage from nematodes occurs in a field. Finally, several seed treatments are available to protect against nematodes that feed on corn. Consult your seed treatment provider for information on specific seed treatment products.