Many species of plant-parasitic nematodes feed on corn throughout the Midwest. Most are commonly found anywhere that corn is grown. Nearly all of these corn nematode species likely are native to Iowa and probably fed upon native grasses long before corn was cultivated in the state. The common and scientific names of the most common genera of corn nematodes are listed in Table 1.
Table 1. Plant-parasitic nematodes that commonly feed upon corn in the Midwest.
|Common Name||Scientific Name|
Most corn nematode species can maintain their populations when soybeans or alfalfa are grown, but repeated cropping of corn may cause nematode populations to flare up. Also, use of transgenic, insect-resistant corn hybrids for corn rootworm control may reduce the amount of soil-applied insecticide used in the state. Some have speculated that these insecticides may have provided some suppression of plant-parasitic nematode populations, and reduction in use of soil insecticides also may lead to increases in corn nematode population densities.
Aboveground symptoms of nematode damage to corn include thin stands, uneven plant height, stunted plants, uneven tasseling, leaf yellowing, and small ears and kernels. Swollen roots, lack of fine roots and root branching, and necrotic lesions (black or dark brown dead spots) are common symptoms of nematode feeding on roots. These symptoms are not unique to nematode feeding and, thus, cannot be used to definitively diagnose nematode damage. The only way to accurately document the occurrence of damage from plant-parasitic nematodes on corn is through collection and analysis of a soil and root sample.
Several points should be considered when collecting a sample for diagnosis of a corn nematode problem.
- Collect 20 or more soil cores, 12 inches deep, from the root zone of plants exhibiting a range of symptoms, not just from the most severely affected corn plants.
- Because some corn nematode species feed entirely within roots, a sample of two or three corn root systems should be submitted for analysis along with soil.
- Soil and root samples should be collected during the middle of the growing season to determine if the detected nematode populations are at densities great enough to cause the damage that is observed.
- It is helpful to collect and submit soil and roots from nearby healthy-looking plants as a separate sample in addition to that collected from sick-looking corn to provide a comparison to assess the damage potential of the nematode numbers recovered from the area of the field showing symptoms.
Once the nematodes are extracted from the soil and roots, identified, and counted, various pieces of information will be considered in deciding whether or not the nematodes present in the sample are partially or primarily responsible for the damage observed in the corn crop. Collecting a good sample and providing pertinent and complete background information about the circumstances in the field are critical steps in making an accurate assessment of the potential for damage. Information about the field history, soil type, and rainfall can be useful in making an accurate judgment as to whether the numbers of nematodes recovered from the sample are sufficient to cause damage to corn.
If it is determined that the corn crop is being damaged by corn nematodes, two management strategies are available: nonhost crops and soil-applied nematicides. Neither of these management options can be used to minimize damage or "rescue" the current corn crop.
If the corn crop is being damaged by needle nematode, certain species of lesion nematode, or a combination of these nematodes, growing nonhost crops such as alfalfa and soybean will reduce nematode population densities and, thus, the potential for damage to future corn crops. One or two years of growing nonhost crops may be sufficient to lower the numbers of needle and lesion nematode to below damage thresholds for corn.
There are only a few nematicides that currently are labeled for use in controlling plant-parasitic nematodes on corn. Damaging population densities of plant-parasitic nematodes in corn fields may occur in discrete patches or "hot spots." Consequently, fieldwide application of these pesticides might not be necessary or economical. Also, the benefits of nematicide use in controlling nematodes usually does not carry over to subsequent cropping seasons. Thus, nematicide use will be a management option that is necessary with each corn crop.
Soil and root samples for analysis of corn nematodes can be sent to the Iowa State University Plant Disease Clinic, 327 Bessey Hall, Iowa State University, Ames, IA 50011. The test for corn nematodes is called a complete nematode count. Samples should be accompanied by a completed Plant Nematode Sample Submission Form  (ISU Extension publication PD 32) and a check for the $30 per sample processing fee.
Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.
This article originally appeared on pages 10-11 of the IC-498 (1) -- February 12, 2007 issue.