Plant-parasitic nematodes can damage corn, and several instances of this are discovered each year in Iowa. More frequent cropping of corn following corn and less frequent use of soil-applied insecticides, which may have provided some nematode control in the past, could result in more instances of nematode damage to corn this year and in the future.
For most plant-parasitic nematodes that feed on corn, numbers increase through the first half of the growing season. And samples should be collected midseason, when nematode numbers likely are greatest, so that the numbers can be compared to damage thresholds established for corn.
But if damage from needle nematode is suspected, samples should be collected in the spring or fall, not in the summer. Needle nematodes migrate down into the soil in the middle of summer, when soils are warmest, and this nematode could be missed in mid-season samples. But the needle nematode is among the largest plant-parasitic nematodes, and because of its large size, needle nematodes only occur in sandy soils, usually those with at least 70 percent sand content.
So unless a needle nematode problem is suspected in a sandy field, samples should be collected during mid-season, not in May or June, to check for possible nematode damage to corn.
To test for needle nematode, collect 20 or more 12-inch-deep soil cores in the spring or fall from the root zone of corn plants within the area suspected of being damaged. The soil cores should be mixed well, then placed in a moisture-proof bag and submitted for processing as soon as possible. Samples can be sent to the Iowa State University Plant and Insect Diagnostic Clinic, 327 Bessey Hall, Iowa State University, Ames, IA 50011. The test that would provide information on needle nematodes is called a complete nematode count.
Samples sent to Iowa State University 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 ($60 per sample for out-of-state samples).
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 191-192 of the IC-498(16) -- June 25, 2007 issue.