There continues to be lots of questions about whether plant-parasitic nematodes are causing damage to Iowa's corn crop. This varied group of microscopic worms has some species that cause damage to corn at very low population densities (numbers) and other species that are not harmful until population densities reach many hundred or more per 100 cubic centimeters (a little less than a half cup) of soil.
It is common for several different species of plant-parasitic nematodes to occur in Iowa cornfields at low numbers, explains Greg Tylka, Iowa State University Extension nematologist. But if nematode numbers increase to damaging population densities, symptoms of corn injury will appear. In this article, Tylka discusses how to identify and diagnose cases where plant-parasitic nematodes are damaging Iowa's corn crop.
* What are symptoms of nematode damage to corn? Nematode damage symptoms on corn include stunting of plants (see photo), yellowing of leaves, and mid-day wilting or leaf curling. Roots may be stunted, fine roots may be lacking (see photo), and there may be discrete areas of black, dead tissue, called lesions, on the roots. Also, some nematodes cause roots to swell.
* When do symptoms of nematode damage appear during the season? It would be very unusual for symptoms of nematode damage on corn to occur in the first month of the growing season - except in fields with very sandy soil. For fields with medium and fine textured soils, the aboveground symptoms caused by nematode feeding likely will appear more in the middle of the growing season.
* When should fields be sampled? Samples should be collected when symptoms of damage are seen. Collect soil and root samples from near plants that are showing obvious symptoms of damage, but avoid sampling near plants that are dead or nearly dead. There is no reason to collect samples from corn that is not showing some symptoms of possible nematode damage.
* What type of sample should be collected? It depends on the growth stage of the corn, as to whether you should collect soil samples or root samples or both.
Up until V6 growth stage of corn: collect soil and root samples
* Use a soil probe and collect cores that are at least 12 inches long.
* Collect 10 or more soil cores to represent an area.
* Collect soil cores from within the root zone of plants showing symptoms of damage. 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 four to six plants with symptoms of damage (see photo). Take care not to strip off the smaller seminal roots. The tops of the plants can be cut off and discarded. Place the root samples in a sealed plastic bag labeled with permanent marker.
* Protect the samples from physical jarring and from high temperatures (above room temperature).