There have been many reports of poor corn growth throughout Iowa this season. One possible cause of the poor corn growth probably not considered very often is plant-parasitic nematodes. Dry conditions in some parts of the state are favorable for nematode damage. There are many species of nematodes that can damage corn. Dagger and spiral nematodes may be the most common and widespread nematodes that feed on corn in Iowa. Needle nematode probably is the most damaging, but is not widespread throughout the state. The most important species that is a parasite on corn in Iowa probably is the lesion nematode.
In general, damage to corn from plant-parasitic nematodes results in poor or uneven stands if high nematode densities occur early in the season. Symptoms also include yellowing or chlorosis of foliage, unevenness in the height of the corn plants, and small or poorly filled ears during mid- to late-season. The symptoms and damage caused by plant-parasitic nematodes can occur in distinct patches or "hot-spots" that often elongate in corn fields in the direction of tillage operations. Damaged corn roots will be stunted, discolored, swollen, and lacking fine roots, and may contain dark brown or black lesions. However, other factors also can cause these types of aboveground and belowground symptoms, so nematode damage easily can be misdiagnosed.
||Uneven stand and yellowing of corn foliage caused by plant-parasitic nematodes.
||Nematode damage to corn roots may look similar to herbicide damage. Left: herbicide damage; right: nematode damage..
How can you identify whether a nematode problem exists? The only way to effectively diagnose most plant-parasitic nematode infestations on corn is to collect an accurate soil sample. You should collect a soil core or small shovel-full of soil from the upper foot of the soil profile from 10 or more places within an area suspected of being damaged by nematodes. In addition, be sure to include numerous fibrous roots from plants suspected of being damaged. Many of the nematodes that damage corn spend much or all of their lives within the corn root tissue and, thus, the roots must be tested for the presence of damaging nematodes.
For a comparison, growers are advised to also collect a companion sample from a comparably sized area of plants that are not showing symptoms. Soil and roots should be placed in a moisture-proof bag and submitted for processing as soon as possible. Be sure to keep the samples cool until they are sent for processing and avoid sending samples late in the week to prevent improper storage over the weekend.
A new Iowa State University Extension publication, Scouting for Corn Nematodes, IPM-53-s, illustrates proper procedures to use when scouting for these pests. Soil samples for analysis of corn nematodes can be sent to several private laboratories in Iowa and surrounding states or to the ISU Plant Disease Clinic, 323 Bessey Hall, Iowa State University, Ames, IA 50011. If samples are sent to ISU, the test for corn nematodes is called a complete nematode count. Samples should be accompanied by a completed Plant Nematode Sample Submission Form (ask for ISU Extension publication PD-32) and a check for the $15 per sample processing fee.
Typically, results from analysis of soil samples will list the scientific and common names of the plant-parasitic nematodes found in the sample, the numbers of nematodes present, whether the detected numbers of nematodes are thought to be capable of causing damage, and some management recommendations. Corn nematodes can feed without causing appreciable yield loss if nematode numbers are low and/or the environmental conditions are such that the corn crop is not stressed. Much is still unknown about the nematode population densities needed to cause damage to the many corn hybrids grown throughout Iowa, and about the environmental and host factors involved in the build-up of nematode densities.
Management options for control of nematodes on corn are limited. Many effective nematicides have been removed from the market and very few new nematicides are being developed, but a few compounds (including some soil insecticides) are still labeled for control of plant-parasitic nematodes on field corn. Cultural control strategies such as crop rotation, delayed planting, and alternative tillage have little effect on corn nematode densities and nematode-resistant corn hybrids are lacking. More information about plant-parasitic nematodes that affect corn in Iowa is available in an ISU Extension publication, Nematodes That Attack Corn in Iowa, Pm-1027.
This article originally appeared on pages 123-124 of the IC-478(15) -- June 30, 1997 issue.