The soil is a battle zone that will largely determine the fate of your corn crop, says George Bird, a Michigan State University (MSU) nematologist. Both soil type and soil health are keys to microscopic worm (or nematode) population levels that influence whether or not corn roots start out strong during the growing season, he says.

While most corn growers understand the relationship between good soils, good root growth and good yields, not as many consider the battle going on underground that can either help or hinder their bottom line, says Bird. He points out that there are many different nematodes present in almost all corn fields – some are plant parasites that can cause significant damage and others are nonparasitic species that help boost root growth and yields (see sidebar on next page).

 “Beneficial nematodes make nitrogen (N) available to plants,” explains Bird. “They feed on soil bacteria and excrete N into the root zone in a plant-available form.”

Anything farmers can do to feed the soil bacteria – whether it be manure, or compost or cover crops or crop residue – will in turn provide more N for corn roots, he says.

In some instances, adding organic matter to soils has helped soil quality and reduced a crop’s susceptibility to plant-parasitic nematode damage, says Bird. Michigan potato growers have recently managed nematodes successfully by spreading compost over problem areas, “but not at high rates – just enough to act as a pulsing agent for the crop,” he says.

Different crops are susceptible to different plant-parasitic nematodes. There are three parasitic-nematode types that cause the most damage to corn in Michigan: needle, root lesion and stubby root, says Bird.

“If your corn has a nematode problem, the corn needle nematode is the worst one that you could get,” he says. “It can be disastrous for corn yields, but it’s usually limited to areas with coarse-textured, sandy soils.”

Unlike other nematodes that feed on corn, the corn needle nematode can be managed reasonably well through crop rotation, says Bird. “On one farm, we doubled the corn yield after rotating away from continuous corn to alfalfa, clover and soybeans,” he says.

The corn needle nematode also avoids warm soils, Bird adds. “They do their damage early in the season and then head deep into soils later on,” he says. “So, it’s best to scout for these nematodes and sample soils early in the spring.”

Other nematodes that feed on corn will either chew inside and remain in the roots (such as root-lesion nematodes) or stay closer to the soil surface where soil samples can detect them from early spring through late summer, says Bird. Crop damage from needle, sting and stubby root nematodes often resembles herbicide injury, he points out. He adds that the sting nematode has never been found on corn roots in Michigan, but that it is a severe problem in other corn-growing states.

Nebraska and Kansas have significant problems with sting nematodes that feed on corn, says Greg Tylka, Iowa State University Extension plant pathologist.

“The two most damaging nematodes to corn per individual worm are the sting and needle nematodes,” says Tylka. “Nebraska and Kansas seem to be ground zero for these two, which are extremely damaging and almost always occur only in sandy soils.”

All the other nematode types that feed on corn can be found in various soil types, he says.

While crop damage from sting and needle nematodes is typically obvious and dramatic, the injury to corn from other parasitic nematodes isn’t often recognizable, says Tylka. Other nematodes feeding on corn are stunted, yellow growth and general stress symptoms or some wilting under dry conditions, he says. “In general, the plant looks unthrifty.”

Nematode species

It is quite common to find nematode species that feed on corn in low numbers throughout the Corn Belt, says Tylka. Much is still unknown about how much yield loss they cause and what population levels trigger measurable yield loss. Better early season weed control might help to prevent parasitic nematodes from ever reaching damaging levels in corn.

“In Iowa, if you harvest an area with only 160-bu. corn when you were expecting 180 bu., you wouldn’t know whether nematodes were the problem without scouting and sampling soil and roots earlier in the year,” says Tylka. “For nematodes that feed on corn, all but needle and sting species are more yield nibblers than yield chompers. So, these nematodes haven’t been on most people’s radar screen yet.”

 Bird advises keeping a year-by-year record of what your corn yields are and where problem areas of the fields are located. “Then, sample soil and root samples for nematodes there.”

In Michigan, one particular root-lesion nematode, Pratylenchus penetrans, causes the most concern to overall corn yields, says Bird. “Root-lesion nematodes probably take 5 bu./acre off the top of all the corn grown in the state,” he says. “They are present in all soil types but are more of a problem on sandy, coarse soils.”

Root-lesion nematodes have a very wide host range, so they’re also difficult to control through crop rotation, he adds. “In four decades, I haven’t found a non-host crop,” says Bird.

Since root-lesion nematodes live inside the roots, include some root material with soil-core samples when sending them to a lab for analysis, he says.

There are different recommendations on when to scout for nematodes that feed on corn, and what samples to take, says Tamra Jackson, University of Nebraska-Lincoln (UNL) Extension plant pathologist. “Check with your local nematology lab.

“In Nebraska, we have documented areas with problems from sting and needle nematodes, which is why we recommend soil sampling in spring, rather than later in the year when some nematodes may have traveled deeper in the soil, beyond our soil probes,” says Jackson. “For most other nematodes, it’s okay to sample later in the season.”

Root-lesion nematodes are present in 93% of Nebraska fields, says Jackson. “Cumulatively, they are the most common and probably cause the most damage,” she says.

If soil and root samples show a significant problem with parasitic nematodes that feed on corn, you’ll have to wait until another year begins to start a control program, Tylka says. “There are no in-season tactics to mitigate yield loss in the current growing season,” he states. “Primary management strategies for future years are soil-applied Counter 15G and 20G nematicides and/or seed treatments such as Avictaand Votivo.”

Nebraska has limited results from studies on nematicidal soil and seed treatments for corn, which have been published on the UNL Plant Disease Central website under Management Trials for Corn, Jackson says.

Control products have had the greatest return in areas of greatest nematode pressure,” she says. “However, none of these products will entirely eliminate the problem. Once you have nematodes in corn, it becomes a chronic problem because we don’t have a way to eradicate them.”

Nematodes: the good, the bad -- the ugly and the beautiful

When most corn and soybean farmers hear the word “nematode,” they tend to think only about the nematode species that can cause a nice-looking, healthy crop to turn sickly and ugly, says Greg Tylka, Iowa State University Extension plant pathologist. These “bad,” plant-parasitic nematode species are the ones that feed on crop roots, weakening plants, making them more vulnerable to disease, reducing their nutrient uptake and lowering yields.

Still, not every nematode is a “bad” nematode, notes Tylka. There are many “good” nematode species that are nonparasitic to crops. These nonparasitic, or “good” nematodes, feed on bacteria, fungi and other nematodes, rather than crop roots. These “good,” nonparasitic nematodes can also generate “nematode manure” that can help boost root growth and crop yields.

“Superficially, it may appear that nematodes are so tiny that they couldn't possibly produce enough ‘manure’ to amount to anything significant,” says Tylka. “But there are billions of nematodes in every acre of soil in a corn field, and their waste products can be significant contributors to the soil environment.”