Corn nematodes may be getting worse — and not just in sandy soils.
Less rotation, less tillage and less use of soil insecticides are all raising the risk of damage from corn nematodes, according to nematologists from the Corn Belt. Yet, for many corn growers, these hard-to-diagnose pests are off the radar screen.
“Plant parasitic nematodes of corn often are overlooked as causes of disease and yield loss,” says Tamra Jackson, plant pathologist at the University of Nebraska. In reality, many producers “face chronic challenges with damage caused by corn nematodes,” she says. “It's important not to disregard them, especially since recent changes in cropping practices may favor nematodes.”
Two significant cropping changes are the adoption of insect-resistant Bt corn and the shift to pyrethroid insecticides, Jackson says. Growers have cut their use of traditional carbamate and organophosphate insecticides, which had the side benefit of suppressing nematodes. These changes, combined with more no-till production and more corn on corn, are likely to encourage the buildup of corn nematodes in the soil, she says.
In Illinois, for example, there's been a big upsurge of requests for corn nematode identification, says Terry Niblack, a University of Illinois nematologist. “My bellwether is the number of phone calls I get, and I'm getting as many calls about corn nematode as soybean cyst nematode.”
A common misconception is that the microscopic roundworms are only a problem in sandy soils. Not true, Niblack says.
While corn growing in sandy soil is more susceptible to nematode damage, “sandy soil is a risk factor for only a few species,” she says. Those include needle, sting, stubby-root and root knot nematodes. Plenty of other species infest heavier soils, including lesion, dagger, lance, ring and stunt nematodes.
“Needle and sting nematodes are the ones people tend to notice,” Jackson says. Even at low numbers they can cause significant damage.
But lesion nematodes, a far more common parasite that feeds inside corn roots, “probably costs us the most yield,” Jackson says. Lesion species are widespread in the Corn Belt and can reach enormous numbers — sometimes topping 80,000 parasites in a gram of dry root, according to research published in the late 1990s by Iowa State University Plant Pathologist Don Norton.
Last summer, Syngenta Seed Care did a survey of corn nematode populations in the Midwest, says Clifford Watrin, technical manager for Syngenta Seed Care, which is developing a seed-applied nematicide for corn.
During the 2007 growing season, the company pulled three random soil samples from every Corn Belt county with at least 25,000 acres of corn. “Nematodes have always been perceived to be a problem in lighter, sandy soils,” Watrin says. “But we're certainly finding them in standard silt-loam soils in the Midwest.”
Unlike soybean cyst nematode, which is a recently introduced pest, corn nematodes are probably native parasites that fed on prairie grasses before corn was cultivated. Every field has them, and their vivid names convey the trouble they can cause.
In the sandy soils of southeast Iowa, for example, “needle nematodes are a known problem,” says Tom Hillyer, a crop consultant from West Liberty. In hot spots within fields, losses can be way over 50%, he says.
IN NEBRASKA, Jackson is working with one grower whose corn yields on irrigated land sank to 40 bu./acre under nematode assault. “In some areas, they cause tremendous damage,” she says.
At lower infestation levels, though, “it's nearly impossible to get an accurate assessment of damage,” she says. “With soybean cyst nematode, a healthy soybean plant won't always show damage, but you can still have up to 30% yield loss without symptoms. We don't know if corn reacts the same.”
Greg Tylka, a nematologist at Iowa State University, describes many corn nematodes as “yield nibblers,” a chronic problem that can easily rob growers of 10 bu./acre “without you even noticing. It's like high blood pressure — you don't know you have it until you get tested.”
Nematodes injure corn plants by feeding on the roots and by creating wounds that allow bacteria, fungi and other pathogens to infect the plant and cause secondary rots. Damage is intensified by environmental stresses, such as seedling diseases, insect feeding and drought.
Symptoms include thin stands, uneven plant height, stunted plants, uneven tasseling, leaf yellowing, small ears and kernels and swollen, stunted or discolored roots. Unfortunately, these symptoms mimic other production problems, such as weather stress, nutrient deficiency, insect injury, herbicide damage or soil compaction, Jackson notes. Because of this, corn nematodes can't be diagnosed from symptoms alone, and must be confirmed by soil and root analysis. But if you have an unexplained problem in corn, the culprit could be nematodes.
There's little that can be done during the growing season to rescue your corn crop from nematode damage. Hillyer, the Iowa crop consultant, advises growers who know they have a nematode problem to delay corn planting for a week or two, apply a good starter fertilizer and irrigate “to keep that plant growing well early on. Once the plant gets quite a few roots, it can withstand nematodes better.”
Other things you can try:
Identify the problem by soil and root sampling. “It's very important to get an accurate diagnosis,” Niblack says. That's because there are so many different types of nematodes that damage corn, and management depends on what type you have.
Rotate to non-host crops. If you have needle nematodes or certain species of lesion nematodes, for example, growing alfalfa and soybeans for a couple of years may reduce the populations, Tylka says. Unfortunately, he adds, many nematodes that feed on corn also feed on soybeans, maintaining their numbers despite rotation.
Fertilize according to soil tests. Plants suffering from nutrient deficiency are more susceptible to nematode injury.
Control weeds, which may serve as corn nematode host plants.
Apply chemical controls. There are only a few soil-applied nematicides currently labeled for use in controlling nematodes on corn, Tylka says, and they are expensive — $15-20/acre — so field-wide application may not be economical. Companies working on new nematicide treatments include Syngenta Seed Care and Bayer CropScience.
Management is complicated by old economic damage thresholds that need updating for today's hybrids, Tylka adds. In addition, treatment recommendations vary by nematode type, soil, time of year, plant health, region and other factors. “That's frustrating for growers,” Jackson says. “It's not at all cut-and-dried.”
Because corn nematodes are probably increasing in many areas, it's important to monitor populations, says TamraJackson, a University of Nebraska plant pathologist. Check with your state Extension plant diagnostic lab for sampling guidelines in your area. Here are some sampling tips.
Many nematode experts suggest sampling in spring, within four to six weeks of planting.
You can also collect samples mid-season, when corn nematode populations are highest. However, mid-season samples might miss some of the most damaging nematodes, such as needle nematode, whichcan migrate deep into the soil in themiddle of summer.
Avoid sampling when the soil is verywet or very dry.
Collect 8-12-in.-deep soil samples from the root zones of 20 or more living corn plants within the area where nematodedamage is suspected. Include feeder roots.A traditional T sampling probe works well.
Samples should not represent morethan 20 acres. Use a zigzag or X sampling pattern across the field.
In severely affected areas, sample around the perimeter of the hot spot — not in the center. (Nematodes move away from the roots of dead or badly damaged corn plants.)
Some corn nematodes live inside corn roots, so check with your lab to make sure that they process roots for nematodes, too.
Package the mixed soil sample and roots in a sturdy plastic bag to keep it moist.Submit about 1 qt. soil/sample.
Collecting a comparison soil and root sample from nearby, healthy-looking corn plants is also recommended.
Keep samples cool until they are sent tothe lab. Protect samples from direct sunlight and treat them gently. Nematodes must arrive at the lab alive for an accurate diagnosis.
Get samples to the lab as quickly aspossible. Don't send samples late in the week; they might sit in hot conditionsover the weekend.
Include pertinent background information, such as field and cropping history, soil type, rainfall, pesticide and fertilizer applications and plant symptoms and distribution.
Sources: Tamra Jackson, University of Nebraska; Terry Niblack, University of Illinois; Greg Tylka, Iowa State University.
A new cyst nematode that feeds on corn has been discovered in Tennessee. Juveniles and cysts — egg-filled dead females — were discovered in a soil sample taken from a field of stuntedcorn in Obion County, TN, in 2006.
“This new cyst nematode on corn isprobably not an immediate threat to U.S. corn production,” says Greg Tylka, an Iowa State University plant pathologist, “butwe don't really know how widespread it is, so it is something to be concerned about.” Growers should be vigilant and keep an eye out for cysts on corn roots.
The spread of cyst nematode on corn could also complicate soybean cyst nematode (SCN) management, because the eggs look the same, Tylka says. “Determining SCN egg population densities for management or research purposes would be impossible using current techniques if other cyst nematodes were present.”