Nematodes Can cause anywhere from negligible yield losses to 100% yield losses in corn, depending on the type and number in a given field. What's more, some nematologists believe there is a growing incidence of injury to corn from nematodes, which may be a result of modern cropping practices.

Terry Niblack, extension specialist, Plant Pathology, University of Illinois, says experts cannot make recommendations for a particular field until the identities and numbers of plant parasitic nematodes in the field are known. “The first step an individual farmer needs to take is to sample cornfields for nematodes and submit samples to a reputable lab for analysis,” she says. She adds that crop rotation can sometimes help.

The needle nematode and some species of root-lesion nematode, for example, cannot reproduce on soybean or alfalfa, so rotating to these crops would decrease population densities, says Greg Tylka, Iowa State University plant pathologist. However, other species that feed and reproduce on corn also are capable of maintaining their numbers by feeding on soybeans or alfalfa, he adds. And there are no nematode-resistant corn hybrids.

Some researchers believe an early planting date may be beneficial. The theory is that the quicker plants emerge, the more likely they are to avoid some nematode injury. But not enough data are available yet to confirm this theory, says Tamra Jackson, University of Nebraska plant pathologist.

Fertilization methods may enable plants to better withstand nematode injury. Spoon-feeding nitrogen to the corn crop early could be effective in protecting against nematode damage because it provides damaged roots additional nutrients, which are often difficult to take up, Jackson says.

Optimal levels of fertilizer and water and other good agronomic practices can help, says Tim Todd, Kansas State University research and extension nematologist.

Countless species

At least a dozen nematode genera affect corn, but the number of species within each genus is often unknown, Jackson says. The root-lesion nematode, however, is common, and at least six species exist that feed on corn. It is estimated that more than 90% of Nebraska fields have lesion nematodes, for example.

The lesion nematode causes significant yield loss that may appear to be caused by other factors, including nutrient deficiency, compaction and root-rotting fungi, Niblack says. But some common corn-parasitic species can occur in the hundreds per cubic centimeter of soil and cause little if any measurable damage.

Kansas State University's Todd notes that growers might not notice a nematode problem unless they are tipped off by a yield monitor. Some nematodes will contribute to a 5 to 10% yield loss.

The needle nematode, on the other hand, can kill corn seedlings and cause 100% loss in oblong-shaped areas within a field, Niblack says.

Iowa State's Tylka adds that the damage threshold for needle and sting nematodes is 1 nematode/100 cc (a little less than one-half cup) of soil. For spiral or root-lesion nematode, the damage threshold is 500 to 1,000. “It is important to note that these damage thresholds were established in the 1970s and 1980s,” he says.

New cropping practices

According to Syngenta Seed Care, nematode damage in the past was suppressed by the widespread use of in-furrow organophosphate and carbamate insecticides. With the switch to pyrethroid insecticides and transgenic rootworm-resistant corn, nematode pressure may become more evident because these products lack the ability to suppress nematode populations.

The increased trend in no-till farming may have helped provide an environment for nematodes to thrive because some nematodes are sensitive to soil disturbance, Syngenta Seed Care reports. The company adds that corn-on-corn cropping, which has become more prevalent in the last few years, also can cause nematode populations to build.

Crop protection

“Historically, treating for corn nematodes alone hasn't been a common practice, in part because nematodes were controlled or suppressed by corn rootworm insecticides,” says Paul Vaculin, marketing manager, Granular Insecticides & Closed Delivery Systems, Amvac Chemical Corporation. “Now that corn rootworm insecticides are used less often, there is no longer a free ride for corn nematode control.”

Counter is the only registered granular soil insecticide in corn that is labeled for nematode control. Amvac acquired the Counter business from BASF in November 2006 when the insecticide was offered primarily as a Counter CR formulation in the Lock 'n Load handling system.

Since that acquisition, Amvac has not offered the Counter CR formulation (which for many years was sold by American Cyanamid and BASF). It offers Counter as a 15G formulation, which had been the primary formulation before the early 1990s.

Counter 15G is now available in the Amvac SmartBox closed handling and application system. Growers with the system on their planters have available to them Counter 15G, Aztec 4.67G, Force 3G and Fortress 5G. If corn nematodes are a primary concern, then Counter 15G would be the product of choice.

Counter is also available in the Lock 'n Load closed handling system. Growers need to obtain modified lids for the planter insecticide hopper box to handle and apply Counter 15G in Lock 'n Load.

New seed treatment

The EPA recently approved Avicta seed treatment nematicide from Syngenta Seed Care for use on corn to provide protection against all major early season nematode species, including root-knot, stubby-root, sting, dagger, ring, stunt, spiral, needle, lesion and lance nematodes.

Syngenta Seed Care will offer the new seed treatment in combination with an enhanced rate of Cruiser seed treatment insecticide and a fungicide seed treatment that combines Apron XL, Maxim XL and Dynasty. Avicta will be launched commercially in 2010.

Growers may or may not choose to invest in nematicides, depending on their individual situations. Because there are so many types of nematodes, economic thresholds vary a great deal and should be used as a guideline, Jackson says, adding that an economic threshold could be different for every field and for every year.