No. 12/June 12, 1998
The cooler weather also provides a more favorable environment for corn nematodes. In Illinois, there are usually three to four different genera of plant pathogenic nematodes found in any corn field. Although most common in fields with greater than 50 per cent sand content, nematodes such as the stunt and lance nematode may build to damaging levels even in the darker and heavier soils of central Illinois.
Nematodes feed by inserting a long, hollow, needle-shaped mouth spear into root cells. They withdraw nutrients and water intended for plant growth. Their feeding typically causes a stunting of corn plants, but this may not become apparent until the plants are under moisture stress later in the season.
A 3-year study of corn nematodes in Whiteside County indicated that populations tended to peak about 50 to 60 days after planting and then to decline until fall, when there was a second, smaller peak. This second peak may have been due to nematodes feeding on grasses growing in the field late in the season. Nematodes such as the dagger and needle nematode parasitize corn very early in the season. Soil samples collected in Whiteside County in early May had above-threshold populations present in grassy areas prior to corn planting. Thus, when corn is planted, nematodes can begin damaging roots very early.
Nematodes are favored by cool and wet soils. As the season progresses and soil heats up and is dry in the upper layers, nematodes may burrow deep into the soils and can sometimes be found at depths of 30 to 40 inches. If soil samples are taken at this time (when symptoms begin to show), nematode populations may be misdiagnosed unless samples are taken much deeper than the typical 8 to 10 inches. Thus, it is important for producers to sample early in the season even if corn has already been planted. These samples will provide information for the next corn crop.
Management of corn nematodes is more complex than for Soybean Cyst Nematodes (SCN) because there are no resistant hybrids to those nematodes found in Illinois. The use of a soil treatment such as Counter CR or Mocap at planting can provide the early season protection needed for establishing a healthy root system on corn. In-furrow applications of insecticides-nematicides are not as effective as a 7-inch band application because not enough soil area is treated. Thus, producers who wish to treat soils for nematodes should use a band application and not an in-furrow to maximize nematode control.
Crop rotation can also be effective against some corn nematodes, especially the needle nematode. The needle nematode is the most damaging of the corn nematodes and has a host range primarily limited to grass crops. Rotation with soybeans for a single year can effectively reduce needle nematode populations. Research conducted by the late Dr. Richard Malek indicates that the needle nematode can feed and damage many grasses, including corn and small grains, but can also parasitize potato plants. Thus, soybeans would be the most effective of the alternate crops because there was no evidence that this crop is affected by the needle nematode.
Control of grasses in the field, both in the current year and in the rotation crop, is important. Some of the highest nematode populations I have seen were from soybean fields with poor grass control. Although the nematodes could not feed on soybeans, they were building up on the grasses not controlled by the herbicide program. This allows corn nematode populations an extra year to build until the corn crop is planted the following year.
Soil testing is needed to identify correctly the nematodes present in corn fields. Symptoms of crop injury from nematodes is not distinct and can resemble those from many other factors. Samples should be taken in the early spring or when cool weather occurs. Keep samples cool until they are sent.
H. Walker Kirby (email@example.com), Extension Plant Pathology, (217)333-8414