Nematodes That Attack Corn in Iowa

Types

• According to plant pathologists Don C. Norton and Robert F. Nyvall in their paper "Nematodes That Attack Corn in Iowa," 11 types of nematodes affect corn production in Iowa. These consist of both ectoparasites, which live on the surface of the corn, and endoparasites, which break through the root and infect the interior of the plant. The most commonly occurring ectoparasitic types are the dagger nematode, ring nematodes, spiral nematodes and stunt nematodes. The first two types occur primarily in sandy soils; spiral nematodes persist in heavier soil types and stunt nematodes can live in both sandy and heavier soil types, such as loam soil. Endoparastic types include lance nematodes, which appear in sandy soil, the six species of lesion nematodes, which occur in high concentrations and cause tremendous damage to crops, and stubby root nematodes, which also exist in sandy soil and cause serious crop damage.

Symptoms

• Symptoms of nematode infection depend on the type of soil and the type of parasite. Farmers may see some symptoms in the corn plant and others in the root. Some commonly observed above-ground symptoms include stunted plant growth, yellow leaves, underdeveloped ears of corn, diminished crop yields and raggedy appearance of plants that occur in patches but not throughout the entire field. Inspection of the root can reveal other symptoms. These signs include shortened or retracted root growth, swelling and discolored roots. The swelling and discolored roots can indicate a fungal or bacterial infection made possible by the nematode causing lesions in the roots.

Diagnosis

• Farmers will not be able to see nematodes; these pests are invisible without the aid of a microscope. Detection requires a soil sample. Different nematodes live in different types of soil, and the type of soil determines when a farmer should take a soil sample to test for these parasites. Nematodes that infest dry, sandy soils strike early in the growing season, and farmers concerned about infestation can take a soil sample in the spring. Nematodes that live in heavier soils become problematic later in the growing season, and the soil will show high concentrations of these parasites about midway through the growing season. Farmers can take a soil sample by digging out soil from a cornfield with a spade or by using a soil probe to recover potentially infected soil. The best soil samples come from about 12 inches below the surface; farmers should take 10 to 20 samples and place these in a waterproof plastic bag and then ship the soil to the Plant and Insect Diagnostic Clinic at Iowa State University. Farmers should take care not to expose the soil to high temperatures, which can kill the nematodes and ruin the sample.

Treatment

• Farmers can use both chemical and preventive measures to remove or treat a nematode infestation. Iowa permits farmers to use carbofuran, ethoprop, fensulfothion and terbufos to kill nematodes. Norton and Nyvall suggest that farmers can also treat a nematode infestation by crop management techniques. These researchers suggest using fertilizer to improve crop health, as nutrient-deficient plants are more susceptible to infestation. Farmers can also plant crops other than corn in infected soil for a season, though Norton and Nyvall note that many corn nematodes will also infect soybeans. Clearing fields of weeds will also help reduce the nematode population in soil, as nematodes often arrive in fields via weeds and weed spores.