Do you have a corn nematode problem? Is your nematicide doing any good?

Unexplained yield losses or patchy areas of low productivity or vigor may indicate not a herbicide, nutrient or environment issue but an established corn nematode population, reports Plant Diagnostic Clinic and IPM coordinator Suzanne Bissonnette, University of Illinois. Corn nematode populations were a bigger issue than previously thought, according to a 2009-2010 survey supported by the U of I National Institute of Food and Agriculture-Extension Illinois Pest Management program.

Corn nematodes include a number of damaging species, such as dagger, lance, lesion, ring, stunt and, occasionally, spiral nematode, which may be found in heavy soils.

“Different nematode situations require different types of product application, so it is best that you send in a sample for analysis before attempting nematode control,” Bissonnette said. “You have no other way of knowing what your initial population is or if the population is being controlled.”

The survey found that about half of the cornfields in Illinois have lesion nematode populations with densities at or above the threshold for moderate to severe risk of injury (yield loss). Lesion nematodes are not only capable of injuring corn roots, but they also frequently act as vectors for the development of root rots.

It is not true that corn nematodes are a problem only in very sandy soils. Sandy soil is a risk factor for only a few species (needle, sting, and stubby-root nematodes throughout Illinois, and southern root-knot nematode in southern Illinois). Although needle nematode can kill corn seedlings, most nematodes will not cause severe injury unless the infestation level is very high.

“Consider sampling for nematodes now, especially in cornfields that are at risk,” Bissonnette suggested. Some risk factors include corn-on-corn, minimal or no tillage, and the absence of nematode-suppressing soil-applied insecticides. The best time to sample for nematode diagnosis is approximately 4 to 6 weeks after planting. The pest management strategy depends on the species involved and how high their numbers are, so it is very important to get a good sample.

Start by examining the physical characteristics of the plants. If there are no symptoms (hot spots) in the field, sample a representative area of the field, perhaps 10 acres or less. Sample in a zigzag or “w”-shaped pattern, and collect 20 to 25 cores in a bucket. If there are hot spots, sample around their edges, not in the centers, and collect a total of 20 to 25 cores. Record the GPS coordinates for the area.

Sample as deeply as possible from within the rows when the soil is moist but not wet to a depth of at least 6 to 8 inches. Use a 1-inch-diameter soil probe if possible.

Treat the samples gently because some corn nematodes are very sensitive to manipulation, and it is important not to kill them before they reach the lab. Do not break up the cores or drop the samples. Put the samples in a plastic rather than paper bag to help preserve moisture during transport, and store them in a cooler. Include the GPS coordinates for the samples along with contact information when submitting samples.

Symptoms of corn injury caused by nematodes look similar to those caused by other production problems, including poor or uneven crop development, yellowing or streaking, and reduced or brushy root systems. The only way to diagnose corn nematode is by direct examination under a microscope following an appropriate extraction method.

U of I Plant Clinic nematologist Alison Colgrove and her staff test soils for nematodes. Services include soil nematode analysis, $40; root analysis, $40; soybean cyst nematode (SCN) egg counts, $20; pinewood nematode, $20. Send samples to the Plant Clinic, 1102 S. Goodwin, S-417 Turner Hall, Urbana, IL 61801. For specialty testing, including SCN Hg typing, variety screening, phytosanitary testing, or other nematode projects and diagnostics, contact Colgrove at acolgrov@illinois.edu or 217/333-9057. More information regarding nematode sample submission can be accessed on the Plant Clinic website at http://web.extension.illinois.edu/plantclinic/.