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Issue No. 4, Article 6/April 29, 2010

Time to Plan for Corn Nematode Sampling

Nematodes are probably a much bigger problem for corn growers than we've thought before. For example, in our 2009 corn nematode survey, we found that about half of the cornfields in Illinois have lesion nematode population densities at or above the threshold for moderate to severe risk of injury (yield loss). Lesion nematodes are capable of injuring corn roots by themselves, and they are also frequently involved in the development of root rots.

The best time to sample for diagnosis is about 4 to 6 weeks after planting. A couple of weeks more or less may not matter very much, but it's time to put sampling on your calendar now.

Corn injury caused by nematodes cannot be diagnosed from symptoms. The symptoms of nematode parasitism can look like 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 of the worms under a microscope following an appropriate extraction method. Some private labs will analyze soil for corn nematodes, as will the University of Illinois Plant Clinic. The Nematology Lab also does corn nematode analysis; contact me for more information (tniblack@illinois.edu). Both the Plant Clinic and the Nematology Lab charge $40 for a corn nematode analysis.

Exactly how and where you sample is determined by the reason you're sampling. Corn nematode management depends on the species involved and how high their numbers are, so it's very important to get a good sample as the basis for a reliable diagnosis. Typically you just want to know whether nematodes are causing yield loss in a given field. Start with how the plants look:

  • If there are no symptoms, focus your sampling on a representative area of the field, perhaps 10 acres or less. Nematodes can reduce yields without causing obvious symptoms. Record the GPS coordinates for the area. Sample in a zig-zag or w-shaped pattern, and collect 20 to 25 cores in a bucket.
  • If there are symptoms (hot spots), sample around the edges of symptomatic areas and collect a total of 20 to 25 cores. Record the GPS coordinates for the area.

Other things to consider for sampling:

  • Sample as deeply as possible from within the rows, when the soil is moist but not wet. Use a 1-inch-diameter soil probe if at all possible.
  • Treat the samples gently while they're being taken and afterward, because some corn nematodes are very sensitive to manipulation and you want to avoid killing them before they reach the lab. In other words, don't break up the cores or drop the samples.
  • Put the sample in a plastic-not paper-bag to help preserve moisture during transport. Take a cooler along to store the sample and keep the nematodes from being cooked!
  • Include the GPS coordinates for the sample along with your contact information when you submit samples.

Again: corn nematode management depends on the species and population densities in the field. A good sample will result in a reliable diagnosis and management recommendations.--Terry Niblack

Terry Niblack

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