Issue No. 12, Article 6/June 15, 2007
Time to Sample for Nematodes in At-Risk Corn Fields
Now is the time to sample for nematodes, especially in corn fields that are at risk. The risk factors are corn-on-corn; minimal or no tillage; and the absence of nematode-suppressing soil-applied insecticides (in other words, anything other than an organophosphate or carbamate). As I have reported previously in the Bulletin, economic infestations of corn nematodes appear to be increasing in Illinois because more farms have one or more of these risk factors; however, awareness of nematode problems in corn is not keeping up with their actual occurrence.
The common belief that corn nematodes are important only in very sandy soils is not accurate. 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). The term "corn nematodes" includes a number of other potentially damaging species, including dagger, lance, lesion, ring, stunt, and occasionally spiral, which may be found in heavy soils. Recent field observations have suggested that infestations of corn nematodes may be causing yield loss through nonspecific interactions with various root-rotting fungi.
Corn injury caused by nematodes cannot be diagnosed from symptoms. Although needle nematode can kill corn seedlings, most nematodes will not cause injury this severe; neither will needle nematode unless the infestation level is high. 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 after an appropriate extraction method. Many private labs will test for corn nematodes, as will the University of Illinois Plant Clinic. The Nematology Lab also does corn nematode analyses; contact me for more information (email@example.com).
For instructions on how to collect the best samples, see RPD 1100 (Adobe PDF). Before you take samples, consider the following points. First, sample as deeply as possible, when the soil is moist but not wet. Second, 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. Third, sample around the edges (not in the centers) of "hot spots" in the field. Fourth, put the sample in a plastic bag--not a paper bag--to help keep it moist during transport.
Corn nematode damage in a corn field "hot spot."
Take a cooler along to store the sample and keep the nematodes from being cooked! Corn nematode management depends on which nematode species is involved and how high their numbers are, so it's very important to get a good sample as the basis for a reliable diagnosis.--Terry Niblack