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Nematodes can cause significant yield losses in a corn or soybean field without any aboveground symptoms being present. When aboveground symptoms do appear, they often indicate a long-term problem that is just being recognized. The only way to accurately diagnose the presence of parasitic nematodes in corn and soybean is through soil and root sampling.
Symptoms of nematode feeding are typically found in irregular patches and not across a whole field (Figure 1). There are often no aboveground symptoms unless populations are extremely high; symptoms may also intensify under stressful growing conditions. Foliar symptoms may be hard to identify as they are not unique to nematode feeding and can be easily confused with nutrient deficiencies and herbicide or insect damage. The only way to accurately diagnose the presence of parasitic nematodes in corn and soybean is through soil and root sampling.
Figure 1. Symptoms of nematode feeding in a soybean field (top), photo courtesy of Edward Sikora, Auburn University, Bugwood.org., and in a corn field (bottom).
To confirm the presence of nematodes, soil and root samples must be collected and sent to a nematode testing laboratory for analysis. It is important to collect both soil and root samples to determine the presence of endoparasitic and ectoparasitc nematodes. Nematode sampling is done in much the same way in corn and soybean.
Timing of sampling is more of an issue in corn than in soybean. For corn, it is recommended to sample during the growing season, ideally when symptoms are present. Sample earlier in the season in sandy soils, while plants are still small (up to V6), or through the R3 growth stage in finer textured soils. Sampling for soybean nematodes can be done at any time. For both corn and soybean nematodes, soil cores should be taken from the top 6 to 8 inches of soil around the edge of symptomatic areas (nematode populations will be highest here), taken at an angle to intersect the root zone. The soil should not be overly wet or dry when sampling. Approximately 20 soil cores should be collected per sampling area (40 acres or less) and mixed together so that a composite of the area can be tested. Roots from 4 to 6 plants per sampling area should also be collected. Root and soil samples should be refrigerated until the time of shipping.
Contact the diagnostic lab you will be using before taking and sending your samples. Different labs may have slightly different sampling and mailing procedures. Visit the Pest and Diagnostic Clinic at the University of Nebraska-Lincoln (UNL) at https://cropwatch.unl.edu/plantdisease/unl-diagnostic-clinic-lincoln for information on their sampling and shipping procedures. The Nebraska Soybean Board has partnered with UNL Extension to provide bags to submit soil samples for a free SCN analysis, a $20 value. These are available at your local UNL Extension office.