Plant parasitic nematodes exist in every field to some extent, ranging from no obvious impact to the crop to severe injury and tremendous yield loss. In soybean, the soybean cyst nematode is well known and has been receiving a lot of attention in Nebraska lately as we monitor its spread to new areas. In contrast, nematodes feeding on corn roots already occur in every field to some extent and are usually referred to as “corn nematodes,” although some may feed on other host plants (such as soybean).

There are many species of corn nematodes from at least 12 genera with common names such as sting, needle, lance, lesion, stunt, dagger, spiral, etc. The extent of serious crop injury and yield loss depends on the species present and their population densities. The only way to assess these two factors is by collecting and submitting samples to a laboratory for plant parasitic nematode analysis.

Unfortunately, strict guidelines for collecting samples for plant parasitic nematode analysis in corn have not been established, but following the tips below can help you collect better samples. The best time to sample for corn nematodes in Nebraska is now through the next couple of weeks.

Corn nematodes are diverse and are not all equally damaging. For example, those that cause the worst injury, such as needle and sting nematodes (Figure 1), tend to be distributed in patches in a field and not across an entire field. Others, such as root-lesion (or lesion nematodes), are far more common, occurring in more than 93% of Nebraska fields. Root-lesion nematodes may be present across an entire field. Lesion and other nematodes tend to cause less severe injury and symptoms (Figure 2) than sting or needle nematodes, but their total losses are probably greater than any other nematodes considering their wide distribution.

How to Sample for Nematodes in Corn

Sampling for some nematodes, such as soybean cyst nematode, can be conducted any time because they remain in the upper inches of the soil. However, some of the nematodes feeding on corn can travel deeper in the soil during the growing season and may be out of reach of most sampling probes. Thus, sampling at the wrong time can result in false negative results if some nematodes were below the sampling area.

We recommend sampling for corn nematodes early in the season, when roots are still shallow, in the upper 8 inches of the soil. During most seasons the roots and root-feeding nematodes should be in reach of soil sampling probes about four to eight weeks after planting.

Because of the cooler temperatures in Nebraska this spring, corn growth and development has been somewhat delayed, so there’s still time to collect samples for nematode analysis. Other laboratories may recommend sampling later in the season when population densities are at their highest, but some nematodes, such as needle and sting, may travel deep in the soil and not be captured by a typical 8-inch sampling probe at that time.

Note that laboratories should extract nematodes from the soil, as well as endoparasitic nematodes (such as lesion nematodes) from root material. Some labs also may require the submission of entire plants or root balls, in addition to soil samples, to conduct these tests. Contact the lab you plan to use to learn their requirements. Nematodes in these samples must be alive and crawl out of root material during extraction procedures. For this reason, it takes several days longer to process corn nematode samples than other types of samples.

Remember, the reliability of your diagnosis depends on the quality of the sample that you submit! And, the nematodes in your sample must be alive for an effective analysis.

Know Your Sampling Strategy

How you sample should be determined by your reason for sampling.

Diagnosing Symptomatic Areas. Nematodes can cause many types of symptoms, such as stunting, yellowing, root lesions, and deformity. All of these are similar to those caused by other common problems such as pH extremes (Figure 3), nutrient imbalances, and insect or herbicide injury. Thus, they are frequently misdiagnosed.

Samples can be collected directly from symptomatic areas of a field, such as those pictured in Figure 2. However, when sampling a severely affected area (Figure 1), you should avoid sampling the center of the area where few roots and nematodes will be found. Instead, collect samples around the perimeter where symptoms are less severe and you are more likely to find more nematodes. It’s also a good idea to collect a second sample from a nearby apparently healthy area of the field. Analyzing both samples for plant parasitic nematodes will allow for comparison of nematode populations and a more definitive conclusion.

Establishing a Baseline. If you don’t have a particular problem spot in a field, but overall yield is not as high as expected and other possible causes, such as fertility issues and other pests have been ruled out, you could collect a sample for analysis. In this case, the most effective strategy would be to collect a random pattern of soil cores from less than 40 acres for a composite sample.

Sampling Nematicide Strips

New seed treatment nematicides, such as Avicta and VOTiVO, provide more management tools. One of the best ways to evaluate the product(s) on your farm is to conduct a replicated strip trial. It’s important to recognize, however, that this process can be complicated and labor intensive. The ultimate success of these trials is determined by how well you’ve planned them and whether they’re influenced by other conditions outside your control.

Product testing usually occurs over several years and often across hundreds of locations to help minimize the impact of variability. Testing in a single or few locations in one or two crop seasons may not provide adequate information to reflect how well the product will perform and may be of limited value during some years or situations.

If you have decided to conduct your own testing, you will need to collect yield data from multiple, replicated strips. Use at least three strips per treatment to account for variability within a field. Use a yield monitor or weigh wagon to measure yield from the strips.

Many people have expressed interest in sampling for nematodes within treatment strips to evaluate product performance on their farm. There are no guidelines for how best to do this and the natural variability of nematode populations in the field may make the data impossible to interpret.

Submitting Samples

Samples collected for corn nematode analysis can be processed at the UNL Plant & Pest Diagnostic Clinic for a fee of $25 per sample. Because of the variability among laboratory procedures, you should contact your lab of choice to find out what they require for sample submission. For example, some labs may request entire plants or root balls in addition to the soil samples, which may provide some clues in their symptoms.

Visit the UNL Plant and Pest Diagnostic Clinic website to view its procedures and to download a copy of the PPDC Sample Submission Form. Be sure to indicate the sample is for corn nematode analysis.

More Information

For additional information, see:

• Plant Disease Central
• Soybean Cyst Nematode, an article by Greg Tylka, extension plant nematologist at Iowa State University

Tamra Jackson
Extension Plant Pathologist
Amy Timmerman
UNL Plant & Pest Diagnostic Clinic