Figure 1. (Left) Severe sting nematode injury to corn in July 2008 . When sampling areas where roots have been severely damaged (right), collect samples from the perimeter edges of damaged spot(s) in the field to find the most nematodes.

June 8, 2010

Plant parasitic nematodes exist to some extent in every field, with injury ranging from none visible to severe with tremendous yield loss.

In soybean, the soybean cyst nematode is well known and receiving a lot of attention in Nebraska as we monitor its spread to new areas.

	Figure 2. Yellowing of plants (above) caused by root-lesion and other nematode injury in June 2006. Yield in the center of these areas was as low as 30 bu/ac with badly damaged roots (left) near the end of the season.

In contrast, nematodes feeding on corn roots already occur in every field. These are usually referred to as corn nematodes, although some may feed on other hosts as well. 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 level of crop injury and yield loss depends on which species are present and their population levels. The only way to determine that 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 there are some tips (see box below) for collecting samples. Now through the next couple of weeks may be the best time to sample for nematodes in corn.

Corn nematodes are very 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. They 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 the losses they cause additively are probably greater than any other nematodes, considering their wide distribution.

How to Sample

Tips for Collecting Samples for Corn Nematode Analysis at the UNL Plant and Pest Diagnostic Clinic Collect soil about 4-8 weeks after planting. Probe at an angle through root zone. Probe at least 6-8 inches deep. Approximately 20 soil cores needed. Samples should represent less than 40 acres. Collect a total sample size of equal to or greater than 2 cups. Double bag in sealable zipper-top bags. Refrigerate if possible until shipping. Package with soft packing material in a sturdy leak-proof container. Print and fill out a Sample Submission Form indicating the sample is for corn nematode analysis Ship early in the week, Monday – Wednesday. If sampling outside of Nebraska, please contact the UNL Plant and Pest Diagnostic Clinic for further instructions.

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 corn nematodes 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 of year can result in false negative results if some nematodes cannot be accounted for because they were missed.

We recommend sampling for corn nematodes early in the season, when roots are still shallow and in the upper 8 inches of the soil. At this time, approximately 4-8 weeks after planting, the roots and the nematodes feeding on them should be within reach of soil sampling probes.

Other laboratories may recommend sampling later in the season when population densities are at their highest, but some nematodes, such as needle and sting, which are both known to be present in Nebraska, may have traveled deep in the soil and not be captured by a typical 8-inch sampling probe at that time. See below for more tips on sampling for corn nematodes.

Note that laboratories should extract nematodes from the soil, as well as endoparasitic nematodes (such as lesion nematodes) from root material. Some labs may also require the submission of entire plants or root balls, in addition to soil samples to conduct these tests, so it is a good idea to contact your lab of choice to determine what they require.

It is necessary for the nematodes to be alive in these samples because they must crawl out of root material during one of the 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.

Sample corn fields for nematodes 4-8 weeks after planting when roots and any feeding nematodes should be accessible with sampling probes.

Diagnosing Symptomatic Areas. Nematodes can cause many types of symptoms, such as stunting, yellowing, root lesions and deformity, etc., all of which are often confused with symptoms caused by other common problems such as pH extremes (Figure 3), nutrient imbalances, and insect or herbicide injury, and are thus, 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 as shown in Figure 1, 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 when trying to diagnose a problem area in a field to collect a second sample from a nearby apparently healthy area of the field. Having both samples analyzed for plant parasitic nematodes will allow for comparison of nematode populations and a more definitive conclusion.

The reliability of your diagnosis depends on the quality of your sample. Nematodes must be alive for an effective analysis.

Establishing a Baseline. You could collect a sample for analysis if 1) you don’t have a particular problem spot in a field, but the overall yield has not been as high as expected and other possible causes, such as fertility issues, other pests, etc. have been ruled out, or 2) you are just curious about what nematodes are present. In this case, the most effective strategy would be to get a composite sample by collecting a random pattern of soil cores from less than 40 acres.

Testing Nematicides. The recent introduction of seed treatment nematicides, such as Avicta and VOTiVO, has increased management tools and many people have expressed interest in trying these products on their own farms.

One of the best ways to evaluate a product(s) on your farm is to conduct your own replicated strip trial, which many producers have chosen to do. However, this process can be complicated and labor intensive. Its ultimate success and ability to answer your questions depends on how well you’ve planned the trial and sometimes by other conditions that might be out of your control.

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

Conducting Your Own On-farm Nematicide Trials

Figure 3. Symptoms of low soil pH (4.4 pH here) and aluminum toxicity can be easily mistaken for nematode injury.

If you decide to conduct your own testing, you will need to collect yield data from replicated (multiple) strips (either with a yield monitor or weigh wagon) of at least three strips per treatment to account for the variability within a field. In addition, many people have expressed interest in sampling for nematodes within treatment strips to evaluate product performance on their farm.

There are no clear guidelines for how best to do this and the natural variability of nematode populations in the field may make the data impossible to interpret. At the very least, it would be necessary to collect nematode samples from treated strips twice — at planting to determine the initial population densities and a few weeks later to evaluate nematode reproduction of some of the species.

Samples collected for corn nematode analysis can be processed at the UNL Plant and 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.

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View Tamra Jackson discussing nematodes in corn with program host Doug Jose on this segment of UNL's Market Journal.

Additional Resources

• UNL's Plant Disease Central website
• Sampling Strip Trials for Nematodes, by Greg Tylka, Department of Plant Pathology, Iowa State University Integrated Management Newsletter, May 26, 2010
• Common Corn Nematode Characteristics, by Greg Tylka, Department of Plant Pathology, Iowa State University Integrated Management Newsletter, August 3, 2009
   

Tamra Jackson
Extension Plant Pathologist, Lincoln
Amy (Ziems) Timmerman
UNL Plant and Pest Diagnostic Clinic