This year nematode damage in some Nebraska fields may be masked by the ample rainfall we’ve received, but rest assured, these plant parasitic nematodes are still there in almost every field. Their impact ranges from no obvious sign to severe crop injury and tremendous yield loss.

In soybean, the soybean cyst nematode is well known and receiving a lot of attention as we monitor its spread to new areas in Nebraska. In contrast, nematodes are already feeding on roots in almost every corn field in Nebraska to some degree. Often simply referred to as “corn nematodes,” they feed on corn as well as other hosts, including soybean, other crops and some weed species. There are more than 12 species of corn nematodes with common names such as sting, needle, stubby-root, lance, root-lesion, stunt, dagger, and spiral.

When Should you Scout?

Now, while plants are small (up to approximately V6 growth stage), may be the best time to sample sandy corn fields for nematodes. Fields with finer textured soils can be sampled for nematodes at almost anytime, including after harvest.

Whether nematodes cause serious crop injury and yield loss is determined by:

• which species are present in the field and
• their population densities.

The only way to determine whether nematodes are a potential risk factor or causing damage is by collecting and submitting a sample(s) to a laboratory for plant parasitic nematode analysis. It’s important to collect, handle, and submit samples carefully to avoid compromising the quality of the sample (and reliability of the analysis).

Timing is Important

Corn nematode species are diverse and cause varying levels of damage. For example, needle and sting nematodes (Figure 1) are relatively large and uncommon, but cause the worst damage to corn in noticeable spots in the field. Because of their larger size, sting and needle nematodes are only present in fields with at least 80% sand. These nematodes can move deep in the soil (up to several feet) as the season progresses, dropping below the reach of traditional soil probes. For that reason now, while plants are small (up to approximately V6 growth stage), may be the best time to sample sandy corn fields. Early in the season these nematodes are expected to be shallow, feeding on corn roots in the upper 8-10 inches of soil. Sampling now will increase the chance of capturing these nematode species.

Most fields, sandy or not, have a mixture of several nematode species at varying population densities. Other nematode species affecting corn are not known to travel deeper in the soil, thus there is minimal risk of missing them by sampling later in the season. Fields with finer textured soils can be sampled for nematodes at almost any time, including after harvest. For example, sampling can be done early in the season when symptomatic areas are more obvious or it can be delayed until after harvest when nematodes are at their highest population densities. Waiting until after harvest to sample may be more convenient for people who plan to collect soil samples for nutrient analyses and can simply collect additional soil for nematode analysis.

Nematodes in Corn, a segment on the June 10 Market Journal.

Some nematodes, such as root-lesion (also called lesion nematodes), are much more common, occurring in more than 93% of Nebraska corn fields regardless of soil texture. Lesion and other nematodes tend to cause less severe injury and symptoms (Figure 2) on corn than the sting or needle nematodes, but considering their wide distribution, when added together, their losses may be greater than many other nematodes considering their wide distribution.

Sampling Sandy Fields

Up to approximately V6 growth stage (within 4-8 weeks after planting)

• PLANTS
• Collect 4-6 plants by carefully digging roots
• SOIL
• Probe at an angle through root zone
• Probe at least 6-8 inches deep
• Approximately 20 soil cores needed
• Collect a total sample size of at least 2 cups

Quality Sample = Quality Analysis

Remember, the reliability of your diagnosis will depend on the quality of the sample you submit. The nematodes in your sample must be alive for an effective analysis.

• Samples should represent
• Double bag in sealable zipper-top plastic bags. Bag soil and plant samples separately.
• Handle gently to avoid rupturing nematodes
• 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 that 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 All Other Fields (NOT Sandy)

• Up to approximately V6 growth stage (within 4-8 weeks after  planting)
• Otherwise, sampling can be delayed until after harvest when collecting other soil samples for nutrient analyses. 
• PLANTS
• IF SAMPLING BY V6 - Collect 4-6 plants by carefully digging roots
• If sampling after V6, collecting additional roots is not necessary if soil cores are collected from the root zone.
• SOIL
• Probe at an angle through root zone
• Probe at least 6-8 inches deep
• Get approximately 20 soil cores
• Collect a total sample size of at least 2 cups
• Samples should represent less than 40 acres
• Double bag in sealable zipper-top plastic bags. Bag soil and plant samples separately.
• Handle gently to avoid rupturing nematodes
• Refrigerate until shipping if possible
• Package with soft packing material in a sturdy leak-proof container
• Print and fill out a Sample Submission Form indicating that 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.

Laboratories should extract nematodes from the soil, as well as endoparasitic nematodes (such as lesion nematodes) from root material. It is a good idea to contact your diagnostic laboratory to determine what kind of sample they request. 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.

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, etc., all of which are often confused with symptoms caused by other common problems such as pH extremes (Figure 3), nutrient imbalances, 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 (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.

Establishing a Baseline. If you don’t have a particular problem spot in a field, but the yield has not been as high as expected and other possible causes such as fertility issues, other pests, etc. have been ruled out, you may want to sample for nematodes. In this case, the most effective strategy would be to randomly collect soil cores from less than 40 acres and make a composite sample.

Samples collected for corn nematode analysis can be processed at the UNL Plant and Pest Diagnostic Clinic for a fee of $40 per sample. If using another laboratory, contact them for their sample submission requirements and fees.

Resources

For more information on corn nematodes view these Nebraska Extension resources:

• Market Journal segment (June 10, 2016)
• CropWatch YouTube video on Corn Nematodes
• Nematodes of Nebraska Field Crops (EC1908)