Sampling corn fields for nematodes

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Plant parasitic nematodes are in every field to some extent, ranging from no obvious crop impact to severe injury and tremendous yield loss. In recent years the soybean cyst nematode has been the focus of much attention as we monitor its spread across Nebraska. In contrast, nematodes are already feeding on corn roots in every field to varying degrees.

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. The degree of crop injury and yield loss in each field depends on

  • which nematode 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 appropriately in order to avoid compromising the quality of the sample and reliability of the results of the analysis. Recently, nematologists from several Midwest universities updated guidelines for collecting samples from corn for nematode diagnoses and management recommendations. This article describes the updated recommendations for collecting and submitting samples for corn nematode analysis.

When to Sample for Corn Nematodes

Corn nematode species are diverse and don't cause equal damage. For example, needle and sting nematodes are relatively large and uncommon, but often cause the worst visible injury. Because of their larger size, sting and needle nematodes are only present in fields with at least 80% sand.

They can be hard to detect since they can move down several feet in the soil beneath the reach of traditional soil probes. For that reason, now is the best time to sample sandy corn fields for nematodes while plants are small (up to approximately V6 growth stage). Early in the season these nematodes are expected to be shallow in the soil, feeding on shallow corn roots still mainly in the upper 8-10 inches of the soil profile. Scouting now will increase your chance of capturing them in a routine sample.

Most fields — whether sandy or not sandy — have a mixture of nematode species of varying population densities. Other nematode species affecting corn are not known to travel deeper in the soil and would be included in any late season soil samples. Fields with finer textured soils can be sampled for nematodes almost any time. In finer textured fields, sampling can be done early in the season when symptomatic areas are more obvious, or it can be delayed until after harvest, when nematodes will be at their highest population densities. Often, waiting until after harvest is more convenient if you’re planning to collect soil samples for nutrient analyses and can simply collect additional soil for nematode sampling.

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 symptoms and injury on corn than sting or needle nematodes, but likely cause greater losses than any other nematodes due to their wide distribution.

How to Sample for Nematodes in Corn

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

Sampling Sandy Fields

  • Sample up to approximately the V6 growth stage (within four to eight weeks after planting).
  • Plants:
    • Collect four to six plants by carefully digging roots.
  • Soil:
    • Probe at an angle through the root zone.
    • Probe at least 6-8 inches deep.
    • Take approximately 20 soil cores.
    • Collect a total sample size of at least two cups.
  • Samples should represent less than 40 acres.
  • Double bag the sample in sealable, zipper-top plastic bags.
    • Bag soil and plants separately.
  • Handle gently to avoid rupturing nematodes.
  • If possible, refrigerate until shipping.
  • Package with soft packing material in a sturdy leak-proof container.
  • Print and fill out a Sample Submission Form indicating in the blank area 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)

  • Sample up to approximately the V6 growth stage (within four to eight weeks after planting). Otherwise, sampling can be delayed until after harvest when other soil samples are taken for nutrient analyses.
  • Plants
    • o If sampling before V6, collect four to six plants by carefully digging roots.
    • o 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 the root zone.
    • Probe at least 6-8 inches deep.
    • Take approximately 20 soil cores.
    • Collect a total sample size of at least two cups.
  • Samples should represent less than 40 acres.
  • Double bag the sample in sealable, zipper-top plastic bags.
    • Bag soil and plants 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 in the blank area 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.

Note that 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 lab to determine what kind of sample they need. 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 it does other types of samples.

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, etc., all of which can be confused with symptoms from other common causes such as pH extremes, nutrient imbalances, insect or herbicide injury. This has frequently led to misdiagnoses.

Samples can be collected directly from symptomatic areas of a field; however, when a severely affected area is sampled, 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 field that is apparently healthy. 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, you may still want to collect a sample for analysis if the yield has not been as high as expected and other possible causes, such as fertility issues and other pests, have been ruled out, or you are just curious about what nematodes are present. In this case, the most effective sampling strategy would be to collect a random pattern of soil cores from less than 40 acres to give a composite sample.

Testing Nematicides. The recent introduction of new seed treatment nematicides such as Avicta and VOTiVO is providing new management tools. One of the best ways to evaluate the product(s) on your own farm is to conduct your own replicated strip trial. However, they can be complicated and labor intensive. Their ultimate success and whether they provide the information you need will depend on how well you’ve planned them as well as some conditions that may be out of your control.

Field Testing

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

If you have decided 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.

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 laboratory procedures can vary from one lab to another, contact your lab for their sample requirements.

Resource

Nematode sample submission form for the UNL Plant and Pest Diagnostic Clinic.


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