Nematode Testing

Latest revision: 
March 2023

The University of Idaho perform nematode tests. A fee is levied for each sample of soil or roots tested.

Sample Contents

Soil A composite sample of approximately 1 pint of soil is best for an accurate routine test to diagnose a plant problem or confirm nematode populations. A soil tube 0.875 inches by 12 inches is the best sampling device, but trowels or shovels may also be used. Take the composite sample from at least 20 locations within no more than 5 acres. If sampling poor areas or weak spots, sample from the edge rather than from the middle.

If plants of the crop in question are available, roots and surrounding soil may be included in the sample. Some crops are susceptible to nematodes found in roots, and others are not, so it is advised to call the lab to determine if collecting roots is necessary before the sample is taken. When diagnosing a plant problem, soil samples from both the healthy and diseased part of the field will aid in interpreting the final results.

Plants It is best to take a composite sample (10 or more root subsamples within the 5-acre area) for an accurate routine test for root-dwelling nematodes. The sampler is responsible for ensuring that the roots in the sample bag belong to the species or cultivar named on the label. If the sampling area does not contain plants of the injured crop, it is possible to include an alternate host plant, such as clover or vetch for root-lesion nematodes.

Information Please fill out a Nematode Test Form which is available online and from your county Extension office. Information should include: state, date, grower's name and address, location of sampling area, name and cultivar of crop last grown, and name and cultivar of crop to be planted. Make sure sample bags are labeled with the same designation as used to describe the sample on the form. Sample bags should be labeled with an indelible ink that will not be altered by moisture or abrasion. Samples that do not have a legible label have no value. Do not let a paper label or form come into contact with the sample.

Packing Standard soil test bags or polyethylene bags are satisfactory containers for samples; they maintain soil moisture. Avoid exposing samples to heat, such as direct sunlight or the inside of a hot vehicle. Because some nematode extractions rely on living, active nematodes, heat and drying must be avoided or results will be compromised. Do not use paper bags, glass jars, cardboard boxes, sandwich bags, grocery store plastic bags, bread bags, or other thin plastic bags. Attach a label or Nematode Test Form to the outside of the sample bag.

Address to:

SW Idaho R/E Center
29603 U of I Lane
Parma, ID 83660-9637

Interpreting Nematode Test Reports

Nematode test reports must be interpreted properly before a management decision can be made. Most laboratories use different forms to report results, but all contain essentially the same information. When interpreting the information on these forms, consider units of nematode density, extraction method, nematode species, time of year, and damage thresholds.

Units of Nematode Density Different laboratories report nematode densities on different bases. In addition, damage-threshold levels are reported in a variety of different units. If you send samples to only one laboratory, interpreting its report of density units is less of a problem. Be aware of the type of density-units expression your laboratory uses to report results. If necessary, ask your lab to explain its system of expressing density units to you.

Nematode densities (or counts) may be expressed on the basis of weight (for example, number of nematodes per gram of soil) or volume (such as number of nematodes per cubic centimeter or pint of soil). Counts also may be expressed on the basis of wet or dry soil. Generally, counts based on weight are more consistent than those based on volume. If an adjustment is not made for the amount of water in the soil, however, counts based on weight can be variable. Nematode counts from roots or other plant material are usually expressed by weight of plant material but may be based on either fresh or dry weight depending on the lab processing the samples.

Understanding density units is critical in making treatment decisions. For example, a field may need treatment when there are more than 100 root-lesion nematodes per 500 grams of soil. If the laboratory reports the density as 50 root-lesion nematodes per 100 grams soil, one might conclude that the field does not need treatment. However, this density converts to 250 nematodes per 500 grams of soil, which indicates a different treatment decision.

Extraction Efficiency Several methods can be used to extract nematodes from soil and plant tissue. Each method produces different nematode counts based on the number of live nematodes, duration of extraction, soil type, and nematode species present. Problems can occur when different laboratories' results are compared with one another or with thresholds reported in the literature. Again, if you send samples to only one laboratory, interpreting extraction methods is less of a problem.

Baermann funnel extraction requires several days and recovers only live nematodes. Extractions using density centrifugation are faster and yield both living and dead nematodes. The number of nematodes extracted from roots after only 1 day will be about 10% of the number recovered if roots are extracted for 1 week. Extraction efficiency is higher for lighter soils than for heavy soils.

Some nematode species are more efficiently extracted by a particular method. For example, ring nematodes are best recovered by density centrifugation and may be underestimated if Baermann funnel techniques are used.

Nematode Species Almost all laboratories provide the common and/or generic name of all plant-parasitic nematodes recovered from a sample. However, different nematode species within a given genus may damage a crop differently. Species identification may therefore be important, but it is more difficult than routine nematode tests and may require an extra fee. In mint, for example, pin nematodes can be very damaging, but there are no data to show that different species produce different levels of damage. Therefore, knowing the species of a particular pin nematode in a mint field is not important. On the other hand, barley root-knot (Meloidogyne naasi), Columbia root-knot (M. chitwoodi), and northern root-knot (M. hapla) may be found on crops grown in rotation with mint. Any of these nematodes may be in a sample taken before mint is planted; however, only northern root-knot nematode will damage mint.

Time of Year for Sampling Population densities of nematodes go through cycles during the year. A count of five nematodes per 100 grams of soil at the low part of a nematode's population cycle represents a greater threat than the same count at the peak of the population cycle. If soil samples for a particular nematode species are always taken at the same time of the year, interpreting the results is much easier.

Damage Level Information. Ultimately, the number of nematodes in a soil sample taken at a certain time relates to the damage the nematodes may cause the crop. Some quantitative damage figures for nematode-crop interactions in the Pacific Northwest are available.

The experience of local growers, crop consultants, and Extension agents also is valuable in making management decisions. In general, a healthy crop can withstand higher nematode densities than crops stressed by factors such as a lack of water or nutrients or the activity of other pests. Damage usually is greater on sandy soils.