IPMP3.0, Oregon State University, Copyright 2000



[Return to Interpreting Nematode Reports]

[Return to Nematode Management]

[Return to Nematode Sampling]

[Table of Contents]


Another, much more difficult, problem is that there are several methods with many variations that can be used to extract nematodes from soil and plant tissue. Any of these procedures may be used by the laboratory in question. Since no method is 100% efficient, the count reported is only some percentage of what was actually present in the sample. Few, if any, labs correct their reported counts for the extraction efficiency of the methods they use. Again, this is generally not a big problem if you use the same lab all the time and learn to interpret the counts you get (however, they are determined) from that lab. The problem originates, as before, if you switch labs, try to compare labs with one another or try to relate counts you receive with those reported in the literature.

The most important factors which can influence extraction efficiency include:

1) Method used: i.e. Baermann Funnel vs Density Centrifugation etc.

Baermann funnel extraction takes longer but only recovers live nematodes. Density centrifugation is faster but extracts living and dead nematodes. Some interpretation of dead nematodes may be necessary since only "live" nematodes in the field are capable of damaging crops. Dead nematodes found in samples from established mint probably represent nematodes which died just before or after the sample was taken and, thus, can be included with live nematodes in making a population estimate. Dead nematodes are excellent resources for soil microbes and decompose quickly. Few products are put on mint that would substantially reduce microbial activity.

2) Duration of extraction period

Some labs specialize in rapid turnaround of test reports back to the submitter. Centrifugation techniques can extract nematodes from soil more rapidly than Baermann funnel methods, but there is no way to adequately speed the extraction of nematodes from roots. Roots extracted for only one day will only yield about 10% of the number of nematodes recovered if roots are extracted for one week.

3) Soil Type

In general, efficiency of extraction is higher in lighter soils than in heavy soils.

4) Nematode Species

Some species are more efficiently extracted by one method and other species are more efficiently extracted by another method.

5) How sample was handled prior to extraction

How a sample is handled after it is removed from the field can substantially influence the recovery of nematodes present in the sample. If an extraction procedure requires nematodes to be alive to be recovered and the sample is handled improperly (see section on sampling), so that nematodes die before extraction, the lab will report a number that is an underestimate of what was in the field.