Interpreting Nematode Reports]
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||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
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.