IPMP3.0, Oregon State University, Copyright 2000



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One of the most frustrating aspects of interpreting nematode reports is that all labs do not report results in the same units. This is further complicated by the fact that researchers working on damage threshold levels, and other aspects of nematology, also use a variety of units to express their results.

Nematode counts from soil may be expressed on a weight basis (#/g, #/100 g, #/250 g, etc.) or volume basis (#/250 cc, #/pint, #/quart, etc.). Counts may be expressed on the basis of wet soil or on the basis of dry soil, i.e. corrected for the amount of moisture in the soil. Generally, counts based on weight are more precise than those based on volume because it is easier to weigh out soil consistently than it is to measure the volume of soil consistently. However, if adjustment is not made for the amount of water in the soil, counts based on weight will be highly variable depending on the amount of moisture in the soil at time of processing.

Nematode counts from roots or other plant material are usually expressed as #/g tissue but this may be fresh weight or dry weight depending on the lab processing the samples.

If you work with only one laboratory, units of expression are less of a problem, as long as that laboratory is consistent, because once you learn how to interpret a particular laboratory's results, nematode counts on different reports are relative to each other. However, determining the meaning for your counts may still be awkward if you have to look into the literature and the research results are reported in different units than your lab results.

Understanding the units that nematode densities are expressed in is extremely important. For example, a grower may know that he needs to treat a field if there are 1000 root-lesion nematodes/quart soil. One year he sends soil samples to a different lab (because they are faster, cheaper, etc.) and receives a count of 300 and concludes that he does not need to treat. However, the new lab expresses counts at #/250 cc, so his 300 converts to 1140/quart and he really should treat. Unless growers, extension agents and crop consultants are aware that counts are expressed in different units, they may only focus on the number reported. This is particularly critical with root counts since 1 nematode/g wet root is approximately equal to 10 nematodes/g dry root! Most unit conversions can be done with simple math. The important point is to be aware of the units for the counts you are trying to interpret and ask your lab to explain their units to you if necessary.