Essentially, a pathogen must be isolated from host tissue, grown in pure culture on artificial media, a susceptible host must be inoculated, and the pathogen reisolated following the development of symptoms.
Most plant parasitic nematodes have never been cultured on artificial media and so cannot be subjected to Koch's postulates to prove pathogenicity.
Even if this were possible, the question can be raised as to how closely sterile laboratory conditons compare with field situations.
If plant parasitic nematodes are known to be present in a field, they are unlikely to disappear from that field whether or not they are found in a particular sample.
Each plant parasitic nematode will have a particular crop or group of crops on which they have been observed to cause problems. There is extensive literature on nematode-host associations. In some cases these are just what the word association indicates and pathogenicity has not been proven even to the extent of conducting greenhouse or microplot trials or extracting nematodes from plant tissue.
In other cases various types of research have been conducted to provide more positive evidence that symptoms observed are actually the result of nematode infestation. Several texts listing nematode-host associations have been written and may be found in public libraries or private collections of nematologists.
Several computerized databases are available on the WWW. At UC Davis, Lownsbery created one such listing from the literature from the 1800's to the 1970's which has since been entered into a computer database and is now available on the WWW. Also at UC Davis, Ferris and Caswell-Chen have created a database of the more recent literature with funding from the UC IPM and USDA-ARS Smith Lever projects which is also now available on the WWW either for purchase or downloading, or to be searched directly.
The UCIPM Pest Management Guidelines also list nematodes likely to cause problems on specific crops.
For a quick reference for California crops, the Phytonematology Study Guide contains a listing of important associations.
One then, can take a laboratory report and consult a listing or database to find out if nematodes are known to be a problem on particular crops of interest.
As discussed in the section on sampling, the time of the year a sample was taken can greatly influence the types of nematodes found and their population levels.
Experienced nematologists will sometimes be willing to predict potential subsequent damage based on this information and in some cases damage thresholds have been developed for nematodes.
If only a single species of nematode known to be a problem is present, the potential for damage may be different than if several parasitic species are present in the same situation.
If nematodes known to be vectors of plant viruses are present, the potential for virus spread or damage in a particular crop should be considered.
The potential for nematodes to interact with fungi and or bacteria is often difficult to interpret. Both the Lownsbery and the Ferris, Caswell-Chen databases contain information on known interactions between nematodes, viruses, bacteria, and fungi.
Nematode infestation places a stress on plants (tree picture). If this is the only stress active on a particular crop, growth reduction will likely be less than if multiple stress factors (e.g. fungi, improper irrigation, poor fertilization are operating).