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.
In the 1990's, several computerized databases were developed for use via 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. 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. NEMAPLEX also has a web searchable version of NEMABASE.
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 (out of print) contained a listing of important associations that is provided here: California Nematode Host 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 campus based security concerns have increased, and funding to continue to update the databases has become more difficult to obtain, it has become more difficult to keep these databases available for public use. A web search for "nematode database" should bring up those currently available.
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)