However, if the report is negative, one must consider if resampling in a different manner or at a different time of the year might have yielded or will yield different results.
As discussed under the topic of sampling, for those instances where the population cycling of nematodes on perennial crops has been examined, it can be observed that there are times of the year when one will be more likely to detect nematodes than others. If the sample was taken at a time when nematodes are known to be at low levels, then resampling at a different time might produce different results.
For annual crops, nematodes are typically highest at or shortly before harvest.
If a field remains fallow for a period of time, numbers of nematodes will start to decrease and may drop below the detection ability of the extraction technique used. In this case, a grower might rely on past experience with a particular field and still utilize some type of nematode management prior to planting a susceptible crop.
It seems to be easier to sample and to extract nematodes from lighter (sandier) rather than heavier (large amounts of silt and clay) soils. Heavier soils are more difficult to penetrate with sampling equipment when dry and more likely to stick to the sampling equipment when wet.
Consider the depth at which the sample was taken, keeping in mind that root parasites are likely to be most prevalent at the rooting depth of the crop.
As reviewed in the discussion on sampling, the smaller the area covered by a sample composed of a number of subsamples, the more representative the sample will be. Choosing the size of the area to represented by a single sample is determined by time and cost to take and process the sample and practicality. If the area covered by the sample is 5 acres or less, most field nematologists feel a multi core sample is fairly representative of what is present or can be recovered at the time of sampling. As the area covered by a single sample increases, confidence in interpreting the results of a negative sample decreases. As the number of subsamples in a sample increases, confidence in the results increases.
If a sample does not yield plant parasitic nematodes when expected, or yields no nematodes at all, the storage conditions and length of time from the time of sampling to processing should be reviewed. If samples become too warm during shipment as could happen if left in a vehicle, or shipped by public or private carriers, nematodes could be killed and not recovered during processing.
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.
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).
A newly discovered situation typically begins with the observation that certain plant parasitic nematodes are always associated with the symptoms present. This is often coupled with knowledge that the same nematode has historically caused similar problems on a similar crop.
If it is possible to raise the nematode and crop of interest in a greenhouse, growth chamber, or microplot, then comparisons between inoculated and noninoculated pots or plots can be compared. One potential problem is that unless sterile cultures of the nematode can be produced (which is actually very difficult to achieve), it is possible that another, contaminating organism is also being transferred along with the nematode.
When chemical nematicides first became available for widespread use, it became possible for nematologists to dramatically demonstrate (lesion on begonia pictures) crop improvement following nematode reductions, an indirect way of showing pathogenicity. This is not always as straightforward as it first appears, because nematicides could potentially be killing other oganisms as well as nematodes, and at times, nematicides have been shown to increase crop growth irrespective of nematode presence.
In the few cases where nematode resistant varieties are available (e.g tomatoes and root-knot nematode), it has been possible to demonstrate growth differences between resistant and nonresistant varieties planted in infested fields.