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.