Mechanical injury results from the penetration of the stylet during feeding or from entry of nematodes into a plant and movement through roots, stems, bulbs, or leaves.
Sometimes this penetration and movement results in the death of cells, or cell growth manifested through stimulation of cell division or nuclear division.
Some nematodes selectively feed in the vascular tissue of the plant which disrupts the flow of water, nutrients, and food which are distributed via these tissues.
Nematode feeding and penetration creates openings in roots which makes it possible for other organisms such as bacteria and fungi to enter when they couldn't normally do so.
The interaction of nematodes with bacteria, fungi and viruses has been documented. In some cases the interaction produces more serious symptoms of disease, sometimes less serious, and sometimes no difference has been observed.
Some nematodes have been shown to be able to transmit viruses from one plant to another.
Increasing the susceptibility to environmental stress is perhaps the most significant effect that nematodes have on plants.
It is common for nematologists to assert "that nematodes don't kill plants". Of course there are exceptions to every statement of this sort, but as a general principle, it is a good thing to keep in mind. If one is assessing a new situation, in which plants are quickly succuming to an unknown disease, the problem is probably not caused by nematodes, or at least not by nematodes alone. One should be looking for other causes for the problem in addition to nematodes.
Nematodes are considered to be "very good parasites". They have been associated with plants for millions of years and have developed methods to survive as plant stressors. A parasite that kills its host is faced with serious survival problems relative to one that "only" stresses its host.