This is the second of a series of short articles describing nematodes that affect plants.
What are plant parasitic nematodes?
Like all nematodes, plant parasitic nematodes are unsegmented, tiny worms (see Blog of Feb. 26, 2019). Plant parasitic nematodes are aquatic organisms that require moisture, fluids, and water in order to be active and to move. Size-wise, most plant parasitic nematodes, especially the worm-shaped species (Photo 1), are very small and are not visible with the unaided eye. A few species form specialized structures that can be observed without magnification, such as the female cysts (Photo 2) of the cyst nematodes (Heterodera and Globodera genera). The entire nematode group (classified as Nematoidea) is extremely large and diverse; some researchers estimate that there are over 27,000 total nematode species. Nematode species that infect plants are a relatively small subsection consisting of approximately 4,000 species; therefore, the parasitic nematodes affecting plants consist of less than 15% of the entire taxonomic group.
Most plant parasitic nematodes are primarily found in the soil, where they survive and persist until they encounter the roots of their host plants. Once in contact with host roots, the nematode can penetrate plant cells and feed on the cell contents by inserting a sharp, spear-like structure (stylet) found in the nematode mouth (Photo 3). Nematodes that do not have stylets (Photos 4 and 5) are not parasitic on plants.
Plant parasitic nematodes can be divided into different feeding groups depending on how they develop in relation to the plant:
- Ectoparasitic: the nematode stays external to the root and feeds by extending its stylet into the root. Example: Xiphinema (dagger nematode).
- Semi-endoparasitic: only the front part of the nematode penetrates the root, while the posterior section remains outside. Example: Tylenchulus (citrus nematode).
- Endoparasitic-migratory: the entire nematode enters the host plant and can move throughout plant tissues. Example: Pratylenchus (lesion nematode).
- Endoparasitic-sedentary: the entire nematode enters the host plant but generally establishes a fixed feeding site and does not migrate in the plant tissues. Example: Meloidogyne (root knot nematode).
Future articles will describe specific plant parasitic nematodes and their impact on specific crops.
Prepared by Steve Koike and Kristi Sanchez
Photos 4 and 5. Non-plant parasitic nematodes lack stylets and have diverse mouth structures such as this bacterial feeding species (above: note the tube shaped mouth cavity) and this predatory nematode (below: note the tooth-like structures in the mouth cavity).
