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Anguina agrostis (female, above and male, below), a plant-parasitic nematode.

Stained, juvenile nematodes (Meloidogyne arenaria) in a peanut root (Arachis hypogeae).

Stained, young female root-knot nematode (Meloidogyne arenaria) in a peanut root (Arachis hypogeae).

Stained, adult female root-knot nematodes (Meloidogyne arenaria) in a peanut root (Arachis hypogeae).

Size comparison among adult female root-knot nematodes (pear-shaped), an adult male root-knot nematode (elongated vermiform shape) and a human eyelash.

Magnified heaf of a plant-parasitic nematode.

Generic life cycle of a root-knot nematode (Meloidogyne sp.)

Definition

Nematodes are generally microscopic, unsegmented roundworms that live either saprophytically in water or soil, or as parasites or predators of plants and animals.  They are bilaterally symmetrical, triploblastic (3 distinct germ lines) and pseudoceolemic (having an unlined body cavity, there is no cell lining derived from an endoderm).

Etymology. 1865, from Mod.L. Nematoda, the class or phylum name, from Gk. nema (gen. nematos) "thread" (from stem of nein "to spin;" see needle) + -ode "in the nature of."

Discussion

All plant parasitic nematodes are essentially aquatic animals; their activity and  movement depends on a film of free water. The nematode body is more or less transparent. Nematodes exist in all climates around the world.Nematodes are one of the most abundant life forms on earth. Nematodes are so universally distributed, that if plant life of any form grows in the soil  it is almost certain to contain several species. Nematodes are obligate biotrophs. Nematodes attack roots, stems, leaves, and inflorescences of plants. Nematodes feed on cells by puncturing cell walls with their stylets. Nematodes are often involved in plant disease syndromes. 

 

Nematode taxonomy is based upon nematode morphology and genetic traits. Some nematodes may remain dormant in the soil for many years (via cryptobiosis, eelwool, quiescence). Nematodes proceed through several life stages: eggs, larvae (e.g., several juvenile stages) and adults.

 

Plant parasitic nematodes are very diverse with regard to their:

 

- mode of parasitism (e.g., sedentary vs mobile)

- morphology (e.g., pear-shaped to vermiform)

- effect on host plants (some cause hypertrophy and hyperplasia, some cause massive cell necrosis).

- mode of reproduction (sexual vs asexual reproduction)

 

Typical symptoms of nematode diseases include:

 

Root symptoms: knots, galls, lesions, excessive root branching, injured root tips, root rots (when accompanied by root-infecting fungi). Root symptoms are generally accompanied by foliar symptoms: stunting, reduced yield, wilting in hot/dry weather, chlorosis.

Foliar symptoms: galls, necrotic lesions, rots, abnormal development, distortion of leaves and stems.