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Crop Protection :: Crop Nematodes :: Introduction

Plant Parasitic Nematodes

Plant nematodes attack all crops grown worldwide and cause considerable economic yield loss.

Morphology and Anatomy

Nematodes that attack plants are worms, mostly microscopic in size, ranging from 0.2 mm to 11.0 mm in length. They are generally cylindrical in shape, tapering towards both head and tail ends. Females of a few species lose their worm shape as they mature, becoming greatly enlarged in diameter and assuming varying forms, such as pear, lemon, or kidney shaped. In spite of their small size, nematodes are complex in organization. Plant parasitic nematodes possess all systems as in higher animals except respiratory and circulatory systems. The body is covered by a multilayered cuticle which bears surface marks which are used when identifying nematode species. The cuticle is transparent complex and made up with three layers. Generally nematodes cannot be seen through naked eyes.

Life Cycle and Reproduction

Plant parasitic nematodes have a simple life cycle of six stages: egg, four juvenile stages and adult. The embryo develops inside the egg to become the first stage juvenile. The first stage juvenile molts inside the eggshell to become a second stage juvenile, which hatches from the egg. The nematode molts three more times to become a fully developed adult. Male and female nematodes occur in most species, and both may be required for reproduction. However, reproduction without males is common, and some species are hermaphroditic. Egg production by the individual completes the Life cycle of the nematode and it is varying between species for example the root knot nematode completes its life cycle in 30 days. Where as Dorylaimid nematodes life cycle ranged from 6 months to 2 years depending up on the hosts. Most of the ectoparasitic nematodes lays their eggs singly, sendentary and semi endoparasitic nematodes lays their eggs in masses.

Nematode Feeding and Host-Parasite Relationships

The feeding / living relationships that nematodes have with their hosts affect sampling methods and the success of management practices. Ectoparasitic nematodes that never enter roots may be recovered only from soil samples. Endoparasitic nematodes often are detected most easily in samples of the tissues in which they feed and live (burrowing and lesion nematodes), but some occur more commonly as migratory stages in the soil (root knot and reniform nematodes). Those stages of endoparasites which are inside root tissues may be protected from nematicides which do not penetrate into roots, such as some soil fumigants.

Ectoparasitic Nematodes

Ectoparasitic nematodes are either migratory or sedentary and feed superficially at or very near the root tip or on root hairs, but a few have stylets long enough to enable them to feed deeper in the root. Those which cause the most widespread and severe plant injury are the stubby root (Trichodorus spp.) and awl (Dolichodorus spp.) nematodes. These nematodes feed at or near root tips and usually inhibit root elongation. Ectoparasites which rarely cause severe injury to their plant hosts include ring (Criconemoides spp.) and spiral (Helicotylenchus spp.) nematodes. They apparently feed primarily on root hairs and superficial cortical tissues and cause serious injury only to plants that are especially sensitive to drought stress. Among plant parasitic nematodes, only stubby-root nematodes and their close relatives, the dagger (Xiphinema spp.) and needle (Longidorus spp.) nematodes, are known to transmit plant viruses.


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