Nematode Pests of Horticultural Crops and their Management (1+1)

General Morphology

"Nematode" is a greek word (nema = thread, oides = form) i.e. thread like organisms as they look like tiny threads moving under microscope.

Female Male

Figure 4.1 Morphological features of typical plant parasitic nematodes (female and male)

Synonyms: Threadworms, eelworms (serpentile eel like body), round worms, nemas, paudh/padap krimi etc.
Body Shape: Nematodes are generally vermiform having a cylindrical body tapering towards both anterior as well as posterior ends and having maximum diameter near mid body.

Figure 4.2 Cylindrical shape of a typical plant parasitic nematode

Some exceptions in shape:
1. Filiform – More elongated towards the body extremities e.g. Xiphinema, Longidorus, Paralongidorus

Figure 4.3 Filliform Xiphinema

2. Sausage shaped or plump- When body length is reduced but breadth remains same giving a plump look e.g.

Criconemoid group

Figure 4.4 Sausage shaped Criconemalla

3. Pyriform or flask shaped - Females of certain Genera swell to acquire saccate, pear shape or flask like structure.
e.g. Females of Meloidogyne spp.and Heterodera cysts

Meloidogyne Heterodera

Figure 4.5 Flask shapes /saccate nematodes

4. Kidney shaped or Reniform- eg. Rotylenchulus reniformis

Rotylenchulus reniformis Tylenchulus semipenetrans
Figure 4.6 Kidney shaped or Reniform nematodes

Sexual dimorphism:
Morphological differentiation between the genders of same species is referred as sexual dimorphism.
Both sexes look alike in most of the nematode species; males being smaller than females. However, sexual dimorphism has been observed in number of genera of order Tylenchida which are sedentary semi-endoparasitic/endoparasitic in their feeding behaviour. while the females of these forms become obese to acquire spherical or semi-spherical shapes, males, if present, remain vermiform. e.g. females of Meloidogyne and the cysts of Heterodera and Globodera are saccate. The females of Nacobbus, Rotylenchulus, Tylenchulus etc. are sub-spherical / kidney shaped.

Male and female of Meloidogyne spp.

Male and female of Heterodera glycines

Figure 4.7 Sexual dimorphism in plant parasitic nematodes

Body posture (Habitus)
The specific property of the various nematodes genera to acquire a definite posture on relaxation/ killing by gentle heat which is specific of the genus is called ‘Habitus’
eg.
Pratylenchus spp. acquire almost straight or slightly irregularly curved posture on relaxation by heat.
Hoplolaimus spp.adopts slightly ventral curve.
Tylenchorhynchus spp., Paratylenchus spp. etc.curve in a 'C' shape.
Helicotylenchus spp. and Rotylenchus spp.attain spiral shape.
The habitus has some taxonomic significance as it gives some indication of the presence of a particular species at low magnification.

Figure 4.8 Body postures (Habitus) in different plant parasitic nematodes

As a rule, the inside curvature of nematodes body is always towards ventral side with only one exception of genus ‘Dorsalla’ which curves in a ‘C’ shape dorsally.
Body size:
Plant nematodes are generally microscopic with their body size ranging from 0.3 to 2.0 mm.
Smallest - Paratylenchus minutus
Longest - Paralongidorus epimikis (up to one cm in length)

Figure 4.9 Comparative size of some nematodes Photo courtsey: G L Schumann, NemaPix

Segmentation:
Nematodes have an unsegmented body. The outer cuticle is often marked with superficial transverse grooves (striations) which form rings round the body. Striae are generally shallow and narrow (fine striations) but in Criconematids the striae are very deep and are referred as annules. In addition to transverse striations, the longitudinal markings known as lateral lines are also present in the lateral fields of the body.

Figure 4.9 Cuticular striations and Annulations

Coloration:
Nematodes are colourless and their body wall is transparent.

Body regions:

• Nematode body is tubular without any distinction of regions. However, for the sake of convenience, the anterior end bearing mouth, lips and stoma is referred as ‘Head’ and the region beyond the anus is rightly termed as ‘Tail’.
• Longitudinally, the nematode body is divisible in to four zones; the ventral bears the openings of digestive (anus), excretory (excretory pore), and reproductive (vulva, cloacal aperture) systems; the side exactly opposite to the ventral is dorsal. The other two sides located at right angles to the dorsal and ventral sides are right and left laterals.

Symmetries:

• The nematode body is basically bilaterally symmetrical i.e. when the body is cut in to two equal halves through the saggital (dorso-ventral) plane, one half is the mirror image of the other.
• Structures like lips show hexa-radiate symmetry and stoma and pharynx show tri-radiate symmetry. Nervous, reproductive and excretory systems show asymmetry.