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Pathogen Biology

Genus: Pratylenchus
The "type" (representative) species of this genus, P. pratensis was described in 1880 before the organization of the genus Pratylenchus in 1936, and the inclusion of Pratylenchus as the type genus in the family Pratylenchidae in 1949. Approximately 70 species have been described within this genus, although the designation of about one-third of the members as discrete species is a point of controversy among nematode taxonomists. The species of main importance on economic crop plants are P. brachyurus, P. coffeae, P. penetrans, P. scribneri, P. vulnus, and P. zeae. All postembryonic Pratylenchus life stages are worm-shaped (vermiform), and they are migratory endoparasites of plants (they feed and move within plants, mainly the roots). All life stages, including the adults, are microscopic and must be extracted from soil or plant material before they can be identified under a microscope.

Morphology:
As with most nematodes, the taxonomic designations of lesion nematode species are based upon morphological characters of the adult nematodes (Figures 1,2,and 8). Juveniles of Pratylenchus generally have the same characters as the adults (on a smaller scale), with the absence of fully developed reproductive organs (see Disease Cycle). The overall body shape of any nematode is determined by the pressure of its internal body fluids pushing against its strong, but flexible, outer "cuticle" (like a water balloon). The entire cuticle has a series of fine rings (annulations), like an earthworm, which allow the cuticle to bend at any point along the nematode's body. The cuticle is composed mainly of a structural protein, collagen, and the cuticle is molted four times to allow growth and maturation of the nematode. Adults of Pratylenchus are 300-900 ľm long (depending upon species) and relatively stout (body length ÷ body width = 20-30, usually). The "head" of the nematode can be recognized by the presence of a short, dark spear with basal knobs (the "stylet") just inside the tip of the head. The stylet is hollow (like a hypodermic needle) and is protruded from the head when used by the nematode for penetrating plant tissues and feeding from cells. The very outer tip of the nematode head above the stylet (called the "lip"region) is characteristically flat and blackened in the genus Pratylenchus. In a relatively clear area just below the stylet can be seen a round, muscular pumping organ called the metacorpus - the metacorpus pumps substances (i.e. food and secretions) up and down the esophagus of the nematode. Just below the metacorpus is another relatively clear area that contains three esophageal glands that overlap the nematode's intestine on the ventral (stomach) side of its body. The intestine can be recognized as a fairly long dark area extending from the esophageal glands to the tail of the nematode.

Figure 1

Figure 2

Figure 8

Females:
The adult female can be recognized by an opening in the cuticle on the ventral side (the vulva) that is about 70-85% of the body length down from the head (depending upon the species) of the nematode (Figure 1). A clear row of cells (superimposed over the dark intestine) that get progressively larger leading down to the vulva is the female gonad that produces the eggs. Below the vulva near the tail is the opening of the intestine, the anus. The female tail tapers like a cone but it is rounded at the end (termed "conoid" in shape).

Males:
Pratylenchus males are generally slightly smaller and more slender than females, with the absence of the gonad and vulva. The males have a row of cells that form the testis. The testes look like the gonads in the female, but they empty at the anal opening. An easy, defining characteristic of males is the presence of two dark hooks (called "spicules") that are always present at the opening of the testis near the tail (Figure 10). The spicules are extruded and used during copulation. The tail of male is more pointed that the tail of females, and it often has two flaps of cuticle attached ("alae" = "bursa").

Figure 10

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