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Pathogen Biology

Early reports of the cyst-forming nematodes on soybeans from Asia classified them as variants of the sugar beet cyst nematode, Heterodera schachtii, and occasionally as the pea cyst nematode, Heterodera goettingiana. Ichinohe described the soybean cyst nematode (SCN) as a distinct species, H. glycines, in 1952. Morphological features of the adults of SCN are used for species identification. SCN adults are sexually dimorphic, meaning that they are dissimilar in appearance. The females are swollen and sedentary, and the males are vermiform (worm-shaped) and motile. Only the swollen female stage on the surface of soybean roots can be seen with the naked eye – the male and juvenile stages must be extracted from soil or plant roots and viewed under a microscope.

The second-stage juveniles (J2) of SCN are worm-shaped, 375-520 µm long, and about 18 µm in diameter. The overall body shape of the nematode is determined by the pressure of its internal body fluids pushing against its strong, but flexible, outer "cuticle" (like a water balloon). The outside of the cuticle has a series of fine rings (annulations), like an accordion, that allows the cuticle to bend at any point along the nematode's body. The cuticle is composed mainly of the structural protein collagen, and the cuticle is molted four times to allow growth and maturation of the nematode. 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 (Figure 6). The stylet is hollow (like a hypodermic needle) and protrudes from the head when used by the nematode for feeding from plant cells and penetrating plant tissues. The very outer tip of the nematode head above the stylet (called the "lip" region) is slightly elevated, rounded, and darkened in J2 of SCN. In a relatively clear area just below the stylet, a round, muscular pumping organ called the metacorpus can be seen - the metacorpus pumps substances (i.e., food and secretions) up and down the esophagus of the nematode. Just below the metacorpus is another relatively translucent 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. The tail of SCN J2 tapers uniformly to a fine, rounded tip that is hyaline (clear).

Figure 6

Adult females of SCN are the most easily diagnosed stage for this species. The females are lemon-shaped, ranging from 500 to 900 µm long and 200 to 700 µm wide (Figure 7a). The female's stylet is slender with knobs that project slightly backward. The vulva and anus collectively form a cone-shaped projection from the rear of the female's body, giving them a characteristic "lemon" shape. The SCN females swell and molt through the juvenile life stages until they protrude from the plant roots (see Disease Cycle). The adult females of SCN are initially white (on the root surface), and their heads are buried within the root for feeding. As the female ages, she turns yellow and eventually brown as she dies. Her body becomes a protective cyst encasing the eggs.

Figure 7a

Figure 7b

Adult males of SCN are worm-shaped and relatively long (Figure 7b), compared to second-stage juveniles. They are 1,200 to 1,400 µm long and 26 to 30 µm wide. Although the head, stylet, and esophagus are similar to second-stage juveniles, the tail is short with a blunt, rounded terminus. 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.

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