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Disease Cycle and Epidemiology

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Like all nematodes, the soybean cyst nematode (SCN) has six life stages - egg, four juvenile stages (J1-J4), and the adult stage. The duration of the SCN life cycle runs from 3-4 weeks, but this may be influenced by environmental conditions (mainly adequate temperature and moisture). Depending upon the environment, several generations of SCN can be completed in a typical soybean growing season. After embryonic development within the egg to the first-stage juvenile, the nematode goes through four molts to the adult stage. The molt to second-stage juvenile (J2) occurs within the eggshell (Figure 8), and it is the J2 that emerges from the egg. Egg hatch seems to have evolved as a survival strategy in SCN. A low percentage of SCN eggs appear to hatch spontaneously -- it has been suggested that these eggs are the ones that are laid in a gelatinous matrix outside the female body. A significant proportion of eggs that are retained within cysts are in a dormant state -- they do not hatch until soybeans are planted for the next growing season. It is hypothesized that exudates from soybean roots provide the hatching signals for dormant SCN. Another proportion of eggs within cysts do not hatch, even when conditions are favorable - they are in a state termed diapause. Diapause appears to be a time-mediated hatching process, the basis of which is not presently understood.

Figure 8

The J2 is the infective stage of SCN. The J2 migrates in soil and penetrates plant roots completely, usually just behind the root tip (Figure 9). The J2 moves intracellularly to the root vascular tissue, often leaving a zone of visible root necrosis along their migratory path within the root. The J2 uses thrusts of its stylet and secretes cell-wall-degrading enzymes (cellulases) to migrate directly through plant cells (Figure 10). When the J2 reaches the root vascular tissue, nematode stylet secretions modify selected plant cells into an elaborate feeding site called a syncytium (Figure 11). The J2 will not continue development unless a syncytium is formed for feeding. The syncytium is a large, metabolically active feeding site that becomes multinucleate as neighboring plant cells are incorporated into the syncytium by cell wall dissolution and cell fusion. The J2 feeds from the syncytium and begins to swell and become immobile (Figure 12).

Figure 9	Figure 10
Figure 11	Figure 12

The subsequent juvenile stages molt and continue to enlarge as the nematode feeds. Approximately half of the juveniles will become swollen females, and the majority of the female body (except for the head) breaks through the root surface and becomes visible on the surface of the root (Figures 13, 14). Males develop coiled within the swollen J4 cuticle (Figure 15), and they emerge from the cuticle and root as motile, vermiform adult nematodes (Figure 7).

Figure 13	Figure 14
Figure 15

It has been documented that a higher percentage of males is produced when the nematodes or host plants are under stress. Males do not feed, but they are required for sexual reproduction (copulation) with females that are exposed on the root surface. After fertilization, the majority of the 200-600 eggs produced by the female are retained within its body, but some eggs may be laid in a gelatinous matrix extruded from the posterior (vulva) of the female. As the gravid female dies, its cuticle becomes a brown, hardened structure (the cyst) that encases and protects hundreds of viable eggs (Figure 16). Cysts often fall from roots and remain free in the soil.

Figure 16

Epidemiology

As with many plant-parasitic nematodes in soil, soybean cyst nematodes do not move far from the root zone that they currently infest. In most cases, the natural migration of SCN within a field is defined as "contagious" – small patches of infested areas that gradually enlarge to encompass significant areas of disease. Diseased areas become much more pronounced in sections of soybean fields that are under environmental stress (i.e., insufficient fertilization or water, extreme temperatures). The spread of these nematodes within fields usually is accelerated by cultural practices of the grower (i.e., dissemination of nematodes by soil cultivation). SCN can be introduced to uninfested sites via poorly sanitized farm equipment and within soil peds (small, seed-size clumps of dried soil) within contaminated seed stocks (Figure 17). SCN is not transmitted inside individual seeds nor on the seed surface (SCN is not "seed borne"). The cysts of SCN are lightweight and can survive desiccation. They can be transmitted in surface water, by wind, and also by the movement of animals. It can take from 5-9 years from the time of introduction of SCN into a field until the SCN populations reach detectable levels. By that time, infestation with SCN has become a permanent problem in that site.

Figure 17

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