Click on any
image for a more
detailed view

Disease Cycle and Epidemiology

Click on image for a more detailed view.

Epidemiology

Root-knot nematodes begin their lives as eggs that rapidly develop into J1 (first-stage juvenile) nematodes (Figure 16). The J1 stage resides entirely inside the translucent egg case, where it molts into a J2 nematode. The motile J2 stage is the only stage that can initiate infections (Figure 20). J2s attack growing root tips and enter roots intercellularly, behind the root cap (Figure 21). They move to the area of cell elongation where they initiate a feeding site by injecting esophageal gland secretions into root cells. These nematode secretions cause dramatic physiological changes in the parasitized cells, transforming them into giant-cells (Figure 11). If the nematode dies, so will the giant-cells upon which it feeds.


Figure 16	Figure 20

Figure 21	Figure 11

J2s do not possess reproductive organs. As with all nematodes, root-knot nematodes undergo four juvenile stages, each progressing through a "molting" process similar to that of insects. As a result of this process, juvenile root-knot nematodes have little resemblance to adult males and females. In the J4 stage, the progression from juvenile to globose adult females or to vermiform adult males becomes clearly visible. They emerge as adults from the J4 cuticle (Figure 22). A single female nematode can produce 500 to more than 1000 eggs (Figures 13, 23).


Figure 22	Figure 13

Figure 23

The length of a root-knot nematode life cycle varies among species but can be as short as two weeks. Nematodes in cooler regions typically have longer life cycles. Eggs may remain inside root tissue or may be released into the soil matrix. Eggs hatch at random, i.e. hatching does not require exposure to root exudates. Under favorable conditions, root-knot nematode eggs have been reported to survive for at least one year in the soil.