Nearly one hundred species of plant-parasitic nematodes have been described. They affect most crops causing important diseases. As plant pathogens, their effects are time-dependent and usually do not (unlike other pathogens) kill crops, but reduce crop yields to noneconomical levels in spite of "good agricultural" practices (fertilization, irrigation, agrochemicals, etc.).
Plant-parasitic nematodes differ regarding their feeding behavior (Figure 1). Migratory ectoparasites (Figure 1A) remain in the soil and feed on the external cell layers of the root. These nematodes cause little damage. Sedentary ectoparasites remain feeding for long periods at the same place therefore causing severe root damage. Migratory endoparasites enter the root and periodically feed as they migrate through the root. Their damage to root tissue can be very important and may cause other soil pathogens, such as fungi or bacteria, to act synergistically causing complex diseases (Agrios 1997). Sedentary endoparasites (Figure 1B) have developed complex feeding strategies. Essentially, their females become saccate once they start feeding and deeply modify the root tissue in their vicinity to obtain nutrients. The sedentary endoparasites consist of the most important plant-parasitic nematode groups: the root-knot nematodes (Meloidogyne spp.) and the cyst nematodes (Heterodera and Globodera spp.).
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