Costly parasites beneath the soil: general knowledge about plant-parasitic nematodes

Plant parasitic nematodes are soil-dwelling worms or thread-like organisms that feed on plant roots, damage plants, and cause yield loss. There are over 4,100 species of plant-parasitic nematodes that have been described. Together, these nematodes are responsible for approximately $80 billion USD in crop damage every year.

The most common and problematic among all plant parasitic nematodes include root-knot nematodes (Meloidogyne incognita, M. hapla, and M. javanica,) soybean cyst nematode (Heterodera glycines), lesion nematode (Pratylenchus spp.), dagger nematode (Xiphinema americanum), reniform nematode (Rotylenchulus spp.), spiral nematode (Helicotylenchus pseudorobustus), and stunt nematode (Tylenchorhynchus claytoni) (Bernard, 1980; Jones et al., 2013). These nematodes can attack and cause a significant crop loss on one or many species of plants.

Plant-parasitic nematodes are broadly classified into four main categories based on their association and how they feed on host plants. The four categories include:

  1. Migratory ectoparasites
  2. Migratory endo-parasites
  3. Semi-endoparasites
  4. Sedentary endoparasites

Migratory ectoparasites: Nematodes in this category live outside of their host plants and use their needle-like mouthpart (known as stylet) to feed on the plant parts. They are mostly root feeders, but some can feed on stems, young leaves, and flowers. The root feeders can possess a long or short stylet. Those with short stylet will feed on the external surface of host roots, while those with long stylet can use their stylet to reach inside the tissue to feed on tissue cells. Examples of short stylet migratory ectoparasites include stunt nematodes, stubby-root nematodes, and spiral nematodes). Examples of long stylet migratory ectoparasite nematodes include sting nematodes, ring nematodes, needle, and dagger nematodes.

Migratory endo-parasites: These nematodes will enter their host roots and migrate within the tissue and upon cortical cells. As they move through the roots, they damage root cells and also introduce pathogenic microorganisms into the plant body. Some examples of migratory endoparasites include lesion nematode, lance nematode, reniform nematode, some spiral nematodes, stem nematode, leaf nematode, pinewood nematode, and yam nematode.

Semi-endoparasites: Nematodes in this category will insert the anterior part of their body into the root of their host, while the posterior part of their body will remain outside swollen. The nematodes use their stylet to form a permanent feeding site inside the plant for long-lasting food. Examples of this nematode include reniform nematode and citrus nematode.

Sedentary endoparasites: These nematodes will completely enter the body of their host plant, develop a permanent feeding site inside the plant tissue, and remain inside the plant until the end of their life cycle. Examples of sedentary nematodes include the soybean cyst nematode and root-knot nematodes.

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