Plant-parasitic nematodes include several groups causing severe crop losses. The most common genera are Aphelenchoides, Ditylenchus, Globodera (potato cyst nematodes), Heterodera (soybean cyst nematodes), Longidorus, Meloidogyne (root-knot nematodes), Nacobbus, Pratylenchus (lesion nematodes), Trichodorus, and Xiphinema (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (e.g. by root-knot nematodes), which are useful characters for his or her diagnostic within the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is the Xiphinema index, vector of grapevine fanleaf virus, a crucial disease of grapes, another one is Xiphinema diversicaudatum, vector of arabis mosaic virus. Other nematodes attack bark and forest trees. The most important representative of this group is Bursaphelenchus xylophilus, the pine wood nematode, present in Asia and America and recently discovered in Europe.
