Nematode transmission of plant viruses is remarkable in involving only two distinct groups of viruses, nepo and tobra, and being limited to longidorid and trichodorid nematodes. Further, only a small proportion of species in the virus vector genera have been shown to be capable of transmitting and these are mainly located in Europe and North America. In a survey of virus vector nematodes in Britain, Taylor and Brown (1976) found that very few vector populations were viruliferous. The association between viruses and vector nematodes possibly originated independently in the two continents as the viruses and vectors are quite distinct between each (Taylor and Brown, 1981; Lamberti and Roca, 1987). Some of the vectors and viruses have been disseminated to other geographical areas e.g. arabis mosaic virus (AMV) and Xiphinema diversicaudatum from Europe to New Zealand (Thomas and Proctor, 1972); grapevine fanleaf virus (GFV) and Xiphinema index from Ancient Persia to Europe and North America (Hewitt, 1968). Also, several of the nepoviruses have been disseminated in planting material e.g. strawberry latent ringspot virus from Europe to California, USA (Hanson and Campbell, 1979); tomato black ring virus (TBRV) from Europe to Kenya, Africa (Kaiser et al., 1978); tomato ringspot virus (TomRSV) from North America to Europe (Martelli, 1975). Some nepoviruses appear to have evolved from transmission by vector nematodes to alternative methods for their efficient transmission e.g. pollen transmission of cherry leaf roll virus (Jones et at., 1981).
