Summary: The aim of this review was to undertake a survey of researchers working with plant‐parasitic nematodes in order to determine a ‘top 10’ list of these pathogens based on scientific and economic importance. Any such list will not be definitive as economic importance will vary depending on the region of the world in which a researcher is based. However, care was taken to include researchers from as many parts of the world as possible when carrying out the survey. The top 10 list emerging from the survey is composed of: (1) root‐knot nematodes ( M eloidogyne spp.); (2) cyst nematodes ( H eterodera and G lobodera spp.); (3) root lesion nematodes ( P ratylenchus spp.); (4) the burrowing nematode R adopholus similis ; (5) D itylenchus dipsaci ; (6) the pine wilt nematode B ursaphelenchus xylophilus ; (7) the reniform nematode R otylenchulus reniformis ; (8) X iphinema index (the only virus vector nematode to make the list); (9) N acobbus aberrans ; and (10) A phelenchoides besseyi . The biology of each nematode (or nematode group) is reviewed briefly.

The origin of plant parasitism within the phylum Nematoda is intriguing. The ability to parasitize plants has originated independently at least three times during nematode evolution and, as more molecular data has emerged, it has become clear that multiple instances of horizontal gene transfer (HGT) from bacteria and fungi have played a crucial role in the nematode's adaptation to this new lifestyle. The first reported HGT cases in plant-parasitic nematodes were genes encoding plant cell wall–degrading enzymes. Other putative examples of HGT were subsequently described, including genes that may be involved in the modulation of the plant's defense system, the establishment of a nematode feeding site, and the synthesis or processing of nutrients. Although, in many cases, it is difficult to pinpoint the donor organism, candidate donors are usually soil dwelling and are either plant-pathogenic or plant-associated microorganisms, hence occupying the same ecological niche as the nematodes. The exact mechanisms of transfer are unknown, although close contacts with donor microorganisms, such as symbiotic or trophic interactions, are a possibility. The widespread occurrence of horizontally transferred genes in evolutionarily independent plant-parasitic nematode lineages suggests that HGT may be a prerequisite for successful plant parasitism in nematodes.