Xiphinema is a genus of ectoparasitic root nematodes commonly known as dagger nematodes.[1] The genus is of economic importance on grape, strawberry, hops and a few other crops. Major species include X.americanum, X.diversicaudatum, X.index, X.italiae and X.pachtaicum.[2] They can be easily recognized by their long bodies and stylets which are long enough to reach the vascular tissue of plants.[3] Different members of the genus have been shown to induce moderate to large amounts of root damage through root penetration, which in some species results in the formation of galls.[2] They are of agricultural concern because they are vectors of nepoviruses, transferring them during feeding.[4] Efforts to study these nematodes in more detail have proved problematic in some species due to difficulties in maintaining populations in a greenhouse environment.[3]
Morphology
Mouth section of a Xiphinema sp. juvenile
Xiphinema are large nematodes, with an adult length between 1.5mm – 5.0mm.[1] They have a long protrusible odontostyle, with 3 basal flanges at the posterior end of the stylet and a relatively posterior guiding ring when compared to the genus Longidorus.[3] The odontostyle is lined with cuticle and alongside the esophagus serves as a good surface for viruses such as arabis mosaic virus to form a monolayer, which can be vectored to healthy plants.[4]Xiphinema have a two-part esophagus, which does not contain a metacorpus. A modification in the posterior end of the esophagus forms a muscular posterior bulb, which can generate a pumping action similar to that of a metacorpus in other plant parasitic nematodes.[4] The number of males varies from abundant to sparse depending on the species.[1] Males have paired spicules but the gubernaculum and bursa are absent. Males of different species can be characterized using the varying number and arrangement of papillae.[3] Females have 1 or 2 ovaries.[3]
Taxonomy
There are 296 nominal taxa, including 234 accepted species, 49 synonyms and 13 species inquirendae.(He 2003)
History
Xiphinema americanum was the first species to be described by Nathan Cobb in 1913, who speculated that it was likely a plant pathogen. .[1] This speculation was experimentally confirmed in 1949 and 1952.[5]
The genus Xiphinema is distributed worldwide. Two economically important Xiphinema species, X.index and X.americanum, are both commonly found in California and tend to be problematic in vineyards. X. diversicaudatum is also found in parts of the U.S, as well as Europe and Australia.[3]
Life cycle and reproduction
Eggs are laid singly in thin water layers in the soil and are not part of an egg mass. After the first-stage juvenile emerges from the egg there are 3 or 4 molts, all of which occur in the soil.[3] Males can be abundant or sparse depending on the species, which may suggest the presence of both parthenogenic and amphimictic species.[3]
Host parasite relationship
Xiphinema has a very wide host range including crops of high economic importance such as grape, hops and strawberry. Other documented hosts include: Nectarine, oak, rose, grapevine, raspberry, carrot, cherry, peach, and soybean.[3]
References
^ abcdWhitehead, A.G. 1998. Plant Nematode Control
^ abEvans, K., Trudgill, D.L., Webster, J.M. 1998. Plant Parasitic Nematodes in Temperate Agriculture.
^ abcTaylor, C.E., Robertson, W.M., 1970. Sites of Virus Retention in the Alimentary Tract of the Nematode Vectors, Xiphinema diversicaudatum (Micol.) and X.index (Thorne and Allen), Annals of Applied Biology (1970),66, 375-380)
^Schindler, A.F., 1957. Parasitism and Pathogenicity of Xiphinema diversicaudatum, and ectoparasitic nematode. Nematologica, II (1957):25-31)
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