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NOTES ON TAXONOMY AND BIOLOGY: The burrowing nematode is an amphimictic species characterized by accentuated sexual dimorphism. Males of this species have a  poorly developed stylet, a distinct elevated lip region set off by a distinct constriction and a coarsely crenate bursa enveloping 2/3 of the tail.  R. similis females have well developed esophagus and stylet [18 (16-21) µm long],spherical spermatheca containing rod-like sperms and an elongate-conoid  tail with a narrow rounded or indented terminus (Esser, et al., 1984; Orton Williams and Siddiqi, 1973). This species has migratory endoparasitic habits and develops and reproduces inside plant tissues of host roots. Radopholus similis populations with different host preferences have been reported in Florida. The populations with citrus-parasitic habits are able to parasitize citrus and banana and are indicated as the citrus race. The citrus race has been reported only in Florida and it is the casual agent of a citrus disease called spreading decline. The populations with non-citrus parasitic habits are able to parasitize banana and are indicated as the banana race. Preliminary attempts to elevate the citrus race to the rank of a sibling species  (R. citrophilus Huettel, Dickson & Kaplan 1984) were not validated by the results of molecular analyses of R. similis and R. citrophilus genomes (Kaplan and Opperman, 2000). The results of these analyses and recent mating studies between R. similis and R. citrophilus have shown that R. similis and R. citrophilus are not sibling species  (Kaplan et al., 1997).

GEOGRAPHICAL DISTRIBUTION: The burrowing nematode has a wide geographical distribution in banana growing regions because this nematode was spread with infected banana rhizomes from Southeast Asia (Myanmar, Thailand, Indonesia) to new banana growing areas in the world (Marin et al., 1998). The nematode occurs in Australia, Bahamas, Barbados, Belau, Belize, Benin, Bolivia, Brazil, Brunei, Burundi, Central African Republic, Colombia, Cameroon, Congo, Congo Democratic Republic, Cook Islands, Costa Rica, Cuba, Dominica, Dominican Republic, Ecuador, Egypt, El Salvador, Ethiopia, Fiji, French Polynesia, French West Indies, Gabon, Gambia, Ghana, Grenada, Grenadines, Guam, Guatemala, Guinea, Guinea Bissau, Haiti, Honduras, India, Indonesia, Ivory Coast Jamaica, Kenya, Lebanon, Madagascar, Malawi, Malaysia, Mauritius,  Mexico, Micronesia, Morocco, Mozambique New Zealand, Nicaragua, Nigeria, Niue, Norfolk Island,  Oman, Pakistan, Papua New Guinea, Philippines, Reunion, Samoa,  Singapore, Senegal, Seychelles, Solomon Islands,  Somalia, South Africa, Sri Lanka, St. Kitts, St. Lucia, St. Vincent  Sudan, Suriname, Tanzania, Thailand, Tonga,Trinidad & Tobago, Uganda,  Venezuela, Zaire, Zambia, and Zimbabwe Yemen. In the United States this nematode occurs, in Florida (banana and citrus races), Hawaii, and Puerto Rico (Marin et al., 1998).

HOSTS: A revision of Radopholus similis hosts include 365 plants (Holdeman, 1986). Primary hosts of burrowing nematodes are banana (Musa spp), black pepper (Piper nigrum), citrus (Citrus spp.), and foliage ornamentals belonging to the Araceae [anthuriums (Anthurium sp.), epipremnum  (Epipremnum sp.), philodendron (Philodendron sp.), spathifillum (Spathifillum sp.), syngonium (Syngonium sp.)], the Marantaceae  [calathea (Calathea sp.) and maranta (Maranta sp.)], and Zingiberaceae [gingers (Hedychium sp. and  Zingiber sp.)]. Other hosts include betelnut palm (Areca cathecu), coconut (Cocos nucifora), coffee (Coffea, sp.), parlor palm (Chamadorea elegans), sugarcane  (Saccharum officinale), tea (Camellia sinensis) and turmeric (Curcuma domestica).

CROP LOSSES: Yield suppression indiced by this nematode on banana and citrus ranges 5-100% and are influenced by many factors, such as soil type and climate (Gowen and Queneherve, 1990; O’ Bannon and Esser, 1985). Severe losses are reported on black pepper (Koshy and Bridge, 1990).

MEANS OF MOVEMENT AND DISPERSAL:  Through root material and by poorly sanitized bare root propagative plant material.

RATING: (M) The burrowing nematode is a restricted pest by many countries such as the European union and Japan. In the United States, quarantines are enforced by the states where citrus are an important component of their agriculture.  Taking into consideration that the nematode is regulated internally by the US citrus producing states, this nematode is given a moderate priority for a complete risk assessment.

REFERENCES:
    CAB International, 2001. Radopholus similisin: Crop protection compendium, global module, 3rd edition. Wallingford, UK: CAB International.
    Esser, R. P., A. L. Taylor, and Q. L. Holdeman. 1984. Characterization of burrowing nematode Radopholus similis (Cobb, 1893) Thorne, 1949 for regulatory purposes. Nematology Circular No. 113, pp. 4. Florida department of Agriculture and Consumer Services, Division of Plant Industry, Gainesville, FL, USA.
    Gowen, S. and P. Queneherve. 1990. Nematode parasites of bananas, plantains and Abaca. Pp. 431-460 in M. Luc, R. A. Sikora, and J. Bridge eds. Plant parasitic nematodes in tropical and subtropical agriculture.  Wallingford, UK: CAB International.
    Holdeman, Q. L. 1986. The burrowing nematode Radopholus similis sensu lato. Nematology Publication, California Department of Food and Agriculture, Division of Plant Industry, Sacramento, CA, USA.
    Kaplan, D. T., and C. H. Opperman. 2000. Reproductive strategies and karyotype of the burrowing nematode, Radopholus similis. Journal of Nematology 32:126-163.
    Kaplan, D.T., M. C. Vanderspool, and C. H. Opperman. Sequence tag site and host range assays demonstrate that Radopholus similis and R. citrophilus are not reproductively isolated. Journal of Nematology 29:421-429.
    Koshy, P.K., and J. Bridge. 1990. Nematode parasites of spices. Pp. 557-582 in M. Luc, R. A. Sikora, and J. Bridge eds. Plant parasitic nematodes in tropical and subtropical agriculture.  Wallingford, UK: CAB International.
    Marin, D. H., T. B. Sutton, and K. R. Barker. 1998. Dissemination of bananas in Latin America and the Caribbean and its relationship to the occurrence of Radopholus similis. Plant Disease 82:964-974.
    O’Bannon, J. H., and R. P. Esser. 1985. Citrus declines caused by nematodes in Florida. I Soil Factors. Nematology Circular No. 114, Florida Department of Agriculture and Consumer Services, Division of Plant Industry, Gainesville, FL, USA.
    Orton Williams K. J., and M. R. Siddiqi. 1973. Radopholus similis. CIH description of plant parasitic nematodes Set 2, No. 27. St. Albans, UK: Commonwealth Institute of Helminthology.