Rev 09/14/2023
Tylenchina Tylenchoidea Pratylenchidae Pratylenchinae
Radopholus similis (Cobb, 1893) Thorne, 1949
Type species of the genus
Similar to Pratylenchus in general shape and size, but female has two ovaries.
Sexual dimorphism is marked in this genus.
Males: Males do not resemble females in appearance. They are 500-600 µm long and more slender than the females.
Morphologically similar physiological races occur in Radopholus similis. They are distinguished on the basis of differences in host range (Kaplan and O’Bannon, 1985).
Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:
Radopholus similis is found world-wide in tropical and sub-tropical regions, and occurs wherever bananas are grown. The following are areas where the burrowing nematode is known to occur, primarily on banana: throughout Africa, parts of Asia, Central and South America, Cuba, Australia and several countries in southern Europe.
Ecological ranges of Radopholus similis and Pratylenchus goodeyi differ in sub-Saharan Africa banana plantations. R. similis appears to be a recent introduction to Africa and is prevalent at lower altitudes with higher soil temperature. It is prevalent in the cooking and dessert banana regions around Lake Victoria in East Africa (Kenya, Uganda, Tanzania),
Pratylenchus goodeyi is the predominant migratory endoparasite of bananas at altitudes above 1000 m with lower soil temperatures. That area produces about 25% of the world's bananas and plantains, mainly for domestic use.
In the United States, the nematode is found in most of the Southeastern US, Puerto Rico and Hawaii.
The principal means of burrowing nematode transmission to new localities, including citrus groves, is by the movement of infected plants. The burrowing nematode is also spread with soil which may be carried from one area to another whether in bulk or on the roots of plants.
A-rated pest in California Nematode Pest Rating System; frequently intercepted on ornamentals in nurseries. Nursery imports sometimes have been routed through Texas or Mexico from Florida.
Radopholus similis was first reported from banana in the Fiji islands in 1893. Spreading decline of citrus in Florida was first recognized as a disease in 1930, but it was not until 1953 that R. similis was reported as the primary cause.
Migratory endoparasite in all life stages.
Oduori et al (2021) demonstrated that tissue of cocoyam (Colocasia esculenta), under sterile conditions, provides an effective medium for monoxenic culture of R. similis for research purposes. This technique porvides a uiseful alternative for nematodes that are not easily cultured in the carrot disc sysmet often used for Pratylenchus app.
Over 350 known hosts. Burrowing nematode may be the most important nematode pest on fruit crops (especially citrus and banana) in the tropics. Most of the banana and plantain cultivars are attacked by R. similis. Other primary hosts include: citrus, coconut, ginger, palm, avocado, coffee, Hibiscus sp., prayer plant, black pepper, sugarcane, tea, vegetables, ornamentals, trees, grasses and weeds.
Races or biotypes have been recognized, and, in 1984, the citrus and banana race were described as sibling species (R. similis and R. citrophilus) with different chromosome numbers (Huettel, Dickson and Kaplan, 1984). There was also evidence of reproductive isolation in mating tests. The separation caused considerable consternation among regulatory nematologists in Florida. They argued that not enough populations had been studied, and that routine separation was very difficult. The species have since been synonymized again as R. similis (ref).
The banana race of Radopholus similis parasitizes banana but not citrus. The citrus race attacks both citrus and banana. There may also be a sugarcane race.
Ecophysiological Parameters:
The nematode is able to complete its life cycle within the root cortex, however, juveniles and adults are also present in rhizosphere soil.
Egg laying begins once the female is within a host. Reproduction is normally assumed to be bisexual, since females recovered from populations with males usually have sperm in their spermatheca (an enlarged portion of the female reproductive system which functions as a reservoir in receiving and holding sperm). However, a population can be initiated from a single egg (i.e., without the presence of any males). Males are present in well-established populations
Grapefruit roots - Eggs hatch occurs in 3-7 days; life cycle completed in 18-20 days at 24-26 C. (fast for a plant-parasitic nematode); life cycle extended at lower temperatures.
Female produces an average of about 2 eggs/day (range is 0.5-6 eggs/day/female).
Males are present, but female can produce eggs without fertilization - parthenenogenetic capability.
Male does not penetrate intact roots and may not feed.
The number of individuals in roots is not particularly high (50 to 120 nematodes/gram of root).
Burrowing Nematode survives less than 6 months in soil free of host roots, but removing all host roots may be difficult to accomplish.
The endosymbiont bacterium Wolbachiaer has been identified within the reproductive tract of female adults of R. similis. Gene sequences showed that the endosymbiont of R. similis is distantly related to the known Wolbachia supergroups. When all nematode expressed sequence tags (EST) in the dataset were mined for Wolbachia-like sequences. The retained sequences belonged to six different nematode species but R. similis was the only plant-parasitic nematode with traces of Wolbachia. Although its function is currently unknown, the endosymbiont was found in all individuals tested. That suggests an essential function of the bacteria (Haegeman et al., 2009).
Radopholus similis is a migratory endoparasite of plant roots and causes spreading decline in citrus.
Citrus:
Spreading decline areas in citrus orchards can be easily delineated by visual symptoms (Christie, 1957; DuCharme, 1954). Symptoms usually appear about a year after infection. Infected trees have sparse foliage, retarded terminal growth, poor color, twig dieback and a general unthriftiness. Leaves may wilt at midday but show temporary rejuvenation with rain or irrigation. There may be little or no new growth during the spring flush. Trees may bloom profusely but bear only a few small fruit. Trees will appear undernourished without exhibiting specific symptoms of malnutrition.
Reduction in citrus fruit yields, as a result of the burrowing nematode infestation, have ranged from 50 to 80 percent for grapefruit, and from 40 to 70 percent for oranges, in Florida (DuCharme, 1968). As a rule, grapefruit trees appear to be more adversely affected than orange trees. Avocado trees show similar spreading decline symptoms when infested with the burrowing nematode.
In the surface 2 1/2 feet of soil, 30% of feeder roots may be destroyed; below that, 90% of roots destroyed, leaving less than half of functional feeder roots. Especially important on non-organic sands in central Florida (less than 0.25% OM). These sands have a moisture holding capacity of 5-7%, and permanent wilting occurs at 2.5%. Roots in moist, deeper soils are 90% destroyed, and upper soil is dry for 5 months, so trees become stressed. Nematodes can be found up to 12 feet below the soil surface in citrus groves.
Above-ground symptoms usually appear 1 year after initial infection of roots (the length of time required for population increase and extensive root injury). Parasitized, but healthy-appearing trees occur 1-3 rows in advance of those visibly declining.
Leaves from infected trees contain less K and N than healthy leaves. Heavy fertilization with KCl causes improvement in K levels.
Some 15,000 acres are seriously infested, but this represents about 1% of Florida citrus, and some say too much attention has been paid to this nematode; however, this attention may have reduced spread and protected other regions from infestation.
Bananas:
In bananas, plants become uprooted and topple over especially those burdened with fruit. Below ground, brown to black lesions are formed at the site of nematode penetration in citrus roots. These lesions coalesce to form a canker.
Feeds in cortex, resulting in lesions and cavities, root breakdown, secondary decays; e.g., Fusarium oxysporum and Rhizoctonia solani in banana. Seldom feeds in vascular tissue.
Causes "Blackhead" or toppling disease of bananas - root system reduced and weakened so that tree falls under weight of fruit or in wind - total crop loss.
Symptoms of toppling disease: Initial entry of the nematode into the root produces a reddish, elongate discolored area parallel to the root axis. The fleck or discolored area enlarges as the nematode and progeny feed. The older parts of the lesion turn black and shrink, with the advancing margin remaining red. Neither the nematode nor the eggs are found beyond the red margin; they are also rare in the older portions of the lesion. Continued feeding causes extensive, deep lesions on roots and rhizomes.
Uninfested bananas in Central America yielded 17,000lbs/acre more fruit than infested.
Other Hosts:
The burrowing nematode can decimate production of several indoor decorative plant species. It is a severe pest of the parlor palm, Chamaedorea elegans, and may preclude commercial production.
Millions of black pepper trees have been lost in Indonesia (approx. 90% of the crop) to "Yellows disease" caused by R similis.
Burrowing Nematode also is a pest of sugarcane in Hawaii.
Tactics for Banana:
A healthy plantation may remain productive for up to 20 years; an infested one for only 2 or 3 years. Thus, there is considerable economic benefit to management.
Tactics for Citrus:
The following methods are recommended by Florida Dept. of Ag. (process about 15,000 samples/year):
If nematode population in citrus is held at 1/g root with nematicides, little damage occurs, but population tends to explode, possibly due to flush of new feeder roots. It requires 3 applications of vydate (oxamyl) per year to hold nematode populations to this level.
The alternative is to "accept" root damage, and to irrigate and attempt to increase yield by 2 boxes fruit/acre (about $500) (Note - Florida citrus not usually irrigated).
O'Bannon feels that improved orchard management can reduce need for "push and treat" programs, except at very high nematode population levels.
Resistance
Host Plant Resistance, Non-hosts and Crop Rotation alternatives:
Exclusion:
The California Department of Food and Agriculture Nematology Laboratory in collaboration with County Agriculture Commissioners, made 16 detections of R. similis in 1994 from shipments sent to California, 27 detections in 1995, 17 detections in 1996, 7 detections in 1997, and 3 in 1998.
The CDFA’s Burrowing Nematode Exterior Quarantine Program has been in effect for several decades. Areas under quarantine includes Alabama, Arkansas, Florida, Georgia, Hawaii, Louisiana, Mississippi, North Carolina, South Carolina, Texas, and Puerto Rico.
Entry of all soil and potting media, all plants and plant parts with roots, all parts of plants produced below ground or at soil level, and all plant cuttings for propagation into California from the area under quarantine is restricted. This includes foreign and domestic areas under quarantine.
In addition to the quarantine program, the nursery certification program may serve as a "secondary screening" mechanism for the burrowing nematode. The nursery stock certification program is both voluntary and mandatory. Certification of stock is mandatory for nurseries if the stock is being marketed for farm planting. The nursery has the option (voluntary) to sell uncertified stock if it will not be used for farm planting.
Eradication is difficult and costly. When R. similis was discovered in a residential area in Huntington Beach in 1996 in established banana corms imported from Louisiana, all infested soil was excavated, removed, and the area subjected to nematicidal, cultural and sanitary treatment. Clearly there are enormous financial cost, inconvenience and potential hazards to eradication.
Sanitation:
Avoiding infestations by R. similis should be the highest management priority. The movement of soil from infected fields should be avoided by using clean equipment and avoiding the movement of soil between fields.
Cultural:
Hot water treatments have been used to control the burrowing nematode. Immersion of roots at 55o C for 20 minutes is effective. Maas (1969), also showed that flood fallowing for 5-6 months eliminated R. similis.
Chemical Control:
Chemical control is difficult to achieve. Eradicative treatments of localized infestations in citrus groves are costly and the results are inconsistent. Trees must be pulled, roots taken out, the ground plowed, fumigated, and allowed to lay fallow for at least six months (or six months fallow followed by fumigation) before replanting (O’Bannon, 1977). Control of the burrowing nematode has been achieved using phorate or phenamiphos (Koshy et al., 1985). The fumigation of fields before planting is also an effective means of plant-parasitic nematode control. However, methyl bromide will be unavailable as of 2005.
Potential for spread in California
Nematodes brought in on infected plants to home sites which border agricultural areas become reservoirs of potential infestation for other crops. In Florida, surface water movement may transport nematodes from grove to grove.
Once the nematode is introduced, the primary contributing factor toward crop loss is a sandy soil texture, second is the soil temperature. In California, sandy citrus soils are present in the Coachella Valley, the Bard Valley near Blythe, the Edison-Arvin citrus district of Kern County, and streaks in various areas up and down the state. Citrus and date palms in the Coachella Valley are planted in soils subject to temperatures which would favor the development of burrowing nematode populations. Citrus found along the California coast experiences temperatures favorable to the development of the burrowing nematode for only three months of the year. Areas intermediate in soil type and climatic conditions would experience a range of impact.
Australasian Plant Pathology Society Factsheets on Plant-parasitic Nematodes (Prepared by Dr. Graham R. Stirling)
(Use your Return Key or click the Index Tab to return to this Nemaplex page)
Chitambar, J.J. 1997. "A brief review of the burrowing nematode, Radopholus similis." California Plant Pest and Disease Report, CDFA. 16:66-70.
Christie, J. R. 1957. "The yellows disease of pepper and spreading decline of citrus." Plant Disease Reporter. 41(4):267-268.
Christie, J. R. 1959. Plant nematodes, their bionomics and control. Gainesville, FL: Agric. Exp. Sta., Univ. of Fla.
CIH Descriptions of Plant-parasitic Nematodes, Set 2, No. 27 (1973).
DuCharme, E. P. 1954. "Cause and nature of spreading decline of citrus." The Citrus Industry. 35(11):6-7, 18; and 35(12):5-7.
DuCharme, E. P. 1968. "Burrowing nematode decline of citrus, a review." Pp. 20-37, G.C. Smart and V.G. Perry (eds.), Tropical Nematology. Gainesville, FL: Univ. Florida Press.
Duncan, L.W. and E. Cohn. 1990. "Nematode parasites of citrus." Pp. 321-346, M. Luc et al. (eds.), Plant-parasitic Nematodes in Subtropical and Tropical Agriculture. Wallingford, England: CAB International.
Kaplan, D. T., M. C. Vanderspool, C. Garrett, S. Chang and C. H. Opperman. 1996. "Molecular polymorphisms associated with host range in the highly conserved genomes of burrowing nematodes, Radopholus spp." Molecular Plant-Microbe Interactions. 9:32-38.
Kaplan, D.T. and J.H. O’Bannon. 1985. "Occurrence of biotypes in Radopholus citrophlus." Journal of Nematology. 17:158-162.
Haegeman, A., B. Vanholme, J. Jacob , T.T.M. Vandekerckhove , M. Claeys , G. Borgonie , G. Gheysen. 2009. An endosymbiotic bacterium in a plant-parasitic nematode: Member of a new Wolbachia supergroup. International Journal for Parasitology 39: 1045-1054.
Huettel, Dickson and Kaplan, 1984. Proc. Helm. Soc. Wash. 51: 32-35.
Kaplan (Journal of Nematology, 1984, 1985)
Koshy, P. K., P. Sundararaju, V.K. Sosamma and K. Ravikumar. 1985. "Efficacy of four systemic nematicides against Radopholus similis in coconut nursery." Indian Journal of Nematology. 15:148-151.
Luc, M. 1987. "A reappraisal of Tylenchina (Nemata). 7. The family Pratylenchidae Thorne, 1949." Revue de Nematologie. 10:203-218.
Maas, P.W.T. 1969. Pp. 149-154, J.E. Peachey (ed.), Nematodes of Tropical Crops. St. Albans, England: Commonwealth Bureau of Helminthology.
O'Bannon, J. H. 1977. "Worldwide dissemination of Radopholus similis and its importance in crop production." Journal of Nematology. 9(l): 25.
Oduori, C.A., Atandi, J., Kisaakye, J., Coyne, D. 2021. Cocoyam (Colocasia esculenta) provides an effective monoxenic culture media for Radopholus similis. Nematology 23:597-599
Poucher, C., H. W. Ford, R. F. Suit, and E. P. DuCharme. 1967. Bull. no. 7. Burrowing nematode in citrus. Gainesville, FL: Fla.Dept. Agric. and Consumer Serv., Div. of Plant Industry.
Suit, R. F., and E. P. DuCharme. 1953. "The burrowing nematode and other parasitic nematodes in relation to spreading decline of citrus." Plant Dis. Reptr. 37(7):379-383.
Tarjan, A. C. and J. H. O'Bannon. 1984. "Nematode parasites of citrus." W. R. Nickle (ed.), Plant and Insect Nematodes. New York: Marcel Dekker Inc.
Thomason, I. J. and E. P. Caswell. 1987. "Principles of nematode control." R. H. Brown and B. R. Kerry (eds.), Principles and Practices of Nematode Control in Crops. New York: Academic Press.