Radopholus similis (Cobb, 1893) Thorne, 1949
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 Âµm 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.
in California Nematode Pest Rating System; frequently intercepted on ornamentals in
nurseries. Nursery imports sometimes have been routed through Texas or Mexico
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
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.
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
Males are present, but female can produce eggs without fertilization -
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
sequences. The retained sequences belonged to six different nematode species
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
(Haegeman et al., 2009).
Radopholus similis is a
migratory endoparasite of plant roots and causes
spreading decline in 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
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
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
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
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
Tactics for Citrus:
The following methods are recommended by Florida Dept. of Ag. (process about
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
Host Plant Resistance, Non-hosts
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
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.
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 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.
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