Download Banana bunchy top virus

Islamic Republic Of Iran
Ministry of Jihad-e-Agriculture
Plant Protection Organization
A Guide for
Diagnosis & Detection
Of Quarantine Pests
Curly top of banana
Banana bunchy top virus
Edited by:
Ahmad cheraghian
Bureau of Plant Pest Surveillance and Pest Risk Analysis
2016
Banana bunchy top virus
Virus Group: Virus
Family: Unassigned virus family
Genus: Nanovirus
Common name:
bunchy top of banana
curly top of banana
cabbage top of banana
Synonyms:
banana bunchy top nanavirus
abaca bunchy top virus
Economic impact:
BBTV is the most serious virus disease of bananas and plantains. Devastating
epidemics occurred in Fiji at the turn of the century and in Australia in the 1920s. The
economic effect in northern New South Wales was dramatic. Between 1922 and 1926,
about 90% of the area under bananas had gone out of production, despite a rapid
expansion of the industry to compensate for abandoned farms. In the Currumbin
district in southern Queensland, the number of plantations fell from 100 to 4 between
1922 and 1925, and production plummeted by over 95% (Dale, 1987). In recent years,
bunchy top disease has been decimating the banana industry in Pakistan. In the early
1990s, statistics revealed that the area under banana cultivation in Sindh province fell
by 55% in one year (Jones, 1994). BBTV is a major constraint to production in many
areas where it occurs.
Hosts:
Major hosts: Musa (banana), Musa acuminata (wild banana), Musa textilis (manila
hemp), Musa x paradisiaca (plantain)
Geographic distribution:
Asia:China,India,Indonesia,Japan,Malaysia,Philippines,Sri Lanka,Pakistan,Vietnam.
Africa: Burundi ,Central African Republic,Congo ,Egypt,Gabon,Malawi,Rwanda.
North America:USA
Oceania:American Samoa,AustraliaSamoa,Fiji,Samoa,Tonga,Tuvalu.
World distribution map of Banana bunchy top virus
Morphology:
Evidence for a Viral Agent
BBTV is presumed to be the causal agent of bunchy top of banana, though
unequivocal evidence, by reproduction of the disease through inoculation of purified
virions or cloned genomic components, is lacking. The virions are intimately
associated with the disease (Harding et al., 1991; Thomas and Dietzgen, 1991) and
have been detected in all symptomatic plants tested (Dietzgen and Thomas, 1991;
Thomas, 1991; Thomas and Dietzgen, 1991; Karan et al., 1994). Dale et al. (1986)
isolated disease-specific dsRNA, suggestive of luteovirus infection, from Cavendish
cultivars and Iskra-Caruana isolated similar dsRNAs from five of six bunchy top
samples (M.L. Iskra-Caruana, Montpellier, 1997, personal communication). However,
neither these, nor any subsequent studies, have identified or established a clear role
for any virus other than the ssDNA BBTV in banana bunchy top disease.
Particle and Genome Properties
The virions of BBTV are icosahedra, ca 18-20 nm in diameter, have a coat protein of
ca 20,000 Mr, a sedimentation coefficient of ca 46S and a buoyant density of 1.291.30 g/cm³ in caesium sulphate (Wu and Su, 1990c; Dietzgen and Thomas, 1991;
Harding et al., 1991; Thomas and Dietzgen, 1991). Purified preparations have an
Asub)260/280> of 1.33 (Thomas and Dietzgen, 1991). The virus possesses a multicomponent genome, consisting of at least six circular ssDNA components each ca
1000-1100 nucleotides long (Wu et al., 1994; Yeh et al., 1994; Burns et al., 1995; Xie
and Hu, 1995). Component 1 encodes two proteins and components 2-6 each encode
one protein (Burns et al., 1995; Dale, 1996; Beetham et al., 1997). Two areas of the
non-coding regions are highly conserved between the six components (Burns et al.,
1995). The first is a stem-loop common region of up to 69 nucleotides. It contains a
nonanucleotide loop sequence conserved amongst ssDNA plant viruses and which
may be involved in rolling circle replication and initiation of viral strand DNA
synthesis. The second, 5' to the stem-loop common region, is a major common region
varying in size between components from 65 to 92 nucleotides and which may have a
promoter function. The initiation factor for endogenous DNA primers is also located
within the major common region (Hafner et al., 1997a). Component 1 encodes a
putative replication initiation protein and contains a second functional open reading
frame internal to this, whilst component 3 codes for the coat protein (Harding et al.,
1993; Dale, 1996; Hafner et al., 1997b, Wanitchakorn et al., 1997). Component 5 has
been shown to produce a gene product containing an LXCXE motif and to have
retinoblastoma protein (Rb)-binding activity. The gene product may be produced very
early in the infection cycle and be responsible for switching the first infected cells to
S-phase in preparation for virus replication. Component 6 appears to encode nuclear
shuttle proteins. The functions of the gene products of Component 2 and the small
internal gene of component 1 are unknown (Wanitchakorn et al., 2000). Recent
research indicates that component 1 is the minimal replicative unit of BBTV and
encodes the 'master' viral Rep (Horser et al., 2001).
Strains of BBTV
Most isolates of BBTV are associated with typical severe disease symptoms.
However, mild and symptomless isolates have been reported from Taiwan (Su et al.,
1993) and may occur more widely, though apparently not in Australia (J.E. Thomas
and A.D.W. Geering, Brisbane, 1998, personal communication). BBTV has been
confirmed in specimens of mild and symptomless infections from Taiwan by both
ELISA and PCR (HJ Su, JL Dale and JE Thomas, Brisbane, personal communication,
1996) and the isolates can be transmitted by Pentalonia nigronervosa (HJ Su, Taipei,
personal communication, 1996). Genomic differences, which correlate with these
biological variants, have not yet been determined.
Two broad groups of isolates have been identified based on nucleotide sequence
differences between some, possibly all, of the six recognized genome components
(Karan et al., 1994; JL Dale, Brisbane, personal communication, 1996). The 'South
Pacific' group comprises isolates from Australia, Fiji, Western Samoa, Tonga,
Burundi, Egypt and India, whilst the 'Asian' group comprises isolates from the
Philippines, Taiwan and Vietnam. These differences are present throughout the
genomes of components 1 and 6, but are most striking in the untranslated major
common region. No biological differences have been associated with these sequence
differences.
Magee (1948) noted that certain plants of 'Veimama', a cultivar originally from Fiji
and growing then in New South Wales, showed a 'partial recovery' from bunchy top
symptoms and produced bunches. After an initial flush of typical severe symptoms in
three or four leaves, subsequent leaves showed few, if any, dark-green flecks. Suckers
derived from these partially recovered plants also displayed a flush of typical
symptoms followed by partial recovery. The origin of the infection, whether from
Australia or Fiji, was uncertain. This partial recovery was noted for some infected
plants of 'Veimama' only, and in Fiji was noted for one sucker only on a single
infected stool from among hundreds of infected stools of 'Veimama' observed. Magee
was not able to transmit the virus from partially recovered plants and was only able to
super-infect them, with difficulty, with high inoculum pressure. This may be an
example of a mild strain of BBTV, possibly a non-aphid transmitted one, propagated
vegetatively, reaching only a low titre and conferring a degree of cross-protection.
Alternatively, 'Veimama' may not be uniform and individual plants with a degree of
resistance may exist. The complete explanation for this phenomenon is unclear.
The inability to transmit bunchy top from abac‫ ل‬to banana (Ocfemia and Buhay,
1934) was originally considered evidence that two distinct strains of the virus existed.
However, as noted by Magee (1953), technical deficiencies with these experiments
mean that the results must be viewed with caution. As first suggested by Magee
(1953), the many similarities between the bunchy top diseases of banana and abac‫ل‬,
including transmission properties with the vector P. nigronervosa, suggests that both
are caused by the same virus. ELISA has recently detected BBTV in a sample of
abac‫ ل‬bunchy top from the Philippines (JE Thomas and N Bajet, Brisbane and Los
Baٌos, personal communication, 1993).
Biology and ecology:
BBTV is transmitted by an aphid vector (Pentalonia nigronervosa) and is
disseminated in vegetative planting material, but is not transmitted by mechanical
inoculation (Magee, 1927).
Distribution and Movement within the Plant
Magee (1927) showed that banana bunchy top was systemic. Following aphid
inoculation, symptoms generally do not appear until a further two or more leaves have
been produced (Magee, 1927). This period can vary between 19 days in summer to
125 days in winter (Allen, 1978a). The virus can only be recovered by aphids from
the first symptom leaf or those formed subsequently (Magee, 1940). Suckers
produced on an infected stool generally develop symptoms before reaching maturity
(Magee, 1927).
Magee (1939) also concluded that the virus was restricted to the phloem tissue.
Microscopic examination revealed hypertrophy and hyperplasia of the phloem tissue
and a reduction in the development of the fibrous sclerenchyma sheaths surrounding
the vascular bundles. The cells surrounding the phloem contained abnormally large
numbers of chloroplasts giving rise to the macroscopic dark-green streak symptom.
Subsequent investigation using RNA probes and PCR (Hafner et al., 1995) has
demonstrated that BBTV replicates for a short period at the site of aphid inoculation,
then moves down the pseudostem to the basal meristem, then finally to the corm,
roots and newly formed leaves. Trace levels of virus were eventually detected by PCR
in leaves formed prior to inoculation, but replication was not demonstrated. This latter
observation is consistent with the inability to transmit the virus by aphids from such
leaves (Magee, 1940) and with the sequential development of single, new leaves from
the basal meristem.
BBTV has been detected by ELISA and/or PCR in most parts of the plant, including
leaf lamina and midrib, pseudostem, corm, meristematic tissues, roots, fruit stalk and
fruit rind (Thomas, 1991; Wu and Su, 1992; Hafner et al., 1995; A.D.W. Geering and
J.E. Thomas, Brisbane, personal communication, 1996).
Aphid Transmission
In Australia, the banana aphid (P. nigronervosa) was considered to have a role in the
etiology of banana bunchy top (Darnell-Smith, 1924) and in 1925 was conclusively
demonstrated to be the vector (Magee, 1927). Banana aphids have a worldwide
distribution with a host range that includes M. textilis and other species in the
Musaceae. Species in several closely related plant families including the Araceae
(Alocasia sp., Caladium spp., Dieffenbachia spp., Xanthosoma sp.), Cannaceae
(Canna spp.), Heliconiaceae (Heliconia spp.), Strelitzeaceae (Strelitzia spp.) and
Zingiberaceae (Alpinia spp., Costus sp., Hedychium spp.) are also colonized
(Wardlaw, 1961; RN Allen, Brisbane, personal communication, 1996). However, a
degree of host preference is displayed and some difficulty can be experienced
transferring them between host species. On banana plants in New South Wales, aphids
are found at the base of the pseudostem at soil level and for several centimetres below
the soil surface, beneath the outer leaf sheaths and on newly emerging suckers. Aphid
numbers decrease during periods of drought (Wardlaw, 1961).
Transmission of BBTV is of the circulative, non-propagative type. The transmission
parameters reported from Hawaii (Hu et al., 1996) and Australia (Magee, 1927),
respectively, are: minimum acquisition access period 4 h/17 h; minimum inoculation
access period 15 min/30 min-2 h; retention of infectivity after removal from virus
source 13 days/20 days; similar parameters have also been reported by
Thiribhuvanamala et al. (2001) from India. No evidence was found for transmission
of BBTV to the parthenogenetic offspring (Magee, 1940; Hu et al., 1996) or for
multiplication of BBTV in the aphid vector (Hafner et al., 1995).
Transmission efficiency for individual aphids has been reported as ranging from 46 to
67% (Magee, 1927; Wu and Su., 1990a; Hu et al., 1996) and the virus is more
efficiently acquired by nymphs than by adults (Magee, 1940).
Colonies of P. nigronervosa from Australia (where bunchy top occurs) and from
Reunion Island (where bunchy top does not occur) both transmitted each of six
isolates of BBTV with similar efficiency (M.L. Iskra-Caruana, Montpellier, personal
communication, 1994).
Vegetative Propagation
BBTV is efficiently disseminated in conventional planting material including corms,
bits and suckers. All suckers from an infected stool will eventually become infected.
Magee (1927) demonstrated 100% transmission of bunchy top through new 'eyes'
(meristematic growing points) even in a plant that had only been expressing
symptoms for 2-3 weeks.
BBTV is also transmitted in micropropagated banana plants (Drew et al., 1989;
Ramos and Zamora, 1990; Wu and Su, 1991) though not always at rates of 100%.
From time to time, apparently virus-free meristems producing apparently virus-free
plants can arise from an infected clone (Thomas et al., 1995).
In Pakistan in the early 1990s, much of the available planting material of 'Basrai'
(AAA, Cavendish subgroup) was infected with BBTV. Plantations established from
infected suckers and corms were completely unproductive.
Epidemiology of Banana Bunchy Top
The epidemiology of banana bunchy top in Australia is simplified by the presence of a
single susceptible host and a single vector species (P. nigronervosa). Dissemination
over long distances is by infected planting material and it is by this means that new
plantings in isolated areas usually become infected. Spread over short distances from
these infection foci is by the banana aphid.
In studies of outbreaks of bunchy top in commercial banana plantations, Allen
(1978b, 1987) showed that the average distance of secondary spread of the disease by
aphids was only 15.5-17.2 m. Nearly two-thirds of new infections were within 20 m
of the nearest source of infection and 99% were within 86 m. Allen and Barnier
(1977) showed that if a new plantation was located adjacent to a diseased plantation,
the chance of spread of bunchy top into the new plantation within the first 12 months
was 88%. This chance was reduced to 27% if the plantations were separated by 501000 m, and to less than 5% if they were 1000 m apart. On average, the interval
between infection of a plant and movement of aphids from this plant to initiate new
infections elsewhere (the disease latent period) was equivalent to the time taken for
3.7 new leaves to emerge. The rate of leaf emergence varied seasonally with a
maximum in summer (Allen, 1987). On the basis of these studies, a computer
program has been developed which simulates epidemics of bunchy top (R.N. Allen,
Brisbane, 1994, personal communication). This program, which is commercially
available, allows epidemiological factors to be varied and their effect on the progess
of the epidemic and disease control to be monitored.
In the Philippines, Opina and Milloren (1996) also demonstrated that most new
infections were adjacent to or in close proximity to primary sources of infection.
Symptoms:
The typical symptoms of bunchy top of banana are very distinctive and readily
distinguished from those caused by other viruses of banana. Plants can become
infected at any stage of growth and there are some initial differences between the
symptoms produced in aphid-infected plants and those grown from infected planting
material.
In aphid-inoculated plants, symptoms usually appear in the second leaf to emerge
after inoculation and consist of a few dark-green streaks or dots on the minor veins on
the lower portion of the lamina. The streaks form 'hooks' as they enter the midrib and
are best seen from the underside of the leaf in transmitted light. The 'dot-dash'
symptoms can sometimes also be seen on the petiole. The following leaf may display
whitish streaks along the secondary veins when it is still rolled. These streaks become
dark green as the leaf unfurls. Successive leaves become smaller, both in length and in
width of the lamina, and often have chlorotic, upturned margins. The leaves become
dry and brittle and stand more erect than normal giving the plant a rosetted and
'bunchy top' appearance.
Suckers from an infected stool can show severe symptoms in the first leaf to emerge.
The leaves are rosetted and small with very chlorotic margins that tend to turn
necrotic. Dark-green streaks are usually evident in the leaves.
Infected plants rarely produce a fruit bunch after infection and do not fruit in
subsequent years. Plants infected late in the growing cycle may fruit once, but the
bunch stalk and the fruit will be small and distorted. On plants infected very late, the
only symptoms present may be a few dark green streaks on the tips of the flower
bracts (Thomas et al., 1994).
Mild strains of BBTV, which induce only limited vein clearing and dark-green flecks,
and symptomless strains have been reported in Cavendish plants from Taiwan (Su et
al., 1993). Mild disease symptoms are expressed in some banana cultivars and Musa
species. The dark-green leaf and petiole streaks, so diagnostic and characteristic of
infection of cultivars in the Cavendish subgroup, can be rare or absent (Magee, 1953).
Some plants of 'Veimama' (AAA, Cavendish subgroup), after initial severe symptoms,
have been observed to recover and to display few if any symptoms.
The symptoms of abac‫ ل‬bunchy top include a reduction in leaf size and lamina area,
rosetting of leaves, upcurling and yellowing of leaf margins and stunting of the
pseudostem. Chlorotic areas on the leaves sometimes collapse, becoming necrotic,
and 'heart rot' often occurs in the pseudostem. No fruit is produced and infected plants
usually die within 1 or 2 years (Magee, 1953; Wardlaw, 1961). Dark-green 'dots and
dashes' on the minor veins, midribs and petioles occurs in only a few abac‫ ل‬cultivars.
Magee (1953) concluded that in abac‫' ل‬the green streak symptom occurs so rarely as
to lose nearly all its value as an aid in diagnosis'. Symptoms of abac‫ ل‬bunchy top in
the field are very similar to those that develop on abac‫ ل‬inoculated with the bunchy
top virus from banana. When enset is experimentally infected with BBTV, symptoms
are similar to those described for abac‫( ل‬Magee, 1953).
Symptoms by affected plant part
Fruits/pods: abnormal shape.
Inflorescence: lesions.
Leaves: abnormal colours; abnormal forms.
Whole plant: dwarfing; distortion; rosetting.....
Means of movement and dispersal:
Plant parts liable to carry the pest in trade/transport
- Bulbs/Tubers/Corms/Rhizomes: borne internally; invisible.
- Fruits (inc. Pods): borne internally; invisible.
- Flowers/Inflorescences/Cones/Calyx: borne internally; invisible.
- Leaves: borne internally; visible to naked eye.
- Seedlings/Micropropagated Plants: borne internally; visible to naked eye.
- Roots: borne internally; invisible.
- Stems (above Ground)/Shoots/Trunks/Branches: borne internally; invisible.
Plant parts not known to carry the pest in trade/transport
- Bark
- Growing Medium Accompanying Plants
- True Seeds (inc. Grain)
- Wood.
Phytosanitary significance:
Great care should be taken to prevent the dissemination of BBTV to areas where it
does not occur. The Americas are free of the disease, but the aphid vector, Pentalonia
nigronervosa, is present and the disease would be expected to cause very serious
problems if it appeared in smallholdings in the Caribbean and Central or South
America. No movement should occur other than by tissue culture. However, the
transmission of BBTV in micropropagated cultures has been demonstrated (Drew et
al., 1989).
Infected plantlets were indistinguishable from uninfected control plantlets in culture
(Drew et al., 1992). When plantlets were deflasked after 12 months in culture (as
proliferating tissue) and allowed to grow, all plants developed severe symptoms
within 1 month and all had died by 4 months. After 16 months in culture, 75% of
plantlets showed symptoms, although it took up to 6 months for all plants to develop
symptoms. The other 25% grew normally and appeared healthy (Drew et al., 1992).
They were later tested for BBTV with negative results.
Recent findings indicate that proliferating tissue cultures initiated from meristems
from BBTV-infected plants may eventually 'lose' the virus after many years under
slow growth conditions (DR Jones, Montpellier, personal communication, 1995), but
it would be advisable to test all germplasm from BBTV-affected areas before
meristems are taken to initiate cultures. BBTV has been eliminated from Lakatan
(AAA) by heat-treating shoot-tip cultures for extended periods (Ramos and Zamora,
1990). Subsequently, it was reported that the uneven distribution and low
concentration of BBTV after exposure of proliferating tissue cultures to heat leads to
BBTV-free primordial cells, which in turn develop into healthy plants (Wu and Su,
1991). BBTV is quarantine pest for Iran.
Detection and inspection:
The presence of 'dot-dash' symptoms on leaves almost certainly indicates that a plant
is infected with BBTV. The rosetted appearance of leaves which are progressively
shorter, narrower and more upright is noticeable at a distance. Yellowing of the leaf
margins is also indicative, though this can be caused by other problems.
Detection and inspection Banana bunchy top virus by ELISA&PCR
References:
CAB International. 2007. Crop Protection Compendium. 2007 Edition . CAB
International. Wallingford, Oxon, UK.
Farzadfar, Sh., Golnaraghi, A. R., Pourrahim, R.2002. Plant viruses of Iran. Saman
co. 203pp.
Salavatean , Mer.1996, Plant quarantine in Iran, Research Institute ,Ministey of
Agriculture pub,279p.
http://www.rtb.cgiar.org/blog/2015/03/25/alliance-reviews-pilot-control-strategies-forbanana-bunchy-top-disease-in-africa/
http://www.nrcb.res.in/gallery-Virology.php
https://www.daf.qld.gov.au/plants/health-pests-diseases/a-z-significant/bunchy-top
http://www.ctahr.hawaii.edu/bbtd/closeup_photos.asp
http://www.ctahr.hawaii.edu/bbtd/
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