Epidemiological and Molecular Characterisation of Citrus Huanglongbing (HLB) Disease in Malaysia
Citrus huanglongbing (HLB) disease is considered as one of the most destructive diseases of citrus in Asian, African and American countries. It is caused by Candidatus Liberibacter species. In Asian countries, the causal agent is Candidatus Liberibacter asiaticus. Surveys of the HLB disease on in...
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| Format: | Thesis |
| Language: | English English |
| Published: |
2008
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/6247/1/FP_2008_10.pdf |
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| Summary: | Citrus huanglongbing (HLB) disease is considered as one of the most destructive
diseases of citrus in Asian, African and American countries. It is caused by
Candidatus Liberibacter species. In Asian countries, the causal agent is
Candidatus Liberibacter asiaticus. Surveys of the HLB disease on infected citrus
trees were carried out in the major citrus growing areas in Peninsular Malaysia
namely Selangor, Pahang, Kelantan and Pahang. The occurrence of HLB
disease was confirmed by polymerase chain reaction (PCR) and transmission
electron microscope (TEM) tests. Candidatus Liberibacter asiaticus was detected
positive in samples collected from honey mandarin (Citrus reticulata), pummelo
(C. grandis), Mexican lime (C. aurantifolia), mandarin (C. suhueinsis),
calamondin (C. madurensis), Cleopatra (C. reticulata), Troyer citrange (Poncirus
trifolia X C. sinensis) and citrirnelo (P. trifolia X C. paradise). Field-infected citrus
trees showed typical symptoms of HLB disease such as intervienal chlorosis,
green vein, dieback of twigs, lopsided fruit shape, small fruit size and premature
fruit drop easily. All the major citrus growing areas in Peninsular Malaysia were
found infected with HLB disease. The percentage mean estimate disease
incidence ranged from 28.3 - 53.8% and 0.0 - 46.03% depending on localities
and citrus species, respectively. The seriousness of the disease justified further
studies on the epidemiology and molecular characterization of the causal agent,
Candidatus Liberibacter asiaticus in Malaysia. HLB vector, Diaphorina citri were
more abundant in the lowland areas such as Selangor and Terengganu. It was
absence in Cameron Highland, Pahang but moderate level was recorded in
Lojing Highland, Kelantan. Further study on characterization and strain
differentiation of Malaysian Candidatus Liberibacter asiaticus isolates was carried
out. Based on their biological (pathological) properties, Malaysian isolates
exhibited six types of HLB symptoms such as olive green of leaves, green vein,
vein yellowing, mottling, stunted and dieback of twigs. In terms of disease
incidence and disease severity reactions, Malaysian isolates showed sigmoid
pattern of disease progress curves and caused polycyclic type of disease. Based
on their aggressiveness, Malaysian isolates could be classified into three groups
i.e. severe, moderate and mild. In terms of molecular properties, all isolates
produced intense accumulation of starch granule inside the tissue of infected
leaves. TEM study revealed that Malaysian isolates were pleomorphic and
consisted of two types of bodies i.e. elongated and spherical forms. The body
lengths ranged from 100-1200 nm depending on their body shape and the
isolates. Differentiation of their modal length and composition ratio revealed that
the Malaysian isolates could be classified into two groups i.e. group 1 comprises
of GFB-T and GFB-S and group 2 comprises of GFB-PK. Characterization of
their 16s rDNA gene sequences revealed that Malaysian isolates produced
about 1 156-1 167b.p of nucleotide sequences. Gene sequences between
Malaysian isolates showed high percentage of nucleotide similarity that ranged
from 96-99%. Similar trends were observed on their genetic distances. Analysis
of outer membrane protein (OMP) gene also showed differences between
Malaysian isolates tested, namely GFB-Mandarin and GFB-Pummelo. A study on
host preference of D. citri and susceptibility of citrus species against GFB-T
isolate indicated that D. citri most preferred to colonize and feed on jasmine
orange (Munaya paniculata) followed by sour orange (C. aurantium), pummelo
and honey mandarin. Host susceptibility study revealed that jasmine orange and
pummelo were resistant to HLB infection while sour orange was found to be
tolerant. Calamondin was susceptible and honey mandarin was very susceptible
to HLB infection. The effect of calcium, zinc and copper application on plants'
recovery and severity following infection of HLB disease showed slight protection
against the HLB bacterium but was inconsistent in some cases. Citrus trees
treated with combination of calcium and zinc at 600 ppm, and 10 ppm
respectively resulted to significant increase (P10.05) in terms of fruit production
and total soluble solid (TSS) content. This treatment also resulted in reduction of
AUDPC value of disease severity and also improved mean leaf length, mean leaf
width and mean leaf area. To date, there is no successful treatment available
throughout the world including Malaysia to control HLB disease in the orchards.
Perhaps the combination of present treatment together with good agriculture
practices could improve the efficiency of HLB management in the orchards by
enhancing tree immunity, delaying disease onset to prolong lifespan of citrus
trees. |
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