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CHAPTER 2
LITERATURE REVIEW
2.1 Biological control
Biological control means the reduction of inoculum density or
disease-
producing activities of a pathogen or parasite in its active or
dormant state. The
activity was performed by one or more organisms, accomplished
naturally or through
manipulation of the environment, host, or antagonist, or by mass
introduction of one
or more antagonists (Baker and Cook, 1974). The term biological
control has been
used in this context to describe the use of living organisms or
their products, to
combat the damaging activities of other organisms which are
potential pests or
pathogens of plants. This was on the basic that pests have
natural enemies and
biological control systems are designed to manipulate and
enhance these phenomenon
in order to reduce the pest populations and to limit their
activities (Isaac, 1992).
The objective can be achieved in a number of ways. For example,
many
pathogenic fungi are poor competitors and may be quickly
excluded from a site, such
as a leaf surface, if species which are more antagonistic are
present. Some such
combative fungi are highly aggressive and produce toxic
metabolites which quickly
affect less competitive individuals. Additionally, some fungal
species are able to
parasitise and directly attack insects, nematode pests, or other
fungal pathogens. The
use of various inoculation systems to encourange these
interactions has been shown to
enhance the effectiveness of such natural biological control
(Isaac, 1992).
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Biological control systems are preferred to the use of chemicals
and in recent
years a great deal of research activity has been directed
towards the development of
efficient and reliable systems. In environmental terms the
effects and long term
consequences of biological control, are much less damaging than
the routine use of
pesticides or fungicides. Treatments can be economic and
cost-effective providing
that good control can be established, particularly when repeated
chemical spraying is
required during the growing season. Biological systems have
great potential in the
control of soil-borne microbes which are particularly difficult
to treat by spraying
alone. Since the pest and antagonist are developing within a
natural situation, there
will be co-evolution between them and the potential for the
development of stable
resistance to the biological control agent is much reduced from
that of chemical
treatments (Briese, 1986).
It is interesting that relatively few instances of biological
control have been
effectively implemented in commercial field situations to date.
Much research has led
to the development of efficient systems on laboratory or
greenhouse scales where the
environment is highly controlled and predictable. However, once
trials are scaled up
the extreme variability and unpredictability of natural field
sites can lead to problems.
In theory, biological control, once established in a balanced
situation, could be self-
perpetuating but in practice such systems are more often used as
part of an integrated
control programme with pesticides and fungicides as
supplementary treatments (Way,
1986; Burge, 1988) in much reduced quantities.
Example of plant protection by biological control including
tomato, bell
pepper, celery and citrus. They were propagated in planting
mixes amended with
formulations of commercial biocontrol agents. Root colonization
by selected
biocontrol agents was evaluated for pepper, tomato and citrus,
and found to be
generally between 76 to 100% in both greenhouse ebb and flow,
and bench-produced
plants. All biological control agents, Trichoderma harzianum,
Bacillus subtilis, G.
intraradices, Gliocladium virens, and Streptomyces griseovirdis
reduced crown rot of
tomato in the field, with T. harzianum and B. subtilis being the
most effective
uniformly among four tests. Four biocontrols reduced
Phytophthora root rot on citrus
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in the field, two applied as a drench to soil in pots reduced
Thietaviopsis root rot on
citrus, and two biocontrol agents in combination reduced celery
root rot caused by
Pythium and Fusarium spp. (Nemec et. al., 1996). Six isolates of
plant growth-
promoting fungi (PGPF), non-pathogenic Fusarium oxysporum, and
five isolates of
bacteria were tested in hydroponic rock wool systems as
potential biocontrol agents of
Fusarium crown and root rot (FCRR) of tomato caused by Fusarium
oxysporum f. sp.
radicis-lycopersici (FORL) (Horinouchia et. al., 2006). In
addition Arthrovacter spp.,
Azotobacter spp., Pseudomonas spp., and Bacillus spp. was used
to control Fusarium
verticillioides in the maize rhizosphere. They were applied
under greenhouse
conditions and it was found that Azotobacter armeniacus
inhibited all F.
verticillioides strains assayed (Cavaglieri et. al., 2004).
2.1.1 Factors involved in biological control
Biological control is important method to plant disease control
because it save
cost and it is more safety than using chemical. Biological
control of plant diseases
can occur via several distinct mechanisms, including competition
for nutrients
between a pathogen and harmless species, parasitism, and
production of antibiotics.
Mechanisms leading to biological control of plant pathogens are
complex and may
occur by many routes. Plant pathogens may be suppressed by
events that reduce the
potential inoculum level of the pathogen in the environment, or
by competitive or
parasitic interactions among organisms, or by competition for
limiting resources.
Competition may occur at any point in the infection cycle, from
its initiation outside
the host, through invasion and growth inside the plant. Some
fungi such as species of
Trichoderma exhibit mycoparasitism, attacking and killing hyphae
or other parts of
pathogenic fungi. Some bacteria, actinomycetes, and fungi
produce chemicals
(antibiotics) that actively repress the growth of other species,
including pathogens.
Some fungi repress the growth of pathogens by out competing them
for key resources
such as minerals, nutrients, oxygen, or water, either at or away
from the site of initial
infection (Van Driesche and Bellows, 1996).
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a. The host ; The host population in its wild state was always
involved in
biological control by being a part of the biological balance
that helped keep the
pathogen suppressed. If the host or crop plant is highly
susceptible to the pathogen,
severe disease losses will occur unless the environment is
highly unfavorable or
antagonists suppress the pathogens. The mechanism of resistance
is known in only a
few instances. In some cases, the host may be resistant because
it stimulates
antagonists to grow in its rhizosphere. Resistant varieties may
be rendered susceptible
by products of decomposition of organic matter in soil.
b. The pathogen or parasite ; Pathogens and parasites are
generally more
sensitive to unfavorable a biotic factors than are saprophytes.
Most pathogens invade
the host early in the disease and, being internal, are generally
protected from
antagonists. In addition, secondary organisms may, however,
invade diseased tissue
and rot it.
c. Physical environment ; Control of a disease through the
inhibitory effects of
the physical or chemical environment directly on the pathogen
would not be a type of
biological control, because it is not then operating through
another organism.
However, the environment may favor the host and causes it to
maintain its resistance
to facultative types of microorganism
Conditions of the environment may be made unfavorable to the
pathogen or
vector. Tillage practices that modify the environment so as to
favor antagonists are
certainly part of biological control.
An admittedly oversimplified schematic diagram of the major
factor-groups
acting to produce plant disease is shown in Figure 2.1. Each
factor-group is
represented by a disk, and the amount of overlapping indicates
the degree of
interaction. Environment is collectively used to include several
factors that might be
operative; it could also refer to a single controlling
factor.
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Figure 2.1 Schematic diagram of interactions in major
factor-groups in plant disease.
A. Severe disease loss. Susceptible crop moderately well adapted
to the
environment; pathogen well adapted; antagonists not well adapted
and
ineffective. Exemplified by the Fusarium wilt diseases in acid
sandy soils.
B. Slight disease loss. Susceptible crop well adapted to
environment; pathogen
poorly adapted; antagonists moderately adapted and quite
effective.
Exemplified by the Fusarium wilt diseases in alkaline clay
soils.
C. No disease loss; biological control. Susceptible crop,
antagonists, and
pathogen well adapted to environment. Antagonists have
suppressed the
pathogen. Exemplified by Phytophthora cinnamomi root rot of
avocado in
Queensland.
D. No desease loss; resistance. Resistant crop, antagonists, and
pathogen well
adapted to environment. Host resistance prevents disease.
Exemplified by
Fusarium wilts in any soil when the crop carries monogenic
resistance (Baker
and Cook, 1974).
C
D B
A
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d. Antagonists ; Any disease control in which antagonists are
involved is
biological control. Antagonism is now considered to include
three types of activity:
* Antibiosis and lysis. Antibiosis is the inhibition of one
organism by a
metabolic product of another. Although it is usually an
inhibition of growth,
it may be lethal. The metabolite may penetrate a cell and
inhibit its activity
by chemical toxicity. Lysis is a general term for the
destruction,
disintegration, dissolution, or decomposition of biological
materials.
Because of the variety of ways it can be produced, and the
number of its
effects on plant cells, confusion has resulted in the
literature.
∗ Competition. Competition was viewed by Clark (Baker and
Snyder, 1965)
as “the endeavor of two or more organisms to gain the measure
each wants
from the supply of a substrate, in the specific form and under
the specific
conditions in which that substrate is presented when that supply
is not
sufficient for both.” In essence, competition is for nutrients,
particularly
high-energy carbohydrates, but also nitrogen, and possibly
certain growth
factors.
∗ Parasitism and predation. Although the existence of this type
of biological
control is not questioned, there is uncertainty about its actual
importance
under field conditions. Fungi known to parasitize other
organisms are
Rhizoctonia solani on Pythium, Trichoderma viride on Armillaria
mellea,
several genera of trapping fungi on nematodes, Fusarium roseum
on rusts.
The free-living nematode (Aphelenchus avenae), a ubiquitous
fungivore,
thrusts its stylet into a hypha and injects digestive saliva
that liquified the
contents, which are then sucked out through the stylet.
Predatory nematodes
such as Seinura rapidly paralyze other nematodes by injecting
saliva and
later sucking out the digested contents (Baker and Cook,
1974).
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Example of antagonistic microbes such as the activities of
Trichoderma spp.
are inhibitory to a range of fungi. Trichoderma harzianum
mycoparasitises the
mycelium of some other fungal species in soils, e.g. Rhizoctonia
and Sclerotinia, and
inhibits the growth of others, e.g. Pythium and Fusarium. The
fungus Gliocladium
virens mycoprasitises mycelium of Sclerotinia sclerotiorum
(Isaac, 1992).
For plant pathogen, biological control is the reduction of
disease by any of
these following action (Baker and Cook, 1974).
1. Reduction of inoculum of the pathogen through decreased
survival
between crops.
2. Reduction of infection of the host by the pathogen.
3. Reduction of severity of attack by the pathogen.
2.1.2 Mechanism of fungal biological control
A number of the natural characteristics of the life-styles of
fungi confer
qualities which make them potentially useful biological control
agents against a
variety of pests and pathogens of plants.
There are several action of fungal biological control by
following action.
(a) Competitive ability
The competitive activities of some fungi render them highly
antagonistic and
ideal as potential combative organisms. In theory, at least,
increased levels of such
species introduced to leaf surfaces would lower the potential
rates of infection from
other pathogenic fungal species, which tend to be less
competitive and aggressive.
(b) Antibiosis
Antibiosis is defined as the inhibition of the growth of a
microbe by
substances produced and liberated by another microbe. The term
most usually refers
to antibiotic activity. However, whilst it is relatively easy to
prove that an organism
produces antibiotic in culture it is difficult to ascertain
whether similar production
occurs under natural conditions, and even more difficult to
establish a role for these
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compounds in competition within natural environments. Little
antibiotic activity has
been detected in the soil environment and it has been suggested
that these compounds
are degraded or adsorbed no to soil particles. Additionally, a
strain newly isolated
from the environment may demonstrate antibiotic activity
although this ability may be
rapidly lost on subsequent subculture.
(c) Mycoparasitism
Fungi which derive most or all their nutrients from another
fungus are termed
mycoparasites. The term used to describe the direct parasitism
of one parasite
(usually a primary parasite) by another is hyperparasitism.
Fungal preparations are
now used and marketed commercially for control of insect pests
and nematodes,
particularly in controlled, greenhouse conditions. All the major
fungal taxonomic
groups contain mycoparasitic species. Biotrophic mycoparasites
may have relatively
long-term associations with living cells of the invaded species;
however, necrotrophic
mycoparasites often kill the target fungal-host cells prior to
penetration and invasion.
Some mycoparasitic species are adapted to the exploitation of
fungal spores,
either asexual or sexual resting spores. Exploitation of this
ability, particularly the
mycoparasitism of sclerotia, would be of great agricultural and
horticultural interest
since these structures are extremely long-lived and very
difficult to eradicate from soil
(Isaac, 1992).
2.2 Endophytes
Endophytes was defined by Petrini (1991) as “all organisms
inhabiting plant
organs that at some time in their life, can colonise internal
plant tissues without
causing apparent harm to the host”. This definition therefore
includes symptom less
latent pathogens and those fungi which also hame an epiphytic
phase of their life
cycle. Wilson (1995) provided a “working definition” of the term
by analyzing the
different levels of endophyte association and stated that
“endophytes are fungi or
bacteria which, for all or part of their life cycle, invade the
tissues of living plants and
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cause unapparent and asymptomatic infections entirely within
plant tissues but cause
no symptoms of disease”. Endophytic fungi are as highly
specialized on their hosts as
pathogenic fungi, but in contrast they colonize internal host
tissue without manifest
symptoms (Petrini and Ouellette, 1994).
Fungal endophytes provide the protection to their host. For
examplae, fungi in
redwood may function as antagonists or stimulators to pathogens
(Espinosa-García et.
al., 1996). However, colonization or infection by endophytic
organisms cannot be
considered as causing disease, because a plant disease in an
interaction between the
host, parasite, vector and environment and symptoms are a result
from the interaction
(Rossman, 1997). The study of endophytes of tropical plants has
received much
attention because endophytes are believed to be both diverse and
to provide an
excellent potential source of biologically active novel
compounds (Dreyfuss and
Petrini, 1984; Hyde, 2001)
2.2.1 Characteristics of endophytes
Endophyte produced alkaloids and other mycotoxins appear to be
responsible
for the resistance of plants. Several reviews discuss secondary
metabolite production
by endophytic fungi in graminicolous and non-graminicolous hosts
(Miller, 1986;
Clay, 1988, 1991; Petrini et. al., 1992). Furthermore,
endophytic fungi alter
relationships between diversity and ecosystem properties as
shown in Figure 2.2 each
species is represented by a circle that shows the amount of
resources it exploits along
two resource axes. Grey circles represent the grass species. If
the grass hosts an
endophyte, it experiences an increase in resource acquisition
relative to its uninfected
state. This increase will be the strongest when resources are
most limiting (i.e. at
greater levels of species diversity). As the infected grass is
superior to the uninfected
grass in resource acquisition, it can more strongly reduce the
amount of resources
available to other species (Jennifer et. al., 2004).
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Endophytes, in culture, can produce biologically active
compounds (Brunner
and Petrini, 1992) including several alkaloids, paxilline,
lolitrems and tetraenone
steroids (Dahlman et. al., 1991). When Eupenicillium spp. was
isolated from healthy
leaves of Murraya paniculata (Rutaceae) after surface
sterilization, the fungus was
cultivated in sterilized white-corn, where the spiroquinazoline
alkaloids
alanditrypinone, alantryphenone, alantrypinene and
alantryleunone were produced
(Barros, 2005). Endophytes, 155 fungi (91.7%) and 52 bacteria
(64%), were found to
produce xylanase. The inside part of plants is a novel and good
source for isolating
xylanase producers in comparison with soil (Suto et. al., 2002).
Ergopeptine,
peramine, and pyrollizadine based loline alkaloids are produced
from Acremonium
spp, these alkaloids are biologically active against numerous
species of insects,
including aphids (Johnson et. al., 1985; Prestidge et. al.,
1982; Siegel and Schardl,
1991). The alkaloids not only serve as feeding deterrents, but
also decrease the
reproduction and growth of the insects. Antibiotic compounds
have also been
produced in culture by endophytes (Fisher et. al., 1984a, b) and
plant growth
promoting factors have been recovered (Petrini et. al.,
1992).
Furthermore, endophyte was produced hormone for growth of plant.
For
example, root growth and morphology are affected by some
auxin-producing
mycorrhizae and Rhizobium bacteria (Leopole and Kriedemann,
1975). Auxin is a key
hormone in regulating plant growth, differentiation, and apical
dominance and is
produced mainly in shoot meristems and expanding leaves. The
endophyte is most
prevalent near these organs. The plants from which the
endophytes were isolated
produced 24% greater biomass than did their nonsymbiotic
isolines. Auxin
concentrations in both plant types were unaffected by endophyte
infection. May be
expressions of differing defects on auxin concentrations or
antagonisms by other
hormones such as gibberellic acid (Bacon and White, 1994).
Endophytic
microorganisms exist within the living tissues of most plant
species. They are most
abundant in rainforest plants. Novel endophytes usually have
associated with them
novel secondary natural products and/or processes. For example,
muscodor is a novel
endophytic fungal genus that produces bioactive volatile organic
compounds (VOCs).
(Demain and Dijkhuizen, 2006)
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Evidence for the presence of endophytic fungi in many plants has
been
provided solely on the basis of isolations of the fungi from
surface-sterilized tissue.
From ecolofical and floristic analyses of a broad range of host
species, it is apparent
that endophytic fungi comprise a unique and complex ecological
group distinct from
obligate parasites and saprobes, although the ecological role of
many endophytes is
not well understood (Petrini, 1994).
Figure 2.2 Graphical model depicting how mutualistic endophyte
symbiosis in a
common grass species can alter the relationship between species
diversity and
ecosystem functioning (Jennifer et. al., 2004).
2.2.2 Host specificity
The degree of host specificity which operates in the endophytic
fungi is not
yet clear. Some species are commonly occurring and may be
isolated from various
host plant species and from different locations with differing
environmental
conditions. In general terms, the geographical occurrence of
endophytes is related to
the distribution of host species. In some cases almost all
individuals in a plant
population may be infected by endophytes. Cladosporium spp.,
Nodulisporium spp.
and Pleospora spp. are common. Some endophytes, however, do not
show such a
wide species range and are often isolated from plants of the
same family or closely
related families. Other species are only rarely detected (Isaac,
1992).
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The degree to which endophytes are tissue or organ-specific is
also not yet
clear. Some species are most commonly isolated from similar
tissues, particularly the
endophytes of conifer needles. In other cases the occurrence is
less distinct.
Howerver, only limited surveys have been carried out to date
(Isaac, 1992).
2.2.3 Isolation of endophytes
The isolation and identification of endophytic species involves
very careful
surface sterilistion of host plant tissues followed by
incubation on a range of media to
encourage outgrowth of isolates and their subsequent
sporulation, so that
identification can be carried out. It is often difficult to
satisfactorily establish
endophytic status for the isolates which are obtained. It is not
easy to ensure the
exclusion of spores or hyphal fragments which may escape the
sterilization (Petrini,
1986). Some epiphytic species may penetrate host tissues and
additionally some
endophytic species sporulate in culture, although many isolates
grow very slowly and
a considerable incubation time (sometimes many months) is often
required. Fungal
endophytes of conifers are common although many belong to little
known species,
probably because these fungi are very inconspicuous and are
rarely collected. Some
species live almost entirely within a host plant cell. Rate of
endphyte infection
increase with ageing of plants and plant populations. It is also
possible that some
plant species may not occur naturally without endophytic fungal
infections (Isaac,
1992).
.
Techa (2001) studied fungal endophytes associated with the
palms
investigated at two sites within Doi Suthep-Pui National park,
from different tissue
types (petiole, leaf, lamina and leaf veins). The endophytic
fungi isolated included
Xylariaceous taxa (20 morphotypes), sterile mycelia (11
morphotypes), eight
unidentified hyphomycetes and twelve identified taxa.
Likhittragulrung (2003)
studied endophytic fungi were isolated from healthy leaves,
braches and petioles of
tomato, chili and devil’s fig plant, collected in Muang,
Saraphi, Mae Rim and Doi
Saket districts, Chiang Mai province, Muang district, Lamphun
province and Thoeng
district, Chiang Rai Province. All 611 endophytic fungi were
recovered and grouped
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in to 20 taxa. Jansa and Vostka (2000) isolate of more than 200
strains of endophytic
fungi from the roots of several host plants belonging to order
Ericales (Vaccinium,
Calluna, Rhododendon, Empetrum, etc.), followed by a successful
attempt to verify
ericoid mycorhiza status of some of these fungal isolates under
aseptic conditions. A
total of 131 endophytic actinomycete strains were successfully
isolated from surface-
sterilized banana roots. These isolates belonged to
Streptomyces, Streptoverticillium
and Streptosporangium spp. The remaining 2 isolates were not
identified. About
18.3% of the isolates inhibited the growth of pathogenic
Fusarium oxysporum f. sp.
cubense on banana tissue extract medium (Cao et. al., 2005). A
total of 150
endophytic fungi were isolated from stems of cacao. The fungal
community was
identified by morphological traits and rDNA sequencing as
belonging to the genera
Acremonium, Blastomyces, Botryosphaeria, Cladosporium,
Colletotrichum,
Cordyceps, Diaporthe, Fusarium, Geotrichum, Gibberella,
Gliocladium,
Lasiodiplodia, Monilochoetes, Nectria, pestalotiopsis,
Phomopsis, Pleurotus,
Pserdofusarium, Rhizopycnis, Syncephalastrum, Trichoderma,
Verticillium and
Xylaria (Rubini et. al., 2005). Ganley and Newcombe (2006)
investigated the
transmission of diverse fungal endophytes in seed and needles of
Pinus monticola,
western white pine. They isolated 2003 fungal endophytes from
750 surface-
sterilized needles. In contrast, only 16 endophytic isolates
were obtained from 800
surface-sterilized seeds. Twenty isolates of the endophytic
actinomycetes were
isolated from the leaves and stems of healthy jujube plants in
Chiang Mai Province by
using IMA-2 medium (Divarangkoon, 2004). Wathaneeyawech (2004)
isolation 283
isolates of endophytic fungi from the corn leaves using triple
surface sterilization
technique. Five hundred and fifty seven of endophytic fungi of
90 taxa were obtained
from nutgrass, cogon grass and common reed (Jailae, 2003).
2.2.4 Biological control of endophytic fungi
Endophytic fungi can be used to control fungal pathogenic in
plant for the
safety of the environment and human. Many research work studies
ability of
endophyte for used control pathogenic in plant. For example,
Streptomyces sp. strain
S96 isolated from surface-sterilized banana roots inhibited the
growth of pathogenic
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Fusarium oxysporum f. sp. on wilt pathogen in banana (Cao et.
al., 2005). Samuels et
al. (2006) isolated Thichoderma theobromicola and T. paucisporum
from cocao to
control Moniliphthora roreri causing frosty pod rot disease in
cacao. Dhingra et al.
(2006) selected an endophytic non-pathogenic isolate of Fusarium
oxysporum (NPFo)
and an antibiotic producing rhizosphere/rhizoplane (RS-RP)
competent fluorescent
pseudomonas to suppress Fusarium yellow (F. oxysporum (Schlecht)
f. sp. phaseoli
Kendrick and Snyder ) of bean (Phaseolus vulgaris L.). Rubini
et. al. (2005) studied
endophytic fungi that can be used to inhibit Crinipellis
perniciosa causes of Witches’
Broom Disease of Cacao which is the main factor limiting cacao
production in the
Americas. They found that Gliocladium catenulatum reduced the
incidence of
Witches’ Broom Disease in cacao seedlings to 70%. Park et. al.
(2005) studied the
Chaetomium globosum strain F0142, which was isolated from
barnyard grass. It was
found that the fungi showed potent disease control efficacy
against rice blast
(Magnaporthe grisea) and wheat leaf rust (Puccinia recondita).
Waller et. al. (2005)
studied potential of Piriformospora indica to induce resistance
to fungal diseases and
tolerance to salt stress in the monocotyledonous plant barley.
The beneficial effect on
the defense status is detected in distal leaves, demonstrating a
systemic induction of
resistance by a root-endophytic fungus. The systemically altered
“defense readiness”
is associated with an elevated antioxidative capacity due to an
activation of the
glutathione-ascorbate cycle and results in an overall increase
in grain yield.
Wathaneeyawech (2004) studied endophytic fungi isolated from
corn leaves for
inhibition Northern leaf blight disease of corn.
2.3 Phytophthora
2.3.1 Phytophthora as plant pathogens
There are about 60 species in the genus Phytophthora, all of
them are plant
pathogens. Phytophthora or the plant destroyer is one of the
most destructive genera
of plant pathogens in temperate and tropical regions, causing
annual damages of
billions of dollars (Drenth and Guest, 2004).
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Phytophthora diseases have been well studied in the temperate
regions of the
world, ever since the potato late blight epidemic in Europe in
1845 - 1847 provided
the impetus for the development of plant pathology as a
scientific discipline.
Throughout the wet tropics, agricultural production of a large
range of crops is
seriously reduced due to the wide range of Phytophthora
pathogens causing a large
number of different diseases (Drenth and Guest, 2004).
There are a number of host and pathogen factors which, together
with features
of their interactions, make Phytophthora diseases so troublesome
in the wet tropics.
One of the important factors to consider is that the genus
Phytophthora does not
belong to the fungal kingdom. It is an Oomycete, closely related
to diatoms, kelps and
golden brown algae in the Kingdom Stramenopila (Beakes, 1998).
These organisms
thrive in the environments found commonly in the wet
tropics.
Classification of Phytophthora was described by Kirk et. al.
(2001) as
followed.
Phylum Oomycota
Class Oomycocetes
Order Peronosporales
Family Pythiaceae
Genus Phytophthora
Mycelium is generally coenocytic with no or a few septa, in host
often with
haustoria. Hyphae 3 – 8 µm up to 12 µm, irregularly swollen
undulate or gnarled;
sometimes with characteristic swellings; initial branching at
right angles to parent
hypha and often swollen for a short distance. Chlamydospores
usually spherical,
intercalary, sometimes terminal, wall smooth, up to 2 µm thick,
hyaline at first.
Sporangiophores usually undifferentiated apart from a few spp.
Where branching is
reminiscent of Peromospora but with nodal swellings; branching
sympodial or
irregular and from below the sporangium or from within an empty
one. Sporangia
usually terminal, single on long hyphae in sympodia or within
(or just beyond) an
evacuated sporangium; ellipsoid, ovoid, obpyriform or limoniform
(when shed); apex
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differentiated by an internal hyaline thickening of the inner
wall of a depth (up to 6
µm) constant for the sp. and sometimes protruding to form a
papilla; wall smooth up
to 2 µm thick; non-caduceus or shed with a pedicel. Germination
by zoospores
emerging individually through apex (free at once or held
momentarily in an
evanescent vesicle) or by a germ tube. Zoospores hyaline, ovoid
to phaseoliform,
biflagellate, anteriorly directed tinsel (shorter) and
posteriorly directed whiplash
(longer); when motility ceases the spherical cyst may show
repetitional emergence but
not diplanetism. Oogonium usually terminal, spherical or
tapering to the stalk,
delimited by a thick septum; wall hyaline, thin, becoming
thicker and often yellow to
brown, mostly smooth occasionally tuberculate or reticulate.
Antheridium usually
single, monoclinous or diclinous, spherical oval, cravat or
short cylindrical (Figure
2.3), often angula; amphigynous or paragynous (sometimes both),
if latter usually
applied to the oogonium close to the stalk. Oospore single more
or less filling
oogonium, spherical, smooth, hyaline (sometimes faintly yellow),
outer wall very
thin, inner wall 0.5 – 6 µm thick, when mature with large
central globule
(Waterhouse, 1963; Holliday, 1980). Phytophthora can produce
both asexual and
sexual spores. Asexual sporangia emergedirectly from the hyphae
through structures
known as sporangiophores. Under optimal conditions of
temperature and moisture,
sporangia release swimming spores. Sexual spores, or oospores,
are formed when the
male structure, antheridium, associates with the female, egg
bearing oogonium. Some
species of Phytophthora are self-fertile or homothallic, whereas
others are self-sterile
or heterothallic. Heterothallic species are divided into A1 and
A2 mating types and
crossing occurs when these two type of strains contact each
other (Kronstad, 2000).
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19
Figure 2.3 Phytophthora sp. A. Sporangiophores penetrating a
stoma of a potato
leaf. B. Sporangial contents dividing an releasing zoospore. C.
Intercellular mycelium
from a potato tuber showing the finger-like haustoria
penetrating the cell walls.
(Webster, 1980)
All Phytophthora species need high humidity for sporulation and
the
germination of sporangiospores and zoospores to initiate
infections. Frequent or
seasonal heavy rainfall, and high levels of humidity, are common
throughout the
tropical lowlands. Tropical highlands have the added problem of
heavy mist and dew
during the morning and/or late afternoon, producing free water
throughout the night
and providing almost daily opportunities for sporangiospores to
be formed,
transported and start new infections.
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20
Another important factor in the pathogenicity of Phytophthora is
that
sporangia release motile zoospores that are attracted by
chemotaxis and electrotaxis to
the roots of their host plants. The ability to seek out
susceptible host tissue, coupled
with zoospore motility, makes these propagules extremely
efficient, even at low
numbers (Drenth and Guest, 2004).
Factors involved Phytophthora to be formidable plant pathogens
are:
a. The ability to produce different types of spores such as
sporangia and
zoospores for short-term survival and spread, and chlamydospores
and
oospores for longer term survival.
b. Rapid sporulation on host tissue within 3 - 5 days of
infection. This results
in a rapid build-up of secondary inoculum in a multicyclic
fashion, leading
to epidemics under suitable favorable environmental
conditions.
c. Ability of zoospores of Phytophthora to be attracted to root
tips through a
chemical stimulus (positive chemotaxis) as well as
root-generated electric
fields (electrotaxis), coupled with the mobility of zoospores to
actually
swim to the actively growing root tips, encyst, and infect
young,
susceptible root tissue.
d. Ability to survive in or outside the host tissue as oospores
are also known
to survive passage through the digestive systems of animals such
as snails.
e. Production of sporangia, which can be airborne and may travel
reasonable
distances in raindrops, run-off and irrigation water, and on
wind currents,
to infect neighboring fields. These sporangia can directly
infect host tissue.
These same sporangiospores also have the ability to
differentiate into 4 -
32 zoospores under humid and cool conditions and cause
multiple
infections from the one sporangium. Nevertheless, zoospores can
travel
only short distances, as they are susceptible to
desiccation.
f. Phytophthora pathogens belong to the Kingdom
Stramenopiles
(Hawksworth et. al., 1995) and as such have different
biochemical
pathways to the true fungi. Many fungicides are therefore not
very
effective against Phytophthora pathogens.
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21
g. Phytophthora pathogens thrive under humid and wet condition,
which
makes them difficult to control, as protectant fungicides are
difficult to
apply and least effective under such conditions.
2.3.2 Plant diseases causes by Phytophthora
Phytophthora spp. cause the most serious soilborne diseases of
citrus. These
fungi are worldwide in distribution and cause citrus production
losses in irrigated, arid
arese as well as in areas with high rainfall. Diseases caused by
Phytophthora spp.
include damping-off in seedbeds and foot rot, gummosis, and root
rot in nurseries and
orchards. Brown rot of fruit occurs in groves and continues to
spread in
packinghouses, causing further losses (Timmer, et al. 2000).
Phytophthora pathogens can cause many different diseases and
disease
symptoms on a wide range of plant species. There are many
species of Phytophthora,
some of which have extremely wide host ranges and are
particularly destructive and
are therefore important plant pathogens. Some of the
economically important
Phytophthora pathogens and their major host plants are listed in
Table 2.1
Table 2.1 Host plant and Phytophthora species (Kronstad, 2000;
Drenth and Guest,
2004)
Phytophthora species Major host plant(s)
P. capsici
P. cinnamomi
P. citrophthora
P. fragariae var. fragariae
P. infestans
P. palmivora
P. sojae
P. botryosa
P. nicotianae
Pepper
Avocado
Citrus
Strawberry
Potato and tomato
Cocoa, papaya, coconut, black pepper, wild
durian, rubber, longan, mango, pineapple and
palm species
Soybean
Rubber
Citrus, durian, pineapple and black pepper
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22
The disease symptoms most of often encountered are discussed as
follow.
A. Late blight disease which cause blackening of leaves and
other plant organs
and has often been applied particularly to infections which
spread very rapidly
Probably the most famous of the blight diseases is that caused
by Phytophtrora
infestans on potato, tomato and Solanaceae Late blight of potato
caused widespread
and devastating famine in Ireland between 1845 - 1850.
Phytphthora late blight kills
the foliage and stems of potential crop plants in the field
(Isaac, 1992). And cracking
is a serious problem since molds and rots find ready entrance to
reduce the quality of
the fruit and the picked product (Work and Carew, 1955). Late
Blight – General and
serious in humid regions and in cool, wet seasons. Irregular,
greenish-black, water-
soaked, rapidly enlarging, greasy spots on leaves, petioles, and
stems. In moist
weather, a whitish-gray downy growth appears, mostly on
underside of leaves
Infected foliage soon dries, turns brown, brown to black round
lesions, with yellow-
green margins first appear at the tips or sides of the leaves
and withers, Tomato fruit
spots are greasy and dark green to brown or pearly black, fruit
are firm with a
corrugated appearance. The infection spreads, the fruits rot and
become covered with
a whitish growth (Figure. 2.4) (Sburtleff, 1966; Centre for
Overseas Pest Research,
1983).
Figure 2.4 Symptom of Late blight disease in tomato caused by P.
infestans.
(www.mtvernon.wsu.edu/phth-team/diseasegallery.htm) [2006,
October 16].
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23
B. Brown rot disease caused from P. citrophthora on citrus.
Phytophtrora may
infect citrus when its population build-up is much more than
that of other species.
Blossoms wilt, turn brown, and rot. Leaves on twig tips suddenly
wither and turn
brown. Twigs may die back from sunken, brown, girdling cankers,
the bark color
darkens and the internal tissues decay extending into the wood
(Figure. 2.5). Soft,
brown, rotted areas was occurred in fruit. Affected areas may
later be covered by
powdery tufts of gray to tan mold (Sburtleff, 1966;
Mukhopadhyay, 2004).
Losses from brown rot result primarily by rotting of the fruit
in the orchard,
although serious losses may also appear during transit and
marketing of the fruit.
Yields may be reduced also by destruction of the flowers during
the blossom blight
stage of the disease. Twig infections do not always cause losses
directly, but may
cause indirect losses by furnishing inoculum for fruit
infections. In severe infections,
and in the absence of good control measures, 50 - 70% of the
fruit may rot in the
orchard and the remainder may become infected before it reaches
the market (Agrios,
1972). If such fruit are packed, they may cause other fruit in
the same container to
become infected (Timmer et. al., 2000).
Figure 2.5 Symptom of Brown Rot disease in citrus caused by P.
Citrophthora.
(www.giswebr06.ldd.go.th/knowledge/agrilib/plant/tangerine/distangl.htm)
[2006, October 10].
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24
C. Black Stripe disease in rubber caused from Phytophtrora.
Several species
of Phytophthora have been reported to be responsible for black
stripe. The common
species are: Phytophthora palmivora (Butl.) Butl., P. meadii Mc
Rae, P. botryosa
Chee (Drenth and Guest, 2004).
The early symptoms of black stripe are not obvious: a series of
sunken and
slightly discolored areas just above the cut. Later, vertical
fissures appear in the
renewing bark; when these are removed, dark vertical lines are
visible. As the
infection progresses, the stripes coalesce forming broad
lesions, finally spreading the
full width of the panel. When the disease is severe, it extends
vertically in the wood
as far as 15 cm below the tapping cut and 2-5 cm upwards on the
regenerating bark.
Pads of coagulated latex sometimes form beneath the bark causing
extensive bark
splitting and bleeding (Figure 2.6).
Occasionally, infection occurs on untapped bark resulting in a
wound, called
"canker". This may arise on bark previously affected by black
stripe or on wounds
caused by spouts or wires. The early symptoms of canker are not
obvious, but, in the
more advanced stage, the bark bursts and latex oozes out. Pads
of coagulated latex
form under the bark causing it to bulge and split open.
Figure 2.6 Symptom of Black Stripe in rubber caused by
Phytophtrora palmivora.
(www.giswebr06.ldd.go.th/knowledge/agrilib/plant/black.html)
[2006, October 10]
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25
D. Root rot disease, seedlings of many plants is very
susceptible to root rot
and damping off caused by Phytophthora. The early symptoms are
the wilting and
yellowing of young seddlings. General symptoms of root rot are
that plants appear
water stressed, chlorotic, and are often stunted in their
growth. New leaves are often
small and show a light green to yellow colour and wilting occurs
even in the presence
of sufficient water. Affected root tissue is soft, water soaked
and discoloured to dark
brown rather than the creamy white colour of healthy roots.
Advanced root rot leads
to the lack of secondary and tertiary roots and a lack of
healthy root tips (Drenth and
Guest, 2004).
2.3.3 Phytophthora infestans
Phytophthora infestans, which is worldwide, causes the classical
and
extensively investigated late blight of Irish potato and tomato
(Holliday, 1980).
Sporangiophores differentiated from the mycelium (in the host)
by being broader and
having a small swelling at the point of formation of each
sporangium. Sporangia
abundant on the host and on solid media, ellipsoid, ovoid or
(when shed) limoniform,
with a tendency to taper to the base, 19x29 (max. 31x59)µ,
deciduous, pedicel short;
papilla not very protuberant, apical thickening less than
hemispherical, usually 3-3.5µ
(Holliday, 1980) (Figure 2.7). P. infestans is a heterothallic
fungus, that is, sexual
reproduction takes place by means of antheridia and oogonia of
opposite mating
types. There is also evidence for hormonal of chemical control
of sexual reproduction
in heterothallic species. In P. infestans, the oogonia penetrate
and grow through the
antheridium developing into a globose structure above the
antheridium. This type of
development is known as amphigynous development. Both the
oogonia and
antheridia are multinucleate in the beginning but as they
mature, single nucleus is left
that probably undergoes meiosis before fertilization. Migration
of a single antheridial
nucleus to the oogonium occurs through the oogonial wall but
fusion between the two
is delayed until the oospore wall is mature. After a rest period
of several weeks, the
oospore germinates by means of a germ tube that usually
terminates in a germ
sporangium. Zoospores produced in the sporangium give rise to
new thalli (Johri,
2005) (Figure 2.8). Below 15๐C uninucleate zoospores are
produced, whilst above
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26
20๐C multinucleate germ tubes arise. With increasing age,
sporangia lose their
capacity to produce zoospores. Direct germination is preceded by
resorption of the
flagella, formed inside the sporangia (Webster, 1980).
Figure 2.7 Sporangium and sporangiophores of P. infestans, (a)
sporangium and
(b) sporangiophores
a
b
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27
Figure 2.8 Life cycle of Phytophthora infestans. Reproduced from
Drenth (1994).
2.4 Production and biological control of tomato, citrus and
rubber
Plant diseases are important because they cause economic losses
to growers,
result in increased prices of products to consumers, and they
destroy the beauty of the
environment (Agrios, 1972).
2.4.1 Tomato
The tomato belongs to the family Solanaceae (also known as the
nightshade
family), genus Lycopersicon. The Solanaceae family includes
other important
vegetable crops such as chilli and bell peppers, potato and
tobacco. The Lycopersicon
genus includes a relatively small collection of species: the
cultivated tomato L.
esculentum Mill. and several closely related wild Lycopersicon
species, namely L.
esculentum var. cerasiforme, L. pimpinellifolium (Jusl.), L.
cheesmannii, L.
parviflorum, L. chmielewski, L. hirsutum Humb., L. chilense Dun.
and L. peruvianum
(L.) Mill. (Taylor, 1986).
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28
Tomatoes are one of the most important horticultural crops both
temperate
and tropical regions of the world, widely produced and consumed
‘vegetables’ in the
world, both for the fresh fruit market and the processed food
industries. Furthermore,
tomato fruits or plants are occasionally used for decoration or
ornamental value
(Heuvelink, 2005). It is easy to grow and nearly every home
garden has it. It is most
gratifying to the palate, fresh or cooked; soft and grainy,
smooth and juicy in texture.
In addition to the condiments, puree and paste are manufactured
in commercial
quantities. A large share of the processed tomato pack is now
sold as juice but
preservation by freezing has not been successful (Work, 1952).
Their popularity
stems from the fact that they can be eaten fresh or in a
multiple of processed forms.
Three major processed products are: (1) tomato preserves (e.g.
whole peeled
tomatoes, tomato juice, tomato pulp, tomato purée, tomtopaste,
pickled tomatoes); (2)
dried tomatoes (tomato powder, tomato flakes, dried tomato
fruits); and (3) tomato-
based foods (e.g. tomato soup, tomato sauces, chillisauce,
ketchup) (Heuvelink,
2005).
Tomatoes are commonly used as a ‘model crop’ for diverse
physiological,
cellular, biochemical, molecular and genetic studies because
they are easily grown,
have a short life cycle and are easy to manipulate (kinet and
Peet, 1997).
The global production of tomatoes (fresh and processed) has
increased by
about 30% in the last four decades. The annual worldwide
production of tomatoes in
2003 has been estimated at 110 million with a total production
area of about 4.2
million ha. In the future, global production is expected to
increase for both fresh-
market and processing tomatoes. Based on investments made in the
processing sector
and improvements in production systems and cultivars. China may
be the main
source of such increases. Expansions in relatively inexpensive
production areas,
together with in creasing production costs in more
industrialized countries, are a
concern to many growers, especially those producing processing
tomatoes
(Heuvelink, 2005).
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29
Plant disease becomes the limiting factor in tomato production
in many parts
of the world when cultivars with resistance to numerous diseases
are not planted.
There are over 100,000 described species of fungi, and 20,000 of
these are pathogenic
to plants or animals. Diseases caused by fungi such as
alternaria stem canker caused
by Alternaria alternata, anthracnose may be caused by several
species of the genus
Collectotrichum, black root rot by Thielaviopsis basicola, late
blight caused by
Phytophthora infestans (Jones et. al., 1993), wilt caused by
Fusarium oxysporum and
Verticillium dahliae, buckeye rot, stem rot, leaf blight caused
by Phytophthora spp.
and leaf spot caused by Septoria lycopersici (Centre for
overseas pest research, 1983).
Protection involves the use of cultural practices, manipulation
of greenhouse
environments and planting time, regulation of soil moisture,
adjustment of soil
reaction and fertility, control of insect vectors, and the use
of protective chemicals
(Jones et. al., 1993). Cultural control measures of disease
caused by Phytophthora
infestans include: (1) eliminating cull piles in the vicinity of
tomato planting; (2)
destroying volunteer tomato plants; (3) using transplants that
have passed a
certification programme and (4) applying fungicides when weather
conditions favour
disease development (Heuvelink, 2005).
Example application of biological control for control disease in
tomato using
Rahnella aquatilis control bacterial spot of tomato caused by
Xanthomonas
campestris pv. This indicates that R. aquatilis reduced the
deleterious effect and the
stress exerted by X. c. pv. vesicatoria on tomato seedlings.
Foliar application of R.
aquatilis was the most effective method in disease reduction
which could be attributed
to the direct effect of the antagonistic bacteria on the
pathogen. The highest amounts
of fresh and dry weight ere obtained from seed treatment, which
might suggest that
bacterial seed inoculation provides earlier protection than
could be achieved with
foliar, soil or root treatment (El-Hendawy et. al., 2005).
Control of tomato late blight
(LB) in Brazil is heavily based on chemicals. However, reduction
in fungicide usage
is required in both conventional and organic production systems.
Assuming that
biological control is an alternative for LB management, 208
epiphytic
microorganisms and 23 rhizobacteria (RB) were isolated from
conventional and
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30
organically grown tomato plants and tested for antagonistic
activity against
Phytophthora infestans. Based on in vitro inhibition of
sporangia germination and
detached leaXet bioassays, four EP microorganisms (Aspergillus
sp., Cellulomonas
xavigena, Candida sp., and Cryptococcus sp.) were selected.
These microorganisms
were applied either singly or combined on tomato plants treated
or not with the RB
Bacillus cereus. On control plants, LB progress rate (r), area
under disease progress
curve, and final disease severity were high. Lowest values of
final severity were
recorded on plants colonized by B. cereus and treated with C.
xavigena, Candida sp.
and Cryptococcus sp. There was no reduction on disease severity
in plants treated
only with RB. Biological control of LB resulted in low values of
r final severity.
Integration of biological control with fungicides, cultural
practices, and other
measures can contribute to manage LB on tomato production
systems (Júnior et. al.,
2006). The nonpathogenic Fusarium oxysporum strain Fo47 is an
effective biocontrol
agent against Fusarium wilt of tomato caused by F. oxysporum f.
sp. lycopersici
(Fuchs et. al., 1999).
To determine whether bacteria isolated from within plant tissue
can have plant
growth-promotion potential and provide biological control
against soil borne diseases,
seeds and young plants of oilseed rape (Brassicanapus L. cv.
Casino) and tomato
(Lycopersicon lycopersicum L. cv. Dansk export) were inoculated
with individual
bacterial isolates or mixtures of bacteria that originated from
symptom less oilseed
rape, wild and cultivated. They were isolated after surface
sterilization of living roots
and stems. The effects of these isolates on plant growth and
soil borne diseases for
oilseed rape and tomato were evaluated in greenhouse
experiments. We found
isolates that not only significantly improved seed germination,
seedling length, and
plant growth of oilseed rape and tomato but also, when used for
seed treatment,
significantly reduced disease symptoms caused by their vascular
wilt pathogens
Verticillium dahliae Kleb and Fusarium oxysporum f. sp.
lycopersici (Sacc.) ( Nejad
and Johnson, 2000).
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31
2.4.2 Citrus
The true citrus fruit trees belong to the family of Rutaceae
(Spiegel-Roy and
Goldschmidt, 1996). Citrus fruits fall in to several groups:
sweet orange, sour orange,
mandarins and their hybrids, pummelos, grape fruit, lemons, and
limes (Timmer et.
al., 2000).
Citrus fruits originated in South East Asia, including South
China, north-
eastern India and Burma (Spiegel-Roy and Goldschmidt, 1996).
Annual world
production of all citrus fruits is currently about 85 million
metrictons. In many
countries, the crop is consumed as fresh fruit, but in some
countries a major part of
the crop is marketed as a lightly processed, pasteurized,
concentrated juice, jams or
confectionaries. (Timmer et al., 2000). Citrus trees and shrubs
occur naturally
throughout the region, and seletions are widely cultivated.
However, little is known
about the domestication process but it most likely started a
long time ago since citrus
already were taken from southeast Asia for growing in the
Mediterranean during the
great Greek civilization (Drenth and Guest, 2004).
Citrus is second only to the grape (of which most is used for
wine) in the area
planted and in the production of fruit trees. Citrus planting
(FAO Statistics) amount
worldwide to over two million hectares with citrus production
estimated in 1992/3 at
76075000 tons. Brazil is by far the largest producers of oranges
(19.7%), followed by
the USA (13.4%), China, Spain, Mexico, Italy, India and Egypt
(Spiegel-Roy and
Goldschmidt, 1996).
Recent trends of citrus production and demand include
all-year-round supply,
the increasing importance of industrial products (mainly
concentrated fruit juice),
demand for seedless fresh fruit with a substantial increase in
easy peeling. Citrus has
many uses, besides fresh fruit and consumer-processed fresh
juice. Some of the uses
are by products of the processing industry and its main product
concentrated fruit
juice. Products include canned fruit segments (mainly grapefruit
and Satsuma
segments), citrus-based drinks, pectin, citric acid, seed oil,
peel oil, essential and
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32
distilled, citrus alcohol, citrus wines and brandies, citrus
jams, jellies, marmalades and
gel products (Spiegel-Roy and Goldschmidt, 1996).
Citrus is subject to numerous diseases, some of which occur only
in certain
environments, while others, like Phytophthora, pose a serious
problem in all citrus-
growing areas. The common diseases, including postharvest fungal
diseases such as
black root rot caused by Thielaviopsis basicola, wilt caused by
Fusarium oxysporum,
Phytophthora spp. cause the most serious soilborne diseases.
They are of worldwide
distribution. Losses are heavy in murseies (damping-off), in the
orchard (foot rot
gummosis) and on the fruits (brown rot) (Spiegel-Roy and
Goldschmidt, 1996;
Timmer et. al., 2000).
Example application of biological control in citrus. Citrus were
propagated in
planting mixes amended with formulations of commercial
biocontrol agents. Root
colonization by selected biocontrol agents was evaluated for
citrus and found to be
generally between 76 to 100% in both greenhouse ebb and flow,
and bench-produced
plants. Trichoderma harzianum and Bacillus subtilis being the
most effective
uniformly among four tests. Four biocontrols reduced
Phytophthora root rot on
citrus, and two biocontrol agents in combination reduced celery
root rot caused by
Pythium and Fusarium spp., however, none improved above-ground
plant growth or
health of citrus (Nemec et. al.,1996).
2.4.3 Rubber
Rubber belongs to the genus Hevea, species Hevea brasiliensis.
The Hevea
brasiliensis is by far the most important of the species of
Hevea. Ninety-nine per cent
of all the natural rubber produced in the world comes from this
one species. Rubber
production in Hevea is entirely from the bark. The roots, wood,
leaves, and other
portions of the plant do not enter into rubber production
directly (Loreng, 1962).
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33
Hevea brasiliensis or natural rubber was first introduced into
Thailand from
Malaysia through Trang province, South Thailand, in 1900 or
nearly a century ago. It
later spread to Chantaburi province, Southeast Thailand, in
1928. Because price of
rubber is fairly high compared to that of other crops, rubber
growing area has,
therefore, increased rapidly from time to time (Anothai and
Wate, 1995). Thailand is
the world’s largest producer of natural rubber since 1991. In
1993, a total rubber
production of Thailand was about 157 million tons or about 29.3
percent of the world
rubber production. Rubber is also one of the major exports and
dollar earners of
Thailand. It contributed 3.1 to 3.8 percent of the country’s
total export earnings
during the period 1992-1994 (Bank of Thailand, 1994). The
subsequent
establishment and development of the rubber plantation industry
to its present
outstanding position in tropical agriculture have been
continuously aided by research
investigations. Production expansion has been maintained to meet
the ever increasing
world demand for natural rubber (Verhaar, 1979).
Before pneumatic tyres came to dominate the world market for
rubber,
important uses had developed for the vulcanized product. The
manufacture and use of
boots and shoes made of rubber outdistanced that of all other
rubber products and
continued to do so into the twentieth century. By 1900, world
consumption lad
increased to 52, 500 long tons a year and consisted essentially
of five general classes
of products (Loreng, 1962). (1) Boots and shoes. (2) Mechanical
rubber good. (3)
Waterproof clothing other than boots and shoes had assumed great
importance in the
Mexican war through the manufacture of rubberized ponchos. (4)
The manufacture
and sale of drug sundries had become an important part of the
rubber manufacturing
industry. Outstanding products were syringes, hot-water bottles,
bandages, air pillows
and cushions, and atomizers. (5) The manufacture of hard-rubber
goods had also
become important.
The majority of the rubber export at present is ribbed smoked
sheet (71%),
block rubber (16%) and concentrated latex (10%), the other types
of rubber (air dried
sheet, crepe and skim rubber) being small (4%). In 1993, about
130-236 metric tons
of rubber was consumed to produce many types of products. The
main rubber
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34
products of Thailand are tires and tubes for motorcars and
airplanes (39%), gloves
(15%), rubber bands (10%), elastic (8%), tires and tube for
motorcycles and bicycles
(7%), canvas shoes and foam candle (7%) and others (Anothai and
Wate, 1995).
A plant disease caused by fungi is problem in rubber and
economic especially
diseases of the stem and crown. The aerial portions of the Hevea
tree are attacked by
numerous pathogens. Mostly of minor importance but some of great
significance. In
general, it is convenient to divide these afflictions into those
of the stem, of branches,
of bark, and of leaves. One pathogen, Phytophthora palmivora,
attacks all of the
acrial parts and causes leaf-fall, twig dieback, and bark
canker. Besides other diseases
such as oidium mildew caused by Oidium heveae and other leaf
diseases caused by
species of Helnimthosporium, Scoletotrichum and Phyllosticta
cause minor leaf-injury
in the East, and species of Alternaria, Pellicularia and
Catacauma in the Americas.
(Loreng, 1962).
The control of diseases is an important function of estate
management. Many
diseases attack Hevea, but the majority of them are of minor
importance as they are
restricted in spread, lead to little mortality, and require
little in the way of control.
Several have seriously affected stands of Hevea in localized
areas, and a few have
resulted in epidemics of serious proportions (Loreng, 1962).
Example application of
biological control in rubber. The fungitoxic effect of
scopoletin was verified in vitro
on Microcyclus ulei where 2 mM concentrations were sufficient to
strongly inhibit
germ tube elongation and conidium germination. In situ, 24 h
after inoculation,
conidium germination and number of infection sites were lower in
the most resistant
clones. The fungitoxic effect of scopolentin was tested on two
other leaf pathogens of
the rubber tree, Colletotrichum gloeosporioides and Corynespora
cassiicola.
Concentrations double or more than those tested on M.ulei were
required for
inhibition of germination and germ tube elongation (Garcia et.
al., 1995).