Agencies for plant pathogens spreading.
Agencies for plant pathogens spreading.
Distribution of a pathogen is very
essential for the spreading of pathogen and establishment of disease epidemics.
The spread of plant pathogen depends on some agencies. Such agencies are as
follows-
Air
:-A lot of pathogens are spreaded by air.
This type of spread was discovered by Gaumenn. All fungal pathogens which
produce spores or cinidi\a on the surface are disseminated by the wind such as
downy mildews, powdery mildews, rust, smuts, and leaf spot causing pathogens.
Spread of plant pathogen by air may take
place in 2 general ways-
(i) by wind
distribution of diseased plant parts and
(ii) by wind dispersal
of spores and vegetative cells. For effective spreading high spore production
is also essential because a large number of spores are wasted during this
process.
Water
:-Water is also act as an agent for
dissemination of pathogens. Water is relatively less important agent as
compared to wind. Water may serve in 2 ways- (i) as a medium in which actively
motile organisms or spores may swim about and (ii) by the mechanical action of
runoff rain, the flowing of irrigation water or by stream flow.
Actively motile cells can be produced by
some bacterial pathogens white rust pathogen and few downy mildews pathogen.
This type of dissemination is very common in case of Phytophthora infestans
(causing late blight of potato) Albengo candira (white rust of Crucifer)
Insects
:Sometimes the insect act as carriers of
infective materials adhering to their bodies. Or the harbour the inoculums
within their bodies and on certain causes they made the inoculation by feeding
injuries or wounds either by chewing or sucking mouth parts. Both of bacterial
and fungal diseases are being disseminated by insects but they are of
outstanding importance as the vector of viruses.
Some omportant diseases of plant,
spreaded by insects are as follows-
Erwinia tracheiphila
the Cucarbit
wilt organism is completely dependent on Cucumber beetles for its spread; Xanthomonas
stewarti the corn wilt pathogen spread by corn flea beetle; Olphidium
brassicae a pathogenic fungus could be transmitted by some aphids.
Spores of Ustilago violaceae causes smut of caryophylaceae is
dessiminated by pollinating insects.
Birds
:The spread of pathogen is also served by
different types of birds. The best known examples are dissemination of
flowering parasites Dendrophthoe falcate and Viscum spp. In these cases the
dissemination is carried through external contamination of their beaks and
feathers or internally through their elimentary canal. Internal transmission of
spores, chest nut blight fungus as i.e Endotheca parracytia is carried over
by many birds which visit diseased plants.
Mites
: Mites are the wingless insects which
transmit some viruses to the plant. Though transmission of fungal pathogen by
mites has not been extensively studied but some experiments reveal that they
may carry the dormant stage of fungal pathogen.
Nematodes
:Some viruses as well as some bacterial
and fungal plant pathogen are transmitted by nematodes. Corynebacterium facians,
leafy gall of various herbaceous plants is carried by ectoparasitic nematodes
and typical cauliflower symptoms develop only when both nematodes and bacterium
are inoculated into Strawberry plants. There is also evidence that some root
infecting pathogens particularly fungi may enter roots through wounds made by
nematodes and this may perhaps be of more frequent occurrence than is at
present realised. Cotton wilt causing pathogen Fusaruim oxysporium is an
example of this.
Human
:Man is to a large extent responsible for
the dissemination of plant pathogens. With the increasing international tred
and transport new plants, plant products, seed, tubers, the propagating stalk
and fruits carry the plant pathogens with them unknowingly from one place to
another distant place. Some common plant pathogen in this way enters into India.
A list of these pathogens is as follows-
|
Disease |
Original home |
Year of introduction |
|
1) Phytophthora infestans (late blight of potato) 2) Xanthomonas campestris (black rot of Crucifer) 3) Viral pathogen of Bunchytop of
Banana. 4) Urocystis tritici (flag smut of wheat) 5) Phytophthora nicotianae |
England Java Srilanka Australia Holland |
1883
1912 1940
1906
1938 |
Direct
spread by plant or Self agency : In addition with
other agents, host also act as designating agent for pathogen. This type of
transmission of pathogen is called as direct transmission. Direct transmission
may be of following types-
(a) Vegetative transmission:-
A large number of plants are propagated
vegetatively by budding Grafting, cutting or by the use of tubers, corms, bubs,
or rhizomes. The pathogens, especially viruses present in the mothes plant,
from which, these organs are taken, will be transmitted to the progeny. Some
fungi also spread by this method. Eg. Ustilago schitamineae (smut of
sugar). Some bacterium like Clavibacter michiganense (ring rot
of potato) is carried through the tuber of potato.
(b) Mechanical
transmission:-
Viruses can be transmitted merely by contact
or slight rubbing on the infected or healthy part of other. Though such
transmission is uncommon and relatively unimportant but it takes place in a
thickly populated filed where one plant is always in close contact with the
other and ever the slight movement on the wind can help in rubbing the organs
one with the other. Potato virus X (PVX) is one of the virus which easily
transmitted by this method.
·
Cheking
of spreading of pathogen:-
To resist the epidemic spreading of
pathogen or pathogenic substance plants also use some mechanism and devices
which are as follows-
(i) Biological control of soil borne diseases:-
Sometimes biological organisms such as
fungi and bacteria are used to check the disease of a plant caused by other
microorganisms. These biocontrol organisms are favoured by plants either by their storage organ or by their condition
of rhizospere. These biological control microorganisms multiply in huge number
and destroy the pathogens. It is also believed that the pathogenic ones in the
rhizosphere but also enhance the resistance.
For example, Trichoderma harzianum, Glioclabium
virens etc. Are effective against Phytophthora, Pythium, Fusarium
etc.
(ii) Control through trap plant:-
Sometimes spreading of pathogens are
checked by some plant which are planted around a crop field. These plants are
effectively resist many of the incoming pathogen bearing vectors including
aphides, termites, and some birds. These plants are called trap plants. Rye,
corn and other tall plants are used as trap plant around the field of beans,
squash, etc.
The trap plants attack these vector 1st
by resist them due to their taller height and disatributiojn in aq row in close
proximity. Then most of the trap plant provide food materials to the pathogens
and reduce the amount of inoculums in the main plant.
For example, Crotalaria plants trap
the juveniles of root knot nematodes Meloidognye.
(iii) Control through antagonism :-
A few kinds of plants for example, Asparagus,
Taggets
etc.are antagonistic to nematodes and some fungus because they release
substances in the soil that are toxic to several plant parasitic nematodes.
Thus when these inter planted with nematode susceptible plants, antagonestic
pants decrease the number of nematode in the soil and in the roots of the
susceptible crops.
(iv) Rougueing :-
The infected plants are uprooted and
burn immediately after appearance of lesions. Persons or farmers removing and
handling infected plants must wash their hands before the treatment with a new
plant. It should noted that not a single infected propagule remain in the crop
field where a new crop is to be planted.
(v) Crop rotation :-
Crop rotation is one of the best and
most old techniques to control the pathogen spreading. A field or seed bed
where infection have appeared re where the possibility of appearance of
infection is most, crop rotation should be applied. Crop rotation is the
practice where instead of a single crop, a series of crops are planted one
after one through 2-3 years. As a result the dormant stages of pathogens decrease
greatly due to lack of specific host. In this way wheat bunt, rice bunt, wilt
of pigeon pea etc. are eradicated.
(vi) Quarantine and seed certification :-
Quarantine is the legal restriction on
export or import of propagating materials like seeds, roots, tubers etc. To
prevent the spread of disease at international level or in other word,
spreading of pathogen. Quarantine regulation is adopted almost all developed
and developing countries. According to quarantine regulation it is compulsory
that infection and certification system before exchange of those materials.
Within 15 days of shiftment, the plant
material should be throughouly examined and providing with a certificate
mentioning data that the plant materials are found to be healthy or not. To
determine this Koch postulate should be used.
The basic steps of Koch postulates are
that-
(i) The organisms in
question must be found constantly associated with a particular symptom.
(ii) It must be
isolated grown and studied in pure culture.
(iii) The organism
grown in pure culture must be inoculated into a healthy plant to produce the
particular disease. The symptom produced in the inoculated plant should be same
as the symptom 1st observed.
(iv) The organisms must
be reisolated from the inoculated plant and compared with the 1st
culture to be shown to be same as the original culture.
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