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PLANT PATHOLOGY
Pathos = suffering; logos = knowledge
Plant pathology is the branch of botany which deals with the study of nature, development and control
of plant diseases. Disease is harmful variations from normal functioning of physiological processes.
The major objectives of plant pathology are to;
 Understand nature of disease
 Study the causative organism – its spreading, life cycle
 Development of disease resistant/tolerant plants
 Develop disease management strategies
 Increase productivity
An agency which induces a disease (Abiotic or biotic) is called an Incitent:
Diseases are of two categories  Non parasitic disease (Caused by environmental factors – Freezing injury of potato, water core
of apple caused by heat etc)
 Parasitic disease: Caused by biotic agents like fungi, bacteria, nematodes etc.
Disease triangle – Disease development is by the interaction of three components – host, pathogen
and environment.
Inoculum – the portion of pathogen responsible for infection. Spore or mycelium or any other part of
the fungus. Entire body act as inoculum for virus and bacteria.
Development of disease involve five steps
Germination – development of mycelium from inoculum
Penetration – entry of pathogen inside host tissue
Infection – establishment of pathogen inside host tissue
Incubation period – interval between infection and symptom expressions
Symptoms – Manifestation of disease: External symptoms or Internal symptoms
Syndrome – Occurrence of a number of symptoms for a particular disease
Sign – If the disease can be identified by observing the pathogen externally on host surface with naked
eye, it is called sign. Eg. Powdery mildew
SYMPTOMS
The manifestation of a disease is known as symptoms. In most parasitic diseases, the growth of the
pathogen takes place inside the host tissue, thus the parasite is invisible. The expression of the disease
in different shape, colours etc. are visible outside as symptoms. Symptoms are classified into three
categories – Necrotic, atrophic and hypertrophic.
1. Necrotic symptoms
These symptoms are evidenced by the death (necrosis) of cells, tissues or organs of the infected host
due to causal agent. Common necrotic symptoms are given below.
1) Spots: Necrotic regions on host surface formed by killing of tissue in a limited area. Spots may
be angular, circular or irregular. Eg. Brown spot of rice caused by Helminthosporium oryzae.
2) Blight: Rapid killing of different plant parts – leaves, blossoms etc. Killed tissue may become
slimy and emit pungent odour. Eg. Late blight of potato (Phytophthora infestans)
3) Die-back: Gradual dying of stem (twigs) from apex towards base due to pathogen attack. Eg.
Die back of chilly (Colletotrichum capsici)
4) Streak or stripe: Narrow elongated necrotic lesions on leaf veins or stems.
5) Canker: Appear as sunken necrotic areas in the bark, leaves or fruits. Sometimes the diseased
portion becomes separated from healthy region by cracks.
6) Blast: Sudden death of inflorescence, young fruits or region of leaf showing burning at some of
the region. Eg. Blast disease of rice (Pyricularia grisea)
7) Damping off: Pathogen attacks the stem of seedlings at the ground level and causes toppling
down. Eg. Damping off of tobacco (Pythium sps.)
2. Atrophic or Hypoplastic symptoms
This type of symptoms develop due to slowing down in development of the affected tissue or organ.
The underdevelopment may be due to hypoplasia (sub normal cell division) or due to degeneration of
cells. Important atrophic symptoms are;
1. Chlorosis: Appears as yellow or light green in colour due to the degeneration of chlorophyll or
its slow synthesis.
2. Dwarfing: Some organs or the entire plant become reduced in size due to infection. Eg. Little
leaf of brinjal (Mycoplasma)
3. Vein clearing: Green colour (chlorophyll) fails to develop along the veins, thus appearing
translucent. Common symptom in virus infection.
3. Hypertrophic or hyperplastic symptoms
Symptoms are produced by the overgrowth of tissue or organ due to enlargement of cells
(Hypertrophy) or due to increase in cell number by increased cell division (Hyperplasia). Important
symptoms are;
1. Galls: These are abnormal, localized swelling or outgrowth developed on the infected region of
the host tissue.
2. Curl: Symptoms appear due to localized overgrowth on one side or certain region of the leaves
or shoots, which become curled. Eg. Leaf curl
3. Hairy roots: Development of large number of compactly arranged fine fibrous roots due to
infection.
4. Witches broom: Development of more branches with vertical growth are developed due to
infection, instead or normal horizontal branches.
SIGN
Pathogen is partially or wholly visible with naked eye and the disease is identified by seeing the
structure of pathogen. Important signs are given below;
1. Mildews: Pathogen grows superficially and appears as white or brown spots on young stem,
leaves or fruits.
Powdery mildew – Fungal spores are seen as spreading powdery mass on host surface
Downy mildew – Pathogen appears as tangled cottony growth or downy growth
2. Smuts: Black powdery mass of spores develop on the inflorescence either by replacing the
ovary – loose smut or spores remain inside ovarian wall – covered smut
3. Rusts: Spores appear as small pustules on the epidermis of leaf and stem of the host. Pustules
may be powdery or compact and are of different colouration. Eg. Black rust, White rust, etc.
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CAUSATIVE ORGANISMS
An organism which generates a disease is known as pathogen or causative organism. Causative
organisms are generally parasites. Etiology is the determination and study of the cause of a disease.
Fungi, bacteria, viruses, nematodes, mycoplasma etc. are the important causative organisms.
1. Fungi: Important group of plant pathogens. Produce a variety of spores causing rapid
dissemination of disease. Eg. Black rust of wheat – Puccinia, Late blight of potato –
Phytophthora infestans, Loose smut of wheat – Ustilago sps. Etc.
2. Bacteria: These are prokaryotes cause a wide variety of diseases on almost all major families
of higher plants. Eg. Citrus canker – Xanthomonas citri, Blight of paddy – Xanthomonas
compestris etc.
3. Virus: Sub microscopic particles which are obligate inter cellular parasites. Plant viruses are
generally RNA viruses. (Cauliflower mosaic virus is a DNA virus). Eg. Bunchy top of banana
– Musa virus I, Tobacco mosaic – TMV
4. Mycoplasma: Small, wall less, pathogenic organisms grow in young phloem of infected plants.
They do not grow on nutritional media and are visible under electron microscope. Eg. Root
wilt of coconut.
5. Nematodes: Soil borne pathogenic organisms causing plant diseases. Eg. Root knot diseases.
6. Green algae: Some green algae act as parasites causing diseases to plants. Eg. Red rust of tea
caused by green algae Cephaleuros.
7. Flowering plants as parasites: Plants like Loranthus, Cuscuta, Orobanche etc. grow as
parasites on other flowering plants causing economic loss.
Mechanism of infection by Pathogen
Pathogens follow different mechanisms for infecting the host. The pathogen has to cross the
physical barriers of plant – cuticle and cellulosic wall – to get entry into the host. The important
mechanisms for infection are discussed below.
1. Direct penetration: Fungi and nematodes are capable of entering the host by direct penetration.
The nematodes have piercing stylets or proboscis. Fungal spores on germination produce a
germ tube. The tip of the germ tube that touches the host surface enlarges and called
appresorium. Appresorium exerts pressure on the surface of cell wall, and cell wall becomes
thin. Through this thin cell wall infection peg enters and establishes within the cell.
2. Penetration through plants natural openings: Pathogen like certain fungi, bacteria enter the
host through natural openings like stomata, hydathodes or lenticels.
3. Entry of pathogen through wounds: Pathogens like nematodes, virus etc. enter into the host
through wounds formed by agricultural implements, insects, birds, strong wind etc.
4. Production of enzymes: Pathogenic microorganisms produce many enzymes which can
disintegrate cell wall. Pathogens produce cellulose, cellobiase, pectinase etc. that hydrolyse the
cell wall facilitating entry into the cell. Pathogens cross waxy layer and cuticle of epidermis
mainly by mechanical means.
5. Production of toxins: Pathogens produce certain specific or non specific toxins which can
make the host susceptible or weak. As a result infection becomes more efficient. Eg.
Pyricularia oryzae produce a toxin – Piricularin.
Defense Mechanism in Host
Defense mechanisms developed in the plants against pathogens can be grouped into two broad
categories.
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1. Mechanical defense (Resistance of host against penetration of pathogen)
2. Biochemical defense (Resistance of host against invasion of pathogen in the protoplasm)
1. Mechanical defense (Histological defense): Divided into two types – pre existing defense and
post infection or dynamic defense.
a) Pre-existing defense: These are defense mechanisms which are already present in the plant that
resist the entry of pathogens. Some of them are;
Deposition of cuticle and wax on epidermis; Epidermis with compactly arranged cells
Structural speciality of natural openings – Small stomata
Internal structural feature – Thick and tough cell walls of internal tissue prevent entry of
pathogen.
b) Post infection defenses: These mechanisms develop after the infection by the pathogen.
i.
Histological changes: Formation of gums, resins or tyloses. Formation of corky layers
and abscission layers etc. are used by plants to prevent entry of pathogens into other
healthy areas.
ii.
Cellular defenses: Structural modification in cell wall. Infection produce swellings on
cell wall and these swellings become lignified resisting the entry of pathogens.
iii.
Necrotic reactions: Structural change due to alteration of physiology by the invasion of
a pathogen. The affected host nuclei disorganize and host cells are killed. This region is
converted into a necrotic area. As a result the parasite cannot receive food and the
spread of the disease is checked.
2. Biochemical defense in plants
a) Pre existing defense: Presence of inhibitors in the cell – Many chemicals which are toxic to the
microorganisms are present in the plant cell. Eg. Chlorogenic acid which act against
nematodes.
b) Post infection defense:
i.
Formation of toxins after infection: Production of chemicals by host which have
antifungal or bactericidal properties. Eg. Phenolic compounds, Phytoalexins etc.
ii.
Inactivation of toxins of pathogen by plat metabolism.
iii.
Hyper sensitivity: The affected and adjoining cells of hose become hypersensitive and
soon convert to a necrotic region. So that the pathogen is prevented from entering the
healthy cells.
iv.
Production of growth hormones: Plant hormones like IAA, Kinetin etc are found to
increase resistance against certain diseases.
Dissemination of Plant Diseases
The spread of a disease from one plant to other plants or from one locality to another is known as
dissemination. Dissemination of pathogen is carried out by several agents like wind, water, vectors,
seed, soil etc.
1. Wind dissemination: Certain inocula are easily carried by wind since they are very small and
light. Eg. Spores of fungi. Wind also disseminates the pathogen indirectly by carrying infected
leaf or floral parts from one place to another.
2. Water dissemination: Motile and non motile spores of many pathogens are disseminated by
irrigation water, rain water or through streams. Eg. Conidium of Fusarium.
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3. Seed dissemination: Pathogens are carried internally or externally on seed and are
disseminated by transfer of infected seeds. Eg. Smut disease
4. Insect dissemination: Insects can disseminate diseases directly or indirectly. Many viral
diseases are disseminated by insects which suck sap from infected plants and then visit other
plants for feeding (Bunchy top of banana). Pathogenic bacteria and spores of fungi get attached
on the body of insects and get disseminated (Ascospores of Claviceps).
5. Dissemination by animals other than insects: Pathogens can be transported to other locality
by organisms like nematodes. Eg. Bacterial brown spot of potato.
6. Dissemination with plant debris: Due to careless sanitary practice, infected plant debris may
remain in the field and pathogens are transferred to different places during preparation of field
for next plantation.
7. Dissemination by field operations: Diseases can be disseminated during field operations like
ploughing, watering, weeding, transplanting etc. Eg. Tobacco mosaic disease
8. Dissemination by propagating stock: Pathogen can be transported to other locations through
infected propagating stock (cuttings). Eg. Fusarium stem rot of sweet potato.
9. Dissemination by export and import of commodities: During export and import of
commodities, diseases can be disseminated with contaminated container or contaminated
commodities itself. Eg. Wart disease of potato is introduced in India through import.
Control of Plant Diseases
Control of plant diseases is a very important aspect in increasing the productivity and also
safeguarding the crop plants from destruction. In general, the disease control involves two aspects.
The first one is the prevention of disease. These methods are adopted before the pathogenic
infection. This method of disease control is called prophylaxis. In the second type disease control
is adopted after the infection. i.e., diseased plants are treated to cure them. This is mainly done by
chemical, physical or biological methods.
Prophylaxis
This method aims at preventing the plant or a crop from being infected. Prophylactic methods
include three aspects namely, exclusion, eradication and protection. In exclusion a pathogen is
prevented from entering a particular area where a susceptible crop is growing. The important
components of exclusion are quarantine measures and seed certification.
Quarantine measures
Quarantine is the legal restriction on export and import of propagating materials like seeds, roots,
tubers etc. to prevent the spread of disease at international level. Quarantine laws are enforced by
almost all agriculturally advanced countries. These laws prohibit the transmission of pathogen
from one country to another. In India, quarantine laws are introduced as Destructive Insects and
Pest Act (DIP Act) in 1914 and it has amended from time to time. At present in India, there are 16
quarantine centres on land, air and sea routes. Entry of propagating materials are allowed only
through these centres when it is accompanied by the certificate from the ministry of agriculture of
the exporting country stating the material is free from any disease. Chennai is the quarantine centre
for south India. The imported material is also thoroughly checked by experts at quarantine centres.
Domestic quarantine is also applicable to large countries like India. For eg. Transportation of
potato tubers from Darjeeling to other parts of India is prohibited to prevent the spreading of wart
disease.
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Seed certification
It is recognized that a large number of diseases are spread through seeds and vegetative propagules
(stock, suckers etc.) that are contaminated with pathogen. This leads to a practice of inspection and
certification on planting material in several countries before they are sent to different parts. This
certification ensures that the transporting material do not contain any pathogen. This seeds are not
only certified for the absence of pathogen but also for their viability, purity, absence of weed seeds
etc. Careful seed certification is very useful in preventing the spread of disease to different areas.
In many countries seed certification is carried through State Department of Agriculture or other
Govt. agencies.
Therapeutic method
These methods are adopted to treat the diseased plants to cure the disease. This is done by three
methods – physical therapy, chemical therapy and biological methods
Physical therapy
Certain physical agents are used to treat the disease. The important ones age given below.
1. Heat sterilization of soil: Soil is sterilized by steam under pressure in a container or passing
steam through perforated pipes under the soil in green house or nursery seed beds. This method
is effective in controlling nematodes and soil fungi.
2. Hot water: It is used to control perennating pathogen in seeds, bulbs, soil etc. This method is
effective against brown spot of rice, loose smut of wheat and many nematode diseases.
3. Low temperature: Most effective and widely used method to control many post harvest
diseases. Low temperature control the spread of existing pathogen and thereby reduce the
disease incident.
4. Radiation treatment: uv light, X rays, γ rays etc. are effective in controlling post harvest
diseases of many fruits and vegetables by killing the pathogen.
5. Surgery: It is the removal of infected tissue to prevent further damage. It is applicable to
localized diseases. The cut end should be covered with a fungicide to prevent further infection.
Chemical therapy
This method involves application of chemicals to control plant diseases. They are variously classified
according to target microorganisms like fungicides, bactericides, insecticides, herbicides etc.
Depending on the mode of action fungicides are classified into contact fungicides, residual fungicides
and systemic fungicides.
Contact fungicides: These groups of chemicals are aimed at the fungus itself either before or after
infection.
Residual fungicides: These are applied to the host before the infection, it forms a protective layer on
host surface.
Systemic fungicides: These chemicals are applied either in the soil or on leaves, it is taken up by the
plant and translocated to all parts of the plants through xylem. Hence the plant becomes fungitoxic.
The chemicals reach all part of the plant and thus destroy the pathogen.
Important chemicals used in plant disease control
1. Copper compounds
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a) Bordeaux mixture: Important fungicide, discovered by Millardet. It is a mixture of copper
sulphate and lime (calcium hydroxide) in water. The mixture is unstable and hence it is
advisable to use immediately after preparation.
b) Burgandy mixture: Mixture of copper sulphate and sodium carbonate in water with the same
ratio as Bordeaux mixture.
2. Sulphur compounds
a) Sulphur: Elemental sulphur is used as dust or wettable powder or liquid to control leaf blights,
fruit rot, rusts etc.
b) Lime sulphur: Prepared by hydrated lime and sulphur. It is used against powdery mildews,
apple scab etc.
c) Organic sulphur compounds: Many organic sulphur compounds are available namely Thiram,
Febram, Ziram etc. These compounds are used against various plant diseases.
3. Mercury compounds
Two important mercuric compounds which are in use are Mercuric chloride (HgCl2) and
Mercurous chloride (Hg2Cl2). They are used for surface sterilization or wounds or cuts, seed
treatment and against soil borne pathogens.
4. Heterocyclic compounds
These are heterocyclic nitrogen containing fungicides. They are available in trade names like
Captan, Folpet, Vancide etc. Used against leaf spot, powdery mildews etc.
Biological control
Biological control is a practice by which the activity of a pathogen or its survival is reduced through
the agency of any other living organisms resulting in the reduced level of disease caused by the
pathogen. The disease can be controlled by different types of biological methods such as;
By antagonists of pathogen: These are microorganisms which act against pathogens. Eg. Trichoderma
sps. is found to be effective against pathogens like Pythium, Sclerotium etc.
Hyperparasitism: Controlling the pathogen with the help of other microorganisms which parasites on
the pathogens. Bacteriophages, mycoparasites and nematophagous fungi can act as hyperparasites.
Trap crops: Certain plants may get infected by different pathogens, but after infection the host secretes
some substances which are inhibitory to the pathogen or any one of its growth stage. Such hosts are
called trap crops. Trap crops are grown in rotation with crop plants for disease management. Eg.
Crotalaria sps., Solanum nigrum etc.
Antogonistic plants: Certain plants like marigold, asparagus etc., release certain toxic substances in the
soil which are effective to control several soil borne pathogens.
Breeding for disease resistance: Many high yielding, disease resistant varieties have been developed
through conventional breeding and genetic engineering.
PLANT DISEASES
BUNCHY TOP OF BANANA
It is a common viral disease affecting banana plants. The disease is caused by Bunchy top virus or
Musa virus or Banana virus. The disease is transmitted through the sap of the plant by an insect
Pentalonia nigronervosa.
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Symptoms
Tender unfolded regions are affected first, leaves do not unfold normally. Leaf margins are rotted,
leaves show chlorosis. All leaves are arranged as a bunch at the top, due to the fact that affected leaves
cannot elongate petiole and leaf sheath. All regions are affected as the disease progresses – dark brown
spots develop on leaf, petiole, rhizome etc.
Control measures
Systemic disease and hence all parts contain virus. All parts of affected plants should be destroyed by
burning including rhizome.
Destruction or control of insect vector. Use of disease free suckers
Field sanitation, removal of other plants like Heliconia, Ravanalia from the vicinity of banana
plantations since insects live on these plants.
BACTERIAL BLIGHT OF PADDY
This bacterial disease is first reported from Japan. It is caused by the bacterium Xanthomonas oryzae
(X. compestris oryzae) which is a gram negative, non spore forming, rod shaped bacteria. The disease
is a vascular disease. The bacterium can perpetuate in different ways – through soil, seeds, wild
grasses or diseased straw. Bacteria enter through wounds or natural openings. After entering, bacteria
multiply and move upward or downward affecting other regions.
Symptoms
Symptoms are visible on all parts except root. Symptoms appear as water soaked tiny spots on the
margins of mature lower leaf of seedlings. Later on leaves become yellow and dry out. Yellow or light
brown colour stripes develop along the margins. These stripes enlarge in size. This is the blight
symptom and extended to all parts of the plant.
The most harmful phase of the disease is the wilt symptom or the Kresek – develop due to the
systemic nature of bacteria. The leaves turn yellow, bacteria move through xylem and affect other
leaves, leaves roll and drop, later the plant withers completely. Plant wilt and float in water.
Control measures
Field sanitation – Burning of straw of previous crop
Seed treatment – Soaking seeds in Agrimycin and Ceresan followed by hot water treatment for 30 min
at 52-54oC.
Foliar spray – Foliar spray of chloramphenicol with phenyl mercury acetate or Agrimycin mixed with
copper oxychloride.
Use of disease resistant varieties – Disease resistant varieties like IR-20, IR-22, BAM 9, Ratna etc can
be used to reduce disease incidence.
ROOT WILT OF COCONUT
It is a serious disease in coconut growing areas of Kerala. Mainly seen in Alappuzha, Ernakulam,
Kottayam and Kollam districts. Also seen in Kanyakumari district in Tamil Nadu. The disease is
considered to be caused by Phytoplasma or Mycoplasma.
Symptoms
Disease usually seen in grown up plants. First symptoms is the wilting of leaflets of leaves of the
middle whorl. Leaflets show bending called ribbing. Later leaflets show yellow colour and margins
show necrotic symptoms. New leaves formed are shorter, nuts formed are also very small and poor in
quality. Large numbers of tender nuts are shed, resulting in decreased yield.
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Control
Complete eradication of disease is not possible. The affected palms can be maintained with steady
yield through integrated management practices (disease and nutrient management). Severely affected
palms should be removed and destructed. Use of resistant and high yielding hybrid varieties.
ABNORMAL LEAF FALL OF RUBBER
It is a major disease affecting rubber plants. The disease is caused by the fungus Phytophthora
palmivora. Fungus produces both inter and intra cellular mycelium. During favourable season the
fungus produces sporangia which contain biflagellate zoospores. Zoospores liberate, encyst and
germinate and thus spread the disease. Affected fruits, defoliated leaves and fallen twigs in soil serve
as the primary inoculums. From this pathogen infects the plants in the next season.
Symptoms
Disease found to occur during the south west monsoon. First symptom is the appearance of dark
brown or black areas on pods. Under humid conditions pathogen spreads on the pods, latex ooze out
from the pods. Leaves get affected. Dark brown spots develop on petioles, pale coloured water soaked
areas develop on lamina. Leaves get defoliated at early stage. After defoliation, young twigs are also
affected and they die out. Due to rapid defoliation, photosynthesis is reduced thus affecting the latex
yield.
Control measure
The most effective method to control the disease is to spray the leaves with Bordeaux mixture.
Fungal infections on man
Like plant diseases, some fungi also cause human diseases. Some of the important human diseases
caused by fungi are given here.
Name of disease
Causative fungi
Ring worm of skin and nails
Trichophyton, Microsporum
Aspergillosis
Aspergillus niger, A. flavus
Allergy
Alternaria, Aspergillus, Trichoderma etc.
Athlets’ foot
Tinea rubrum
Some fungi produce certain chemicals which are toxic to the human beings. The most common
example is aflatoxin produced by Aspergillus flavus and A. fumigatus.
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