CHAPTER 2: CONCEPT OF PLANT DISEASE
CONTENT
2.1. What is a plant disease?
2.2. Importance of plant disease
2.3. Development of Plant Pathology as a science
2.4. Causes of Plant Disease
2.4.1. Non-infectious disease
2.4.2. Infectious disease
2.5. Classification of Plant Disease
2.6. Conditions for disease development
2.7. Pathogenesis
2.7.1. Survival of pathogen
2.7.2. Inoculum
2.7.3. The spread of plant disease
2.8. General symptoms of plant diseases
2.9. Disease management
2.10. Some important definitions
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2.1. WHAT IS A PLANT DISE ASE?
The term DISEASE is coined by combining the words DIS + EASE = DISEASE.
The prefix DIS means negative, reverse, or opposite, and the word EASE means
comfort, or freedom from pain or discomfort. DIS-EASE therefore means not well,
and the cause can be many. A plant disease may therefore be defined as: Any
harmful deviation or alteration from the normal functioning of physiological
processes. It is also defined by some as: Disease is a malfunctioning process
that is caused by continuous irritation which results in suffering. A more
practical definition of a disease would be: A plant is diseased when its
systems are not normal and, therefore, it is not producing as well as it
should according to normal expectations of the farmers.
2.2. THE IMPORTANCE OF PL ANT DISEASE
Plant diseases are important because of the loss they cause. Bhutan
experienced for the first time one of the worst rice blast epidemic in 1995 during
which most of the rice growers in Paro and Thimphu suffered heavy losses.
Similarly the late blight disease of potato which is caused by a fungus
(Phytophthora infestans) is quite endemic and appears again and again when the
monsoon is heavy. Once the disease spread it will inflict heavy loss to potato
growers. Apple scab is another disease in apples grown in the high altitudes of
Thimphu, Paro, Haa and Bumthang. For this, most of the orchard owners in the
above regions carry out regular spraying with fungicides. Chilli wilt disease has
become another headache for the agriculture extension. We have not yet found
out any reliable solution. While the incidence of diseases on minor crops may not
be of much concern, when diseases bring about a heavy loss on important cash
crops like potato and apple and staple crops like rice, it is a cause for much
concern. One cannot afford to neglect the plant disease aspects.
2.3. DEVELOPMENT OF PLANT PATHOLOGY AS A SCIENCE
The origin and cause of plant diseases became clear only with the invention of
microscope in the 17th century that enabled the discovery of hitherto unknown
world of microorganisms. Leeuwenhoek discovered bacteria in 1675. An Italian
botanist Micheli in 1729 made an extensive study of fungi and their reproductive
structures.
The devastating epiphytotic(epidemic) of the potato disease in the middle of the
19th century was one of the most tragic events in human history. It swept over
the whole of Europe and USA but in Ireland it was catastrophic. Over one million
people died and one and a half million migrated. This tragic event was a blessing
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in disguise, in so far as it hastened the realization of the importance of plant
diseases. Many committees and commissions were formed to report on the
disease. The causal organism was identified by Speerschneider(1857) and De
Bary (1861-63-67). It was Phytophthora infestans. The battle against the
spontaneous origin of disease was largely won. The science of plant pathology,
evolving for a long time, was born.
2.4. CAUSES OF PLANT DISEASE

Non-infectious disease - caused by natural agencies

Infectious disease - caused by pathogens
Infectious means that which tends to spread from one plant to another or from
one part of the plant to the other.
2.4.1. Non-infectious Plant Disease
Possible causes include:
a) Weather
b) Nutrient deficiency
c) Toxic substances
a) The Weather:
1. Lightning - can scorch plants, and break and burn old shrubs and
trees.
2. Rain/hail stone - heavy rain cause physical damage to young plants,
and hail stones cause flower and fruit drops.
3. Wind - breaks leaves and branches and when strong, uproots whole
shrubs and trees and cause lodging of field crops (buck wheat, rice,
maize etc. are prone to lodging)
4. Drought - causes yellowing, leaf curling, wilt and eventual death of
plants.
5. Flooding - causes yellowing, slow development and death through lack
of air at roots.
6. Strong sun - yellowing and scorching of leaves, wilting and death.
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7. Frost - most crops cannot tolerate frost bites
b) Nutrient Deficiencies
Deficiency of nutrients can cause symptoms like yellowing, reddening, spotting,
stunting, distorted growth and death. However, symptoms vary with the element
involved.
c) Toxic substances in the soil or air
Too much elements required by plants can cause death, with similar symptoms
to deficiencies.
2.4.2. Infectious Plant Disease
These can be caused by:
a) Fungi
b) Bacteria
c) Viruses
d) Nematodes
e) Algae
f) Parasitic Plants
a) Fungi
Fungi are plants without chlorophyll and therefore they must live on other plants
which make their own food. Fungi are generally microscopic but there are large
ones like mushrooms. Most fungal bodies consists of thin delicate filaments or
very small tubular structures called hyphae, which grow in or on the host’s tissue.
Sometimes these threads collect together to give big structures like mushrooms
and toad stools.. Eventually they produce spores(fungus fruits/seeds) which are
often visible on the affected parts of the plants. E.g. rust spores on beans, smut
on wheat or maize , and mildews on cucurbits. There are thousands of species of
fungi with endless forms of spores, symptoms and fruiting bodies. Some fungus
live on the living host where as others live on dead plant and animal matter. The
former ones are known as parasites and the latter, saprophytes.
The spores serve to spread the fungus to new plants. Fungal spores have certain
specific characteristics that help the Plant pathologist to identify. Examples of
diseases caused by fungi are, Late Blight of Potato, Wheat rust and smut, apple
scab etc.
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b) Bacteria
Many destructive plant diseases are caused by bacteria, e.g. wilts (tomato and
potato), soft rots of fruit and vegetables, many cancer-like growths of galls of
fruits and stems and several leaf spots, e.g. halo blight of beans.
Bacteria are small single celled structures, which can aggregate to form, often
characteristic, cream or yellow slime. There are 3 basic groups - the bacillus or
rod shape, the coccus or round shaped and spirillum or spiral shaped. The
majority that cause plant disease are the rod shaped.
Bacteria cannot enter directly through the epidermis, as can fungi, but go through
wounds or stomata.
Control of bacterial disease is by chemicals, resistant varieties and good
sanitation.
The identification of bacteria is a complicated process involving many
biochemical tests, although in many cases, symptoms of the disease on plants
are fairly characteristic.
Bacteria multiply by dividing simply in to 2. Some can divide every 20 mins and in
24 hrs, it would be theoretically be possible for a single bacterium to produce
more than 300,000,000,000 individuals.
c) Viruses
Smallest of all and invisible under an ordinary microscope, are the viruses. For
practical purposes they are recognizable only through symptoms they produce
together with the fact that such symptoms can be transmitted to a healthy plant
by various means. This makes virus identification a difficult matter and even
more so because some chemical damage and deficiency symptoms can also
resemble those of virus diseases.
Virus are not living organisms in the normal sense, bit are related to the basic
protein-like components of living matter - the nucleic acids. As a result they are
normally not controlled by chemical methods applicable to fungi and bacteria, but
some can be killed by heat treatment without killing the plants as well. Some
virus forms such as mycoplasmas can be treated with antibiotics, but generally
this is not economic.
d) Nematodes
Nematodes are small worms which can just be seen with the naked eye. Most of
them leave in the soil without doing any damage but some are parasitic. Most of
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the parasitic ones damage roots by burrowing in to them and causing galls or just
feeding on them and sucking the contents out of the root cells.
e) Algae
Algae re single-celled or thread-like plants which contain chlorophyll. Some algae
in the tropical areas are plant parasitic while in general they are not likely to
cause any serious damage except under special circumstances or in neglected
plantations of tree crops.
f) Parasitic Plants
Some plants like the mistletoe grow on other plants and derive nutrition thereby
causing the host plant to die.
2.5.
CLASSIFICATION OF PLANT DISEASES
Plant diseases can be classified in various ways depending upon the purpose of
discussion
1. Based on plant part affected: Localized disease- affecting only a part of the
plant; systemic disease affecting the entire plant
2. Based on symptoms: rusts, smuts, wilts, blights, cankers, mildews, rots,
damping-off, die-back, scab etc.
3. Based on the host plant: cereal crop disease, root crop disease, forage crop
disease, plantation crop disease etc.
4. Based on their occurrence:
Endemic disease: The word endemic means prevalent in, and confined to, a
particular country or district and is applied to disease. These diseases are natural
to one country or part of the earth. When a disease is more or less constantly
present in one form or other or less constantly present form year to year in a
moderate to severe form, in a particular country or part of earth, it is classed as
endemic.
Epidemic or epiphytotic diseases: The term ‘epidemic’ is derived from a Greek
word meaning ‘among the people’ and in true sense applies to those diseases of
human beings which appear very virulently among large section of the
population. To carry the same sense in the case of plant diseases, the term
epiphytotic has been coined. An epiphytotic disease is one which occurs widely
but periodically. It may be present constantly in the locality but assumes severe
form only on occasions.
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Sporadic diseases: Sporadic diseases are those diseases which occur at very
irregular intervals and locations and in relatively few instances.
A given disease may be endemic in one region and epidemic in another. When a
disease is prevalent throughout the country, continent or the world it is known as
a pandemic disease.
5. Based on the cause: Infectious disease (fungal disease, bacterial disease,
viral disease), Non-infectious disease(caused by nature-frost, rain, wind, sun, hail
storm et.)
2.6. CONDITIONS FOR DISEASE DEVELOPMENT
The pathogens, especially the animate ones, will not always be able to cause a
disease unless environmental conditions and suitability of the host are also
favourable for survival, multiplication, and entry of the pathogen into the plant
and further development of the disease. For a pathogen to cause a disease, the
pathogen has to be virulent in the first place; the host has to be susceptible, and
the environmental conditions favourable. Disease will not develop if anyone of
the 3 conditions is not fulfilled. This is known as the disease triangle.
2.7. PATHOGENESIS
Pathogenesis can be defined as the entire chain of events leading to the
development of a disease. In other words it can be termed as a disease cycle. It
tells us about the source of perennation of the pathogen, mode of its dispersal
from the source of survival and during the spread of disease. It also helps us to
formulate effective control measures.
The events constituting pathogenesis occurs in several well defined steps, one
after the other, to complete the disease cycle.

the pathogen perennates at some location during the absence of cultivated
host

it gets transported to the cultivated host through some agencies

the pathogen breaks the host barrier to establish infection

effects the host physiology, damage the plant and the plant expresses
symptoms

finally the pathogen finds an exit from the host through its propagules
Infection chain- chain of events leading to the completion of pathogenesis.
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2.7.1 Survival of Pathogen:
In the absence of their cultivated host, pathogens must find some alternate
source of their survival; otherwise the infection chain will remain incomplete. The
same holds true for viral disease.
The sources of survival can be grouped into:
1. Infected host as reservoir of inoculum
a) cultivated host
b) wild host of the same family (collateral host)
c) wild host of other family (alternate host)
2. Saprophytic survival outside the hostsoil and plant debris serve as the media
for survival. e.g. Pythium, Rhizoctonia
3. Dormant organs of pathogen as a source of survival and primary inoculum.
(virus and bacteria have no resting stage); only fungi and nematodes
have(spores, cysts etc.)
2.7.2. Inoculum
Inoculum is defined as the part of the pathogen, which on contact with a suitable
host can cause infection. E.g. of a fungal inoculum: Chlamydospores, sclerotia,
oospores, teleutospores, zoospores, conidiospores etc.)
Sources of inoculum
1. sowing and planting materials such as seeds, tubers, bulbs, rhizomes
2. affected plants (diseased) in the field
3. debris left on the land or used in compost heaps
4. host plants in a dormant stage when conditions are not favourable
5. soil infested with certain soil-borne pathogens
Types of inoculum
Mycelium: The vegetative growth by means of hyphae is only significant for the
spread and hence the dispersal of a fungus within the host plant or in the
ecosystem of the soil. e.g. Chlamydospores, sclerotia.
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Spores: Spores play a main role in the survival of the fungus. e.g. oospores,
teleutospores, ascospores, zoospores and conidiospores.
2.7.3. The spread of plant diseases
Plant diseases are spread in various ways:
1. Spread through seeds and other planting materials such as tubers, rhizomes
etc.
2. Spread by natural agents such as wind, water, rain
3. Spread by animals and insects
4. Spread by people
Following the infection, the pathogen will continue its growth, produce spores or
reproductive units, which will find exit through host surface and spread to repeat
the same process. Many pathogens spread through crops in a special and often
spectacular way. Some cover tremendous distances at a remarkable speed,
especially pathogens with airborne propagules, like rusts. Pathogens which infect
subterranean plant organs exhibit a restricted pattern of dispersal. Then there are
pathogens which depend for their dispersal of spores and propagules on rain or
water, while others need a vector, like man, insects and nematodes. Some
parasites are transported with seed, thus depending on the hosts’s own dispersal
processes.
There are many dispersal mechanisms. Some of these mechanisms are both
elegant and highly efficient in that they are closely adapted to the biology of the
host, like the synchronization of spore release in certain pathogens like Venturia
and Claviceps. Other parasites produce millions of spores, saturating the
environment with propagules in an apparently haphazard and wasteful way.
2.7.4. The infection process of a fungal disease
A parasitic fungus must establish an intimate relationship with the host tissues to
absorb the desired nutrients. Successful establishment of this relationship is
called infection. The term can be defined as the establishment of a pathogen
within the host body.
The infection process has three distinct stages
1. Pre-penetration stage
2. Penetration
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3. Post-penetration stage
During the pre-penetration stage the fungus hyphae or spores come in contact
with the host surface. The spores will germinate to produce germ tubes or
infection threads. This stage may be abortive if environments do not favor growth
of the hypha or germination of spores. During penetration, the infection thread
enters the host by any of the following methods:
1. Entry through natural openings like stomata & lenticels
2. Entry through rupture of the host surface due to development of organs like
prop roots.
3. Entry through wounds due to mechanical injury or insect injury
4. Direct penetration:
a) Mechanical pressure by the infection thread. The pathogen exerts its
own effort to break the host barrier and directly enter through cuticle or
epidermis without seeking the wounds or natural openings. Host barriers
can be either structural barriers or chemical barriers.
b) Chemical action - enzymes produced by the pathogen will break down
the epidermal cells and thus gain entry.
The entry of the infection thread into the host is dependent upon environments.
After entry it is possible that physiological conditions of the host are not found
suitable and the infection thread may die. In case conditions are favourable the
infection thread will approach the nearest cell and enter it directly or send a
haustorium. This will result in the absorption of food from the host protoplasm.
Supported by these nutrients the infection thread will develop into inter- or
intracellular mycelium or will support the mycelium. In addition to absorption of
food the fungus may interfere with the host activity in various ways. It may
excrete pectinolytic or cellulolytic enzymes to dissolve the cell-wall or toxins may
be produced which, when translocated to other parts may cause various types of
damage to plant tissues. Due to these interferences with the normal existence of
the host, symptoms of disease will develop. The fungus will continue growth,
produce spores or reproductive units which will find exit through host surface and
spread to repeat the same process as explained above.
Dispersal of pathogens depends to a great extent on the type of inoculum.
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2.8. GENERAL SYMPTOMS OF PLANT DISEASES
1. Mildew: Mildews are plant diseases in which the pathogen is seen as a growth
on the surface of the host. They appear as white, gray, brownish, or purplish
patches of varying size on leaves, herbaceous stems, or fruits. In downy mildews
the superficial growth is a tangled cottony or downy layer, while in the powdery
mildews enormous numbers of spores are formed on superficial growth of the
fungus giving a dusty or powdery appearance. Black minute fruiting bodies may
also develop in the powdery mass.
2. Rusts: These are diseases with rusty symptoms. The rusts appear as
relatively small pustules of spores, usually through the host epidermis. The
pustules may be either dusty or compact, and red, brown, yellow, or black in
colour.
3. Smuts: The word smut means a sooty or charcoal-like powder. The affected
parts of the plant show a black or purplish-black dusty mass. These symptoms
usually appear on floral organs, particularly the ovary but they can also be found
on stems, leaves and roots.
4. Scab: The term scab refers to a roughened or crust-like lesions or to a
freckled appearance of the diseased organ. In some diseases of this type the
parasite appears at a certain stage, in others it is never seen.
5. Sclerotia: A sclerotium is a compact, often hard, mass of dormant fungus
mycelium. Sclerotia is most often black, or they may be buff or dark brown or
purplish in colour. E.g. ergot of grasses.
6. Blotch: This symptom consists of a superficial growth giving the fruit a
blotched appearance as in sooty blotch and fly speck disease of apple fruits.
7. Tar spots: These are somewhat raised, black coated fungus bodies with the
appearance of a flattened out drop of tar on leaves.
8. Exudations: In several bacterial diseases, masses of bacteria ooze out to
surface of the affected organ where they may be seen as drops or as thin smear
over the surface.
9. Chlorosis: Development of yellow colour as a result of low temperature,
nutrient deficiency, excess of lime or alkali, lack of iron, disturbances by fungal
and bacterial diseases, viral infection etc. is known as chlorosis. When yellowing
is localized, it is known as chlorotic spots.
10. Overgrowths or Hypertrophy: This is the abnormal increase in size of plant
organs or the entire plant as a result of stimulation of the host tissues. This may
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be brought about either or both of the two processes, hyperplasia and
hypertrophy. Hyperplasia is the abnormal increase in the size of a plant organ
due to increase in number of cells of which the organ is composed. In
hypertrophy, the increased size of the organ is due to increase in size of the
cells.
a) Galls: These are malformations of more or less globose, elongated, or
irregular shape. They may be fleshy or woody. Small galls are called warts,
tubercles, etc. while larger ones are called knots. E.g. crown gall, club root, root
knot, etc.
b) Curl: Leaves are arched, puckered, twisted, curled, and distorted due to
growth in tissues in localized area of the leaf. Examples are peach leaf curl,
papaya leaf curl, etc.
c) Witches broom: Numerous slender branches arise from a limited region in
rather close clusters appearing like a broom.
d) Hairy root: Numerous fine fibrous roots are produced which are abnormal.
11. Atrophy or hypoplasia or dwarfing: This is a disease symptom in which the
plants remain stunted or dwarf because of growth inhibition (retardation). It is
possible that hypertrophy and atrophy both can occur in the same organ.
12. Necrosis: The term necrosis is used to indicate the condition in which the
death of cells, tissues and organs has occurred as a result of infection. The
following are the different necrotic symptoms:
a) Spots: The death of cells or tissues (necrosis) occur in definitely limited areas.
The shape of the lesions(necrotic spots) may be round, angular, or irregular. The
dead areas are often surrounded by a purple, red, yellow or brown margin.
Fruiting bodies may also develop in the dead areas. Leaf necrosis may lead to a
dead leaf tissue falling away, causing a shothole effect.
b) Streaks or stripes: They are prominent symptoms consisting of an elongated
but relatively narrow lesions. These streaks or stripes are usually some shade of
brown colour.
c) Canker: A canker is a dead area in the bark or cortex of the stem, especially
woody plants. They are usually rather large with well defined margins. The
surface may be smooth or rough and usually somewhat sunken. In some cases
only the superficial layers of cells are affected while in others all the tissues
except the fibers are destroyed. In many cases the dead bark splits and finally
peels away leaving the wood naked.
d) Blight: This term means a burnt appearance. Extensive death of tissues
associated with browning of leaves and flowers is known as blight. It also refers
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to a sudden death of a plant or its conspicuous parts, such as leaves, blossoms,
or twigs. The dead organ usually turns brown or black and may soon
disintegrate. Early and late blights of potato are good examples.
e) Damping-off: It is a condition in which the stem is attacked near the soil
surface. The affected portion becomes constricted and weak, incapable of
bearing the load of the upper portion. As a result seedlings topple down and die.
Damping-off of vegetable and ornamental seedlings are common examples.
f) Scald or scorch: Areas in the succulent organs of plants die and turn brown
due to effect of high temperature such as in sun scald of apple.
g) Rots: The affected tissues die, decompose to greater extent, and turn brown.
In most cases this condition is brought about by fungi and bacteria which
dissolve the cell walls more or less completely by means of enzymes. According
to the plant organ attacked the rot may be called root rot, leaf or stem rot, bud
rot, and fruit rot. Depending upon the type of dissolution brought about by the
pathogen the rots may be grouped as soft rot, wet rot, or dry rot.
13. Wilts: The leaves and other green or succulent parts lose their turgidity,
become flaccid and droop. This effect is usually seen first in some of the leaves.
Later the young growing tip or the whole plant may suddenly or gradually dry up.
Wilting may be the result of an injury to the root system, to the partial plugging of
water conducting vessels or to toxic substances secreted by the pathogen and
carried to delicate cells with water.
14. Die-back: Such diseases are characterized by dying of plant organs,
especially stem or branches from the tip backwards.
15. Gummosis: Excessive gum formation in certain trees affected by fungi
2.9. DISEASE MANAGEMENT
1. Use healthy, clean or disease-free planting materials
2. Grow resistant varieties
3. Provide better care and management
4. Follow crop rotation
5. Maintain proper field sanitation
6. Adjust sowing and planting dates
7. Rogue out diseased plants
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8. Follow plant quarantine regulations
9. Use chemicals if necessary
2.10. SOME IMPORTANT DEFINITIONS
Disease: Any harmful deviation or alteration from the normal functioning of
physiological processes.
Pathogen: The agent responsible for inciting pathos (ailment or suffering) or
damage. In other words it is the disease causing agent.
Pathogenesis: The chain of metabolic events that bring about the disease is
known as pathogenesis. It involves the action of the pathogen, susceptibility of
the plant and impact of the environment.
Pathogenicity: It is the ability of a pathogen to cause disease under a given set of
environments.
Virulence is a measure or degree of pathogenicity of an isolate or race of the
pathogen. Successful expression of pathogenicity is virulence. A pathogen is
avirulent when it fails to cause disease.
Inoculum: The part of the pathogen, which on contact can cause infection in host.
E.g. spores, conidia, sclerotia etc.
Predisposition: The environmental effects that make plants more amenable to
infection are called predisposition.
Host: The plant that is diseased is the host.
Parasite: Organisms which derive the materials they need for growth from living
plants are called parasites.
Saprophytes: Organisms which derive their nutrition from dead organic matter.
Immune: Immunity of a plant against a given disease is an absolute quality. It
denotes that the pathogen cannot establish a parasitic relationship with the host.
Infection: is establishment of parasitic relationship between the pathogen and the
host, following entry or penetration. Or in other words: it is the process by which
the pathogen establishes contact with the host cells and draws nutrients from the
host and involves growth and reproduction of the pathogen.
Incubation period: is the time lapsing between penetration and completion of
infection, that is , development of symptoms.
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Invasion and colonization: After infection the pathogen grows through the tissues
of the host to varying extents.
Symptom: A visible or otherwise detectable expression of abnormal physiology,
development, or behaviour in a plant resulting from disease. Symptoms often
involve changes in form, colour, odour, texture, and structural integrity. Or Sign
or evidence of disease or disorder as shown by plant or any objective evidence
of disease or bodily disorder is called symptom of the disease.
Sign: Any observable part or remnant of the causal agent (pathogen) in disease.
Common signs include vegetative or reproductive structures of pathogens such
as fungal mycelium or spores.
Syndrome: The set of varying symptoms characterizing a disease are collectively
called a syndrome. Or in other words: the sum total of all the symptoms and
signs is syndrome. For example, a disease may be characterized by necrosis of
tissues, hypertrophy, as well as wilting. All these symptoms will be collectively
called the disease syndrome.
REFERENCES:

Singh, R.S. (1983). Plant Diseases.Oxford and IBH, N. Delhi.

IAC, Wageningen, The Netherlands (1992). 21st International course on
IPM - Handouts on Mycology.

Bilgrami,K.S. & Dube, H.C. (1976). A Text Book of Modern Plant
Pathology. Vikas Publishing House.
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