disease develpopment

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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 DISEASE?

The term DISEASE is coined by combining the words DIS + EASE = DISEASE.

The prefix DIS means negative, reverse, oropposite, 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 PLANT 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

(Phytophthorainfestans) 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 unknownworld 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

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people died and one and a half million migrated. This tragic event was a blessing

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 fromone 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.

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6. Strong sun - yellowing and scorching of leaves, wilting and death.

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 hosts 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 funguslive 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 manybiochemical 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 producetogether 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 algaein 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: cerealcrop disease, rootcrop 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

apandemicdisease.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, thepathogen 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 cultivatedhost

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 expressessymptoms

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. Thesame 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 mediafor 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 hostss 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 pathogenwithin 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 threadenters 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 growthon 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. Tarspots: 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.

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10. Overgrowths orHypertrophy: 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

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 thecells.

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) Hairyroot: 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.

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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

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 orscorch: 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

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7. Rogue out diseased plants

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.

Incubationperiod: 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. OrSignor 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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