2. DISEASE
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.
3. 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.
4. 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 (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.
5. CAUSES OF PLANT DISEASE
A) NON-INFECTIOUS DISEASE/ ABIOTIC DISEASE- Caused by
natural agencies
B) INFECTIOUS DISEASE/ BIOTIC 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.
A) NON-INFECTIOUS PLANT DISEASE: Possible causes include:
1. Weather
2. Nutrient deficiency
3. Toxic substances
6. 1) THE WEATHER:
I. Lightning - can scorch plants, and break and burn old shrubs
and trees.
II. Rain/hail stone - heavy rain cause physical damage to young
plants, and hail stones cause flower and fruit drops.
III. 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)
IV. Drought - causes yellowing, leaf curling, wilt and eventual death
of plants.
V. Flooding - causes yellowing, slow development and death
through lack of air at roots.
VI. Strong sun - yellowing and scorching of leaves, wilting and
7. 2) NUTRIENT DEFICIENCIES
• Deficiency of nutrients can cause symptoms like yellowing, reddening, spotting,
stunting, distorted growth and death. However, symptoms vary with the element
involved.
2) TOXIC SUBSTANCES IN THE SOIL OR AIR
• Too much elements required by plants can cause death, with similar symptoms to
deficiencies.
B) Infectious plant disease: These can be caused by:
I. fungi
II. bacteria
III. viruses
IV. nematodes
V. algae
8. CLASSIFICATION OF PLANT DISEASES
Plant diseases can be classified in various ways depending upon
the purpose of discussion
1. Based on plant part affected:
I. localized disease- affecting only a part of the plant;
II. 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.
9. 4. Based on their occurrence:
I. 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.
II) 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
10. III) 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
11. 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.
12.
13. 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
14. 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. (Infection
chain- chain of events leading to the completion of pathogenesis)
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 host soil 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
15. 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.
16. 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.
17. 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.
18. 4. Scab:
The term scab refers to a roughened or crust-like lesions or
to a freckled (a small, pale brown spot on the skin, usually
on the face, especially of a person with pale skin )
appearance of the diseased organ.
In some diseases of this type the parasite appears at a
certain stage, in others it is never seen.
19. 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
20. 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.
21. 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 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.
22. 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
B) streaks or stripes
C) Canker
D) Blight
E) Damping-off
F) Scald or Scorch
23. 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.
24. 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.
25. 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.
26. 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.
27. 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.
28. 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.
29. 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
30. DISEASE MANAGEMENT
• Use healthy, clean or disease-free planting materials
• Grow resistant varieties
• Provide better care and management
• Follow crop rotation
• Maintain proper field sanitation
• Adjust sowing and planting dates
• Rogue out diseased plants
• Follow plant quarantine regulations
• Use chemicals if necessary
