2. Disease comprises of two terms viz., ‘dis-ease’ (uneasiness),
which is self explanatory. Disease is a malfunctioning process
that is caused by continuous irritation, which results in suffering
producing symptoms in the plant.
It is a condition in which the functions of the organism are
improperly discharged or it is a state which is physiologically
abnormal and threatens the life of the being or organ (H. M.
Ward, 1896)
It from normal physiological activity which is sufficiently
permanent or extensive to check the performance of natural
functions by the plant or completion of its developmis a variation
ent (E. J. Butler, 1918)
3. Disease is a deviation from normal functioning of physiological
processes of sufficient duration or intensity to cause disturbance
or cessations of vital activities (American Phytopathological
Society, 1940)
A plant is said to be ‘diseased’ when there is a harmful
deviation from normal functioning of physiological process
(Federation of British Plant Pathologists, 1973).
It is a physiological disorder or structural abnormality that is
deleterious to the plant/ to any of its parts/ products that reduces
their economic value (Stakmann and Harrar, 1957)
It is the abnormal changes in physiological processes which
disturb the normal activity of the plant organ (J. Kuhn, 1958)
4. Diease Epidemics
• Quantitative and qualitative loss
• Scarcity of produce
• Affect agriculture based industries and transport industry
• Excessive usage of chemicals - environmental hazards,
mammalian toxicity,
residual effects affecting human/ animal health.
• More expenditure incurred on plant protection measures
• In addition to direct yield loss and monetary returns, many
plant diseases affect the society in various ways.
• Escalation of prices which leads to economic threat.
• Instability in farm income reflects in overall national income.
5. Significance of major plant diseases
Potato late blight: Irish famine (1845).
First, the millions of people died out of starvation as potato was
the staple food for them.
Second, the complete destruction of potato crop by Phytophthora
infestans continuously for two years in 1845 and 1846.
Approximately 1.5 million people migrated between 1846 and
1850, which represented 25% of the total population.
6. Black (Stem) rust of wheat (Puccinia graminis tritici)
It caused havoc in wheat growing countries of Europe when it
appeared in epidemic form. It damaged more than 20% of U.S.
wheat crops several times between 1917 and 1935.
In India (Madhya Pradesh), wheat stem rust appeared in epidemic
form in 1946-47 resulted in food shortage and low seed supply.
7. Brown spot of Rice: Bengal famine in 1943 because of the
occurrence of leaf spot caused by Helminthosporium oryzae.
In 1942, weather conditions were highly favourable to encourage
an epidemic of rice brown spot following a cyclone and flooding.
The outbreak caused severe reduction (one-third) in yield which
resulted in death toll of about 3.5 million people. This disease also
appeared in epidemic form in Bihar in 1963.
8. Bacterial Leaf Blight of Rice:
In 1951, it appeared in a severe
form in Maharashtra, India.
This disease is reported to have
reduced Asia’s annual rice
production by as much as 60%.
9. Coffee Leaf rust: Before 1860, Sri Lanka was topper in coffee
production. Coffee rust caused by Hemileia vastatrix was first
noticed in 1860s in Sri Lanka and it slowly invaded all the
plantations and affected the entire industry. By 1893, the export
of coffee was reduced by 93%. These area were later replaced by
tea plantation.
10. Sigatoka disease of
Banana:
The entire banana
industry in South and
Central America during
1900s was threatened by
Sigatoka leaf spot of
banana caused by
Mycospherella musicola.
11. Downy mildew of grapevine: The wine industry in France was
under threat during 1878-82 because of a fungus, Plasmopara
viticola, which was introduced from USA. Nearly 2.5 million
acres of vines were affected.
To control this disease, Bordeaux mixture was discovered
accidently by Prof. P. M. A. Millardet of Bordeaux University.
12. Downy mildew of cereals
Downy mildew of sorghum (Peronosclerospora sorghi) – epidemic
(1968) in Karnataka, Maharashtra, Rajasthan and Tamil Nadu –
caused 30-70% crop loss.
Philippine downy mildew of maize (Sclerospora philippinensis)
caused a crop loss of 60% in Bihar, Punjab and Delhi.
Downy mildew of bajra (Sclerospora graminicola) - epidemic form
in 1975 in Karnataka and Maharastra with almost 100% yield loss.
Brown stripe downy mildew of maize (Sclerophthora rayssiae var.
zeae) occurred in severe form in mid-1960s in about 13 states out of
20 states in India.
13. Corn Southern Leaf blight: It is one of the most devastating
epidemics occurred in USA during 1969-70 due to the infection by
Helminthosporium maydis.
Chestnut Blight: The fungus, Endothia parasitica devasted the
chestnut trees in USA in 1904. As a result, heavy loss in wood
industry occurred. The tannin industry dependent on this wood was
also severely affected.
14. Red rot of Sugarcane: Epidemic form in Godavari delta of
Andhra Pradesh during 1895-1900.
During 1938-39, the sugarcane growers of Bihar, Punjab and UP
suffered heavy loss due to red rot, Physalospora tucumanensis
(Colletotrichum falcatum). Most of the sugar factories could
crush only one third of the normal quantity.
15. Lethal yellowing in coconut: During 1900, the lethal
yellowing of coconut palms occurred in severe form and
devastated coconut plantation in Cuba and because of this
reason coconut was replaced by sugarcane. It also spread to
other islands where coconut is a major crop.
16. Bunchy top of banana: This viral disease was first observed in
Hawaii in 1889. It caused severe damage to bananas in Fiji islands,
Pacific islands, Queensland, New South Wales regions. During
1921, it appeared in epiphytotic form in New South Wales and
Queensland in Australia.
In India, it wiped out nearly 1.5 lakh acres of banana in Sindh,
Punjab during 1989-91, which had created an urgent need for
introduction of citrus to replace banana.
17. Storage diseases: Wheat, corn, sorghum
and other small grains are generally
infected with various species of mould
fungi viz., Aspergillus, Penicillium,
Cladosporium, Cochliobolus, Curvularia,
Dreschslera, Fusarium, Colletotrichum,
etc. that remain dormant in the grain at
low moisture level.
In case of ergot disease, it produces
mycotoxins that can be fatal when
consumed.
The loss due to post harvest diseases of
fruits and vegetables during storage and
transit ranges as high as 40-50% with an
average loss of 24%. Apart from
quantitative loss it also affect the
commodities qualitatively.
18. Diseases of Forest trees: Several types of pathogens attack
important forest trees and lead to huge loss in various bye-
products for which these trees are known for.
Forests have a great significance in human welfare by their
contributions including fuel, timber, fodder, prevention of soil
erosion, conservation of water, soil and wild life, etc.
Occurrence of root and stem diseases, foliage diseases leads to
staining and rotting of wood and wood products which result in
heavy losses.
19. Components of disease: Three important components of the plant
disease are the pathogen, host and the environment.
Study of causal agent of the disease and its relation with host is
known as etiology.
Host is the organism or plant that harbours or supports the activities
of a pathogen.
Temperature, moisture, wind, rainfall, relative humidity, soil pH, etc.
are the environmental factors that affect disease development.
The host should be susceptible; the pathogen should be virulent and
the environment must be favourable to cause the disease. These
three factors constitute the traditional Disease triangle.
The morphological (external) or physiological (internal) reactions or
alterations as a result of disease are referred as symptom. These
symptoms are collectively called as syndrome. Visible structure of
the pathogen produced in or on the diseased tissues is called as sign.
20. Classification of plant diseases
1. Type of infection
a. Localized diseases: These diseases
are limited to a definite area of an organ
or part(s) of a plant. e.g. leaf spots and
anthracnose caused by fungi.
b. Systemic diseases: In these diseases
the pathogen spreads from a single
infection point to infect all or most of
the host tissues. e.g. Downy mildews
caused by fungi and mosaics and leaf
curls caused by viruses.
21. 2. Type of perpetuation and spread
a. Soil-borne diseases
e.g. Damping off caused by fungi Pythium spp. and
root rot caused by Rhizoctonia spp.
b. Seed-borne diseases
e.g. Loose smut of wheat caused by Ustilago nuda
tritici (internally seed-borne) and blast of rice
caused by Magnoporthe grisea (externally seed-
borne).
c. Air-borne diseases
e.g. Early leaf spot and late leaf spot of groundnut
caused by Cercospora arachidicola and
Phaeoisariopsis personata respectively.
d. Vector-borne diseases
Many of the virus and phytoplasma diseases are
transmitted through insect vectors. e.g., Yellow
mosaic diseases of pulses by whiteflies.
22. 3. Extent of occurrence and geographic distribution
a. Endemic diseases: It is also known as enphytotic disease. When a
disease is more or less constantly occurring year after year in a moderate
to severe form in a locality then it is called as an endemic disease.
E.g. Wart disease of potato caused by
Synchytrium endobioticum in Darjeeling
Citrus canker in Asia and Sorghum rust in India
b. Epidemic or epiphytotic diseases: An epidemic or epiphytotic refers
to sudden outbreak of a disease periodically over a widespread area in a
devastatingly severe form causing extensive losses or complete
destruction.
E.g. Late blight of potato (Phytophthora infestans)
Wheat stem rust (Puccinia graminis tritici)
Sugarcane red rot (Physalospora tucumanensis),
Downy mildew of grapevine (Plasmopara viticola)
Rice blast (Pyricularia oryzae)
23. c. Sporadic diseases: Sporadic diseases are those, which occur at
irregular intervals over limited areas or locations.
E.g. Fusarium wilt of cotton
(Fusarium oxysporum f.sp. vasinfectum),
Wheat loose smut (Ustilago nuda tritici)
Grain smut of sorghum
d. Pandemic diseases: A disease is said to be pandemic when it is
prevalent throughout the country, continent or world involving mass
mortality.
E.g. Late blight of potato and wheat stem rust.
24. 4. Multiplication of inoculum
a. In simple interest disease (monocyclic epidemics) the disease
increase is like simple interest in money. Amount of inoculum for a
given season's crop is fixed. So there is no repetition of the disease
cycle within the crop season. Hence the disease spread will be
slow.
e.g. Soil pathogens (Pythium , Rhizoctonia , Sclerotium , etc.)
b. In compound interest disease (polycyclic epidemics) the
disease increase is like compound interest in money. Inoculum gets
multiplied several times during crop growth in a season. So the
disease spread is fast.
e.g. Wheat stem rust, rice blast, powdery mildew, downy mildew
25. 5. Based on the host plants
Diseases are classified based the host plants viz.,
Cereal crops diseases,
Pulse crops diseases,
Oilseed crops diseases,
Fiber crops diseases,
Sugarcane diseases,
Fruit crops diseases,
Vegetable crops diseases,
Plantation crops diseases,
Spice and condiment crops diseases,
Medicinal crops diseases,
Aromatic crops diseases,
Flower crops diseases
26. Damping off
Wilt
Root rot
Stem rot
Leaf spot
Leaf blight
Rust
Downy mildew
Powdery mildew
Anthracnose
Smut
Bunt
Fruit rot
Canker
Soft rot
Tumours and
galls
Scab
Leaf spot
Blight
Wilt
6. Based on kinds of symptoms
Mosaic
Yellow mosaic
Leaf curl
Leaf crinkle
Leaf roll
Rosette
Bud blight
Vein clearing
Chlorosis
Streak
Necrosis
Dwarf
Stunt
Sterility
Wilt
27. 7. Based on plant parts affected
Root diseases (Dry root rot of pulses)
Stem diseases (Stem rot of paddy)
Foliar diseases (Leaf spot, Leaf blight diseases)
Fruit diseases (Jack fruit rot)
Seed diseases (Loose smut of wheat)
28. 8. Based on causal agents
A. Parasites
i. Biotic agents: These are also called as animate or living organisms.
1. Prokaryotes
a. Bacteria e.g. Citrus canker.
b. Fastidious Vascular Bacteria
e.g. Citrus greening, Pierce's disease of grapevine
c. Candidatus Phytoplasma (MLO/ Phytoplasma)
e.g. Little leaf of brinjal
d. Spiroplasma e.g. Corn stunt, Citrus stubborn
29. 2. Eukaryotes (organisms with membrane bound true nuclei)
i. Fungi e.g. wilt of banana
ii. Protozoa e.g. hart rot of coconut
iii. Algae e.g. red rust of mango
iv. Parasitic flowering plants or phanerogamic parasites
e.g., Cuscuta
v. Animals e.g. Nematodes
ii. Mesobiotic agents: They are infectious agents. They can be
crystallized and are considered non-living. But their multiplication
in the living plants ensures that they are living.
1. Viruses e.g. tobacco mosaic
2. Viroids e.g. spindle tuber of potato
30. B. Non-parasites or Abiotic agents: These are also called as non-infectious or
physiological disorders.
• Too low or too high temperature (Freezing injury in potato and sweet potato;
heat canker of flax, tip burn in betelvine)
• Lack or excess of soil moisture
• Lack or excess of light
• Lack of oxygen (Black heart of potato) and reduced supply of oxygen to the
roots (Ukra/ Pansukh – dry leaf disease/ tip burn in paddy)
• Air pollution (Toxic gases in the atmosphere- Apple scald or black tip in
mango)
• Lightning injury (Damage to potato plants, coconut trees, etc.)
• Mineral deficiencies or toxicities (Excessive minerals leads to internal bark
necrosis in red delicious apple; deficiency of magnesium (Red leaf in
cotton), zinc (Khaira disease of rice, white bud in maize, Foliocellosis
in citrus), molybdenum (whip tail of cauliflower), calcium (blossom
end rot in tomato), micronutrients (coconut pencil point disease).
• Soil acidity or alkalinity
• Toxicity of pesticides
• Improper agricultural practices
