Lecture 1. Plant diseases – Abiotic , biotic diseases, classification based on mode of
infection, multiplication of inoculum, spread, symptoms, occurrence and
distribution
Plant Pathology also known as Phytopathology is a branch of agricultural, biological or
botanical science which deals with the study of diseases in plants - their causes, etiology,
resulting losses and control.
Objectives of plant Pathology
To study living, non-living and environmental causes of diseases or disorders of the
plants
To study the mechanism of plant disease development
To study interaction between host and the pathogen
To develop systems of management of plant diseases and reducing losses caused
by them
Disease: Disease may be defined as a malfunctioning process that is caused by continuous
irritation by a pathogen and/or environmental factor resulting in some suffering producing
symptoms. The diseases caused by the deficiency of nutrients or unfavorable environments are
sometimes termed as disorders or physiological disorders. The diseased plants does not
function or look normal showing structural abnormality and / or physiological disorder and can
not grow, develop and reproduce to its genetic potential.
Plant disease management: Any measure that reduces the disease is called plant disease
management. The goal of plant disease management is to reduce the economic and aesthetic
damage caused by plant diseases.
Causes of plant diseases
1. Biotic diseases
Infectious (biotic) diseases are caused by pathogen which could spread from diseased to
healthy plants. The pathogens are
i. Protozoa: Protozoa are a diverse group of unicellular eukaryotic organisms, many of
which are motile. Protozoa are defined as unicellular protists with animal-like
behaviour, such as movement. Eg. Club root of cabbage – Plasmodiophora brassicae
ii. Fungi: Fungi are microscopic, non chlorophyllous, belonging to lower most ring in the
ladder of plant kingdom. They are unicellular or multicellular filamenous bodies
(thallus) which reproduce by division of vegetative cells or by well defined asexual or
sexual spores. Eg. Root rot – Marcophomina phaseolina.
, iii. Bacteria: Bacterium (pl. bacteria) is a unicellular, microscopic, prokaryotic (lacking
true nucleus) organism, which lacks chlorophyll and multiplies by fission. Eg. Bacterial
leaf blight of rice – Xanthomonas oryzae pv. oryzae.
iv. Virus: Viruses are ultramicroscopic disease producing entities with no metabolic
enzymes which contain only one type of nucleic acid (either DNA or RNA and never
both), obligate parasite of varying shape and small enough to pass through bacterial
filters. Eg. Banana bunchy top – Banana bunchy top virus
v. Viroid: Viroids are the circular single stranded RNA without protein coat. Eg. Potato
spindle tuber – Potato spindle tuber viroid.
vi. Candidatus Phytoplasma: Phytoplasms are small, unicellular, gram-positive non-
motile bacteria like prokaryotes intermediate between viruses and bacteria. They differ
from true bacteria in the absence of cell wall. Eg. Gingelly phyllody
vii. Spiroplasma: Spiroplasmas are spherical to slightly ovoid mollicutes. The first known
spiroplasma is com stunt (Spiroplasma kunkelii) and spiroplasma first cultured in vitro
is citrus stubborn (S. citri).
viii. Algae: Algae are eukaryotic, unicellular or multicellular, chlorophyllous and
photosynthetic organisms and mostly occur in aquatic environments. Eg. Red rust –
Cephaleuros virescens
ix. Phanerogamic parasites: Phanerogamic parasites are flowering plants or seed
plants, which lead a parasitic life on other living plants. Eg. Partial root parasite –
Striga spp.
2. Abiotic diseases or Non-parasitic diseases
The abiotic diseases are caused by abiotic factors like deficiency of nutrients, adverse
environmental conditions etc. They are also called as non-infectious or physiological
disorders. When no pathogen is found, cultured from or transmitted from a diseased plant,
then the disease is said to be caused by a non-living or environmental factor. These diseases
occur because of disturbances in the plant system by the improper environmental conditions
in the air or soil or by mechanical influences. They are listed below.
i. Too low or too high temperature
ii. Lack or excess of soil moisture
iii. Lack or excess of light
iv. Lack of oxygen
v. Air pollution (Toxic gases in the atmosphere etc.)
vi. Mineral deficiencies or toxicities
vii. Soil acidity or alkalinity
viii. Toxicity of pesticides
ix. Improper agricultural practices.
,Classification of Plant Diseases
Plant diseases are classified differently based on different criteria as follows:
i. Based on Plant part affected; they are classified as localized if they affect only
specific organs or parts of the plants. They may be systemic if entire plant is affected
or they can be classified as root diseases, stem diseases or foliage/foliar diseases.
ii. Based on perpetuation and spread or mode of infection, they can be classified as
soil borne (e.g. Fusarium wilt, Dry root rot caused by Macrophomina phaseolina etc) or
seed borne (e. g. Loose smut of wheat caused by Ustilago nuda tritici), when the
pathogen perpetuates through the agency of soil or seed (or any propagation material).
They can be referred to as air borne when they are disseminated by wind e.g. rusts
and powdery mildews.
iii. According to the signs and symptoms produced by the pathogens, diseases are
classified as rust, smuts, powdery mildews, downy mildews, root rot, wilt, blight, canker,
fruit rot, leaf spot etc. In all these examples the diseases are named after the most
conspicuous symptom of the disease appearing on the host surface.
iv. According to the host plants affected; they can be grouped as cereal crop diseases,
forage crop diseases, millet diseases, plantation crop diseases, fruit crop diseases,
vegetable crop diseases or flowering plant diseases etc.
v. Plant diseases may be classified according to the major causes as fungal diseases,
bacterial diseases, viral diseases, phytoplasma diseases etc.
vi. They may be classified as infectious or non-infectious. All the diseases caused by
animate causes, viruses and viroids can be transmitted from infected host plants to the
healthy plants and are called infectious. Non-infectious diseases cannot be transmitted
to a healthy plant. They are also referred as non-parasitic disorders or simply
physiological disorders and are incited by abiotic or inanimate causes like nutrient
deficiency or excess or unfavorable weather conditions of soil and air or injurious
mechanical influences.
vii. Based on pathogen generations / multiplication of inoculums, the plant diseases are
classified into Simple interest / monocyclic diseases or Compound interest/ polycyclic
diseases. Those diseases the increase of which is analogous of increase in money due
to simple interest i. e. those diseases which have only generation in one cropping
season. E. g. loose smut of wheat are known as simple interest / monocyclic diseases.
In those diseases, which have more than one generation in a cropping season. Eg. Late
, blight of potato Polyetic diseases these are also polycyclic diseases but they complete
their disease cycle in more than one year over years e.g.Cedar Apple Rust.
viii.Diseases caused by animate/biotic and virus or virus like pathogens are often classified
in relation to their occurrence as endemic , epidemic, sporadic or pandemic diseases.
a. Endemic diseases are those which are more or less constantly present from year to year
in a moderate to severe form in a particular geographic region i.e. county, district or
location. Eg. Wheat rust, Rice blast.
b. Epidemic or epiphytotic diseases are those which occur widely but periodically
particularly in a severe form. They might be occurring in the locality every year but
assume severe form only on occasions due to the favourable environmental conditions
occurring in some years. Eg. Brown spot of rice (Bengal famine)
c. Sporadic diseases occur at irregular intervals and locations and in relatively few
instances. Eg. Bacterial leaf blight of rice
d. Pandemic diseases: A disease may be endemic in one region and epidemic in another.
When epiphytotics become prevalent throughout a country, continent or the world, the
disease may be termed as pandemic. Eg. Late blight of potato.
Lecture 2. Epidemiology – Disease triangle/ Disease Pyramid - Role of weather factors
in plant disease development. Boom and bust cycle in disease outbreak
Epidemiology or Epiphytology deals with the outbreaks and spread of disease in a
population. It is the study of rate of multiplication of a pathogen which determines its capacity to
spread a disease in a plant population. It is the most important part of the study of plant
diseases from practical point of view. Epidemiologically, the diseases have been described as i)
Simple interest diseases and ii) Compound interest diseases.
1. Simple interest diseases
In these diseases, increase is mathematically analogous to the simple interest in money.
There is only one generation of the pathogen in the life of the infected crop. The primary
inoculum is seed or soil borne and secondary infection is rare. All the infections noticed in the
field are from the pre-existing inoculum in the soil. Most important example is loose smut of
wheat (U. tritici), where the inoculum is internally seed borne or is carried in the seed resulting in
the infected ears in the season showing black powdery loose mass in place of grains. In wilt and
root rot diseases, the primary inoculum is important and there are very remote chances of
secondary spread, even if the pathogen sporulates due to the soil barriers and other factors.
infection, multiplication of inoculum, spread, symptoms, occurrence and
distribution
Plant Pathology also known as Phytopathology is a branch of agricultural, biological or
botanical science which deals with the study of diseases in plants - their causes, etiology,
resulting losses and control.
Objectives of plant Pathology
To study living, non-living and environmental causes of diseases or disorders of the
plants
To study the mechanism of plant disease development
To study interaction between host and the pathogen
To develop systems of management of plant diseases and reducing losses caused
by them
Disease: Disease may be defined as a malfunctioning process that is caused by continuous
irritation by a pathogen and/or environmental factor resulting in some suffering producing
symptoms. The diseases caused by the deficiency of nutrients or unfavorable environments are
sometimes termed as disorders or physiological disorders. The diseased plants does not
function or look normal showing structural abnormality and / or physiological disorder and can
not grow, develop and reproduce to its genetic potential.
Plant disease management: Any measure that reduces the disease is called plant disease
management. The goal of plant disease management is to reduce the economic and aesthetic
damage caused by plant diseases.
Causes of plant diseases
1. Biotic diseases
Infectious (biotic) diseases are caused by pathogen which could spread from diseased to
healthy plants. The pathogens are
i. Protozoa: Protozoa are a diverse group of unicellular eukaryotic organisms, many of
which are motile. Protozoa are defined as unicellular protists with animal-like
behaviour, such as movement. Eg. Club root of cabbage – Plasmodiophora brassicae
ii. Fungi: Fungi are microscopic, non chlorophyllous, belonging to lower most ring in the
ladder of plant kingdom. They are unicellular or multicellular filamenous bodies
(thallus) which reproduce by division of vegetative cells or by well defined asexual or
sexual spores. Eg. Root rot – Marcophomina phaseolina.
, iii. Bacteria: Bacterium (pl. bacteria) is a unicellular, microscopic, prokaryotic (lacking
true nucleus) organism, which lacks chlorophyll and multiplies by fission. Eg. Bacterial
leaf blight of rice – Xanthomonas oryzae pv. oryzae.
iv. Virus: Viruses are ultramicroscopic disease producing entities with no metabolic
enzymes which contain only one type of nucleic acid (either DNA or RNA and never
both), obligate parasite of varying shape and small enough to pass through bacterial
filters. Eg. Banana bunchy top – Banana bunchy top virus
v. Viroid: Viroids are the circular single stranded RNA without protein coat. Eg. Potato
spindle tuber – Potato spindle tuber viroid.
vi. Candidatus Phytoplasma: Phytoplasms are small, unicellular, gram-positive non-
motile bacteria like prokaryotes intermediate between viruses and bacteria. They differ
from true bacteria in the absence of cell wall. Eg. Gingelly phyllody
vii. Spiroplasma: Spiroplasmas are spherical to slightly ovoid mollicutes. The first known
spiroplasma is com stunt (Spiroplasma kunkelii) and spiroplasma first cultured in vitro
is citrus stubborn (S. citri).
viii. Algae: Algae are eukaryotic, unicellular or multicellular, chlorophyllous and
photosynthetic organisms and mostly occur in aquatic environments. Eg. Red rust –
Cephaleuros virescens
ix. Phanerogamic parasites: Phanerogamic parasites are flowering plants or seed
plants, which lead a parasitic life on other living plants. Eg. Partial root parasite –
Striga spp.
2. Abiotic diseases or Non-parasitic diseases
The abiotic diseases are caused by abiotic factors like deficiency of nutrients, adverse
environmental conditions etc. They are also called as non-infectious or physiological
disorders. When no pathogen is found, cultured from or transmitted from a diseased plant,
then the disease is said to be caused by a non-living or environmental factor. These diseases
occur because of disturbances in the plant system by the improper environmental conditions
in the air or soil or by mechanical influences. They are listed below.
i. Too low or too high temperature
ii. Lack or excess of soil moisture
iii. Lack or excess of light
iv. Lack of oxygen
v. Air pollution (Toxic gases in the atmosphere etc.)
vi. Mineral deficiencies or toxicities
vii. Soil acidity or alkalinity
viii. Toxicity of pesticides
ix. Improper agricultural practices.
,Classification of Plant Diseases
Plant diseases are classified differently based on different criteria as follows:
i. Based on Plant part affected; they are classified as localized if they affect only
specific organs or parts of the plants. They may be systemic if entire plant is affected
or they can be classified as root diseases, stem diseases or foliage/foliar diseases.
ii. Based on perpetuation and spread or mode of infection, they can be classified as
soil borne (e.g. Fusarium wilt, Dry root rot caused by Macrophomina phaseolina etc) or
seed borne (e. g. Loose smut of wheat caused by Ustilago nuda tritici), when the
pathogen perpetuates through the agency of soil or seed (or any propagation material).
They can be referred to as air borne when they are disseminated by wind e.g. rusts
and powdery mildews.
iii. According to the signs and symptoms produced by the pathogens, diseases are
classified as rust, smuts, powdery mildews, downy mildews, root rot, wilt, blight, canker,
fruit rot, leaf spot etc. In all these examples the diseases are named after the most
conspicuous symptom of the disease appearing on the host surface.
iv. According to the host plants affected; they can be grouped as cereal crop diseases,
forage crop diseases, millet diseases, plantation crop diseases, fruit crop diseases,
vegetable crop diseases or flowering plant diseases etc.
v. Plant diseases may be classified according to the major causes as fungal diseases,
bacterial diseases, viral diseases, phytoplasma diseases etc.
vi. They may be classified as infectious or non-infectious. All the diseases caused by
animate causes, viruses and viroids can be transmitted from infected host plants to the
healthy plants and are called infectious. Non-infectious diseases cannot be transmitted
to a healthy plant. They are also referred as non-parasitic disorders or simply
physiological disorders and are incited by abiotic or inanimate causes like nutrient
deficiency or excess or unfavorable weather conditions of soil and air or injurious
mechanical influences.
vii. Based on pathogen generations / multiplication of inoculums, the plant diseases are
classified into Simple interest / monocyclic diseases or Compound interest/ polycyclic
diseases. Those diseases the increase of which is analogous of increase in money due
to simple interest i. e. those diseases which have only generation in one cropping
season. E. g. loose smut of wheat are known as simple interest / monocyclic diseases.
In those diseases, which have more than one generation in a cropping season. Eg. Late
, blight of potato Polyetic diseases these are also polycyclic diseases but they complete
their disease cycle in more than one year over years e.g.Cedar Apple Rust.
viii.Diseases caused by animate/biotic and virus or virus like pathogens are often classified
in relation to their occurrence as endemic , epidemic, sporadic or pandemic diseases.
a. Endemic diseases are those which are more or less constantly present from year to year
in a moderate to severe form in a particular geographic region i.e. county, district or
location. Eg. Wheat rust, Rice blast.
b. Epidemic or epiphytotic diseases are those which occur widely but periodically
particularly in a severe form. They might be occurring in the locality every year but
assume severe form only on occasions due to the favourable environmental conditions
occurring in some years. Eg. Brown spot of rice (Bengal famine)
c. Sporadic diseases occur at irregular intervals and locations and in relatively few
instances. Eg. Bacterial leaf blight of rice
d. Pandemic diseases: A disease may be endemic in one region and epidemic in another.
When epiphytotics become prevalent throughout a country, continent or the world, the
disease may be termed as pandemic. Eg. Late blight of potato.
Lecture 2. Epidemiology – Disease triangle/ Disease Pyramid - Role of weather factors
in plant disease development. Boom and bust cycle in disease outbreak
Epidemiology or Epiphytology deals with the outbreaks and spread of disease in a
population. It is the study of rate of multiplication of a pathogen which determines its capacity to
spread a disease in a plant population. It is the most important part of the study of plant
diseases from practical point of view. Epidemiologically, the diseases have been described as i)
Simple interest diseases and ii) Compound interest diseases.
1. Simple interest diseases
In these diseases, increase is mathematically analogous to the simple interest in money.
There is only one generation of the pathogen in the life of the infected crop. The primary
inoculum is seed or soil borne and secondary infection is rare. All the infections noticed in the
field are from the pre-existing inoculum in the soil. Most important example is loose smut of
wheat (U. tritici), where the inoculum is internally seed borne or is carried in the seed resulting in
the infected ears in the season showing black powdery loose mass in place of grains. In wilt and
root rot diseases, the primary inoculum is important and there are very remote chances of
secondary spread, even if the pathogen sporulates due to the soil barriers and other factors.
