Download Final SY Pathology Intro Classi

H. N. G. U.
New Syllabus effective from June – 2009
S. Y. B. Sc Botany
Paper – III
Unit – III
PLANT PATHOLOGY
INTRODUCTION
The word pathology has been derived from two Greek words pathos - suffering and
logos - discussion. Plant pathology is therefore, a discussion on the suffering plant. Therefore
the ‘Plant Pathology’ is a branch of Botany deals with the various diseases or troubles in
plants as well as suggests the ways and means of their remedy. In this sense, the phytopathologists are the qualified plant doctors, who try to cure or control the plant diseases. It is
closely related with the study of fungi, bacteria, mycoplasma and viruses like pathogens.
Plant pathology has the following four main objectives: (i) etiology, (ii) pathogenesis, (iii)
epidemiology and (iv) control. Thus the field of plant pathology comprises both the art of
treating the sick plant and the science of understanding the nature of the diseased plant.
CLASSIFICATION OF PLANT DISEASES
The plant diseases may be classified in several ways:
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On the basis of the extent to which diseases are associated with plant, may be
classified into: 1. Localized diseases. These diseases are limited to a definite area of varying
extents of an organ, or only to a particular part of the plant and 2. Systemic diseases. In these
diseases pathogen spreads throughout the entire plant to varying extents and is associated
with almost every stage of plant's life cycle.
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Sometimes the diseases are grouped according to the host plant, e.g., diseases of
cereals; diseases of legumes and pulses; diseases of vegetables; diseases of cash crops;
diseases of fruit crops etc.
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In the other way, the diseases may be classified according to their symptoms, e.g.,
root rots, powdery mildews, downy mildews, rusts, smuts, wilts, leaf blights, hyperplastic
diseases, hypoplastic diseases, necrotic diseases, damping off diseases, canker diseases,
mosaic diseases, etc.
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The diseases also classified according to the parts of plants affected such as diseases
of roots, diseases of stems, diseases of leaves, diseases of flowers, diseases of fruits etc.
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Parasitic and viral diseases are often classified according to their occurrence, into
following categories; (l) endemic, (2) epidemic or epiphytotic and (3) sporadic.
(1) Endemic diseases
The endemic disease is more or less constantly present from year to year in a moderate to
severe form in a particular area. In such cases the pathogen or virus is well established and
survives from one crop season to the next in the soil, on crop shelter or in wild hosts. It is also
implied here that environmental conditions remain favorable for the development of
inoculum and for the infection and spread of disease. The common examples are - flag smut
of wheat, onion smut, cabbage yellows, club root of cabbage etc.
(2) Epidemic or epiphytotic diseases
The disease which commonly occurs widely but periodically is termed as epidemic. The term
epiphytotic has been coined for the plant diseases. The epiphytotic diseases are usually very
responsive to environmental conditions. Epiphytotic diseases occur where the environmental
conditions are favorable but pathogen occurs irregularly. The common examples are - Late
blight and early blight of potato are epiphytotic disease because of its sensitivity to
environmental factors such as temperature and humidity.
(3) Sporadic diseases
When the epiphytotic diseases occur at very irregular intervals and locations and in relatively
few instances, it is termed as sporadic. A particular disease may be endemic in one region and
epidemic in another.
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Plant diseases Classified According to Major Causal Agents under three heads, viz.
(1) Nonparasitic diseases; (2) Parasitic diseases and (3) Virus diseases.
In nonparasitic diseases, no living organism is involved and they are noninfectious, whereas
in parasitic diseases, the causal agent is a living body and the disease is infectious. The
diseases may be summed up as follows:
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The plant diseases are caused by several micro-organisms and other agencies.
According to the nature of causal agents, they may be grouped as follows: l. Diseases Caused
by Animate Pathogens, 2. Diseases Caused by Virus Pathogens and 3. Diseases Caused by
Inanimate Pathogens.
1. Animate Pathogens
(1) Diseases incited by the various classes of Fungi (Myxomycetes, Oomycetes,
Ascomycetes, Basidiomycetes and Deuteromycetes)
(2) Diseases incited by Bacteria
(3) Diseases incited by Mycoplasmas (MLO's)
(4) Diseases incited by Algae (e.g. Cephalewos an intercellular parasite in the leaves
of a Magnolia, Rhododendron, Camellia sinensis (tea) and Piper)
(5) Diseases incited by Phanerogams (e.g. Cuscuta, Orobanche, Viscum, Dendrophthoe
and Santalum)
(6) Diseases incited by Nematodes (e.g. animals of Phylum - Nemathelminthes)
2. Virus Pathogens
Diseases incited by various viruses.
3. Inanimate Pathogens
It includes the 'physiologic diseases' caused by low temperature effects; high
temperature effects; light effects; nutritional disorders; effects of atmospheric impurities etc.
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The most satisfactory way to classify plant diseases is on the basis of their cause. This
knowledge helps understanding the probable development and spread of the disease and also
devising methods of its control. Thus according to causes the diseases are classified as
Infectious (parasitic) diseases and Non-infectious (non-parasitic) disorders.
1. Infectious (parasitic) plant diseases
They are caused by living agents, the pathogens. They may spread by contact of a diseased
plant with healthy plants. All pathogens are parasitic on plants. On the basis of kind of
pathogen, these are further classified as follows:
- Diseases caused by fungi
- Diseases caused by bacteria
- Diseases caused by mycoplasma or mollicutes (phytoplasma, spiroplasma)
- Diseases caused by viruses and viroids
- Diseases caused by nematodes
- Diseases caused by insects
- Diseases caused by protozoa
- Diseases caused by parasitic higher plants
2. Non-infectious diseases (Physiological disorders)
These do not spread from plant to plant. These are caused due to improper conditions of soil,
air or mechanical injury. Common examples are tip rot or necrosis of mango fruits, tip burn
of paddy, black heart of potato etc. On the basis of nature of the cause, these are classified as
follows:
- Too low or too high temperature
- Lack or excess of soil moisture
- Lack or excess of light
- Lack of oxygen
- Air pollution
- Nutrient deficiencies
- Mineral toxicities
- Soil acidity or alkalinity (pH)
- Toxicity of pesticides
- Improper agricultural practices.