Different causing agents to cause plant diseases.

1. Pat 201 - Fundamentals of plant pathology (2+1)
Presented by,
K. Vignesh., M. Sc., (Ag.), Ph. D., DCA., DABM., PG – DOLG.,
Classification of plant diseases

2. Classification of Plant Diseases
Diseases may be classified in various ways on the basis of:
A. Based on cause;
B. Based on occurrence; and
C. Based on symptoms.
A. Based on Cause
On the basis of cause, the plant diseases are classified into two categories viz., Parasitic and Non-
Parasitic.
a. Parasitic Diseases
These are the diseases incited by biotic factors, i.e., other living organisms under a set of suitable
environmental conditions. Association of a specific pathogen is essential with these diseases. These diseases are
always infectious, sometimes contagious and are transmitted from diseased to healthy plants in the field and
from one place to another through various agencies.

3. Examples
 Fungi: Downy mildew of Grapevine – Plasmopara viticola
 Bacteria: Citrus Canker – Xanthomonas campestris pv. citri
 Viruses: Bhendi Yellow Vein Disease - Bhendi Yellow Vein begomovirus
 Viroids: Potato spindle tuber - Potato spindle tuber viroid
 Fastidious vascular bacteria (RLO's): Citrus greening - Liberobacter asiaticus
 Phytoplasma: Brinjal little leaf – Brinjal Little Leaf Phytoplasma
 Spiroplasmas: Citrus stubborn - Spiroplasma citri
 Protozoa: Hart-rot and sudden wilt of coconut - Phytomonas spp.
 Algae: Red rust on mango - Cephaleuros parasiticus
 Parasitic higher plants (phanerogams): Dodder - Cuscuta spp.

4. b. Non-parasitic Diseases
These are the diseases with which no parasite is associated; hence they are called as non-parasitic or
abiotic diseases. They remain non-infectious and cannot be transmitted from one diseased plant to another
healthy plant. These non-parasitic, non-infectious diseases are due to disturbances in the plant body caused by
lack of proper environmental conditions like suitable soil, air, temperatures, oxygen, moisture, pH, atmosphere
etc., are the major causes of non-parasitic diseases.
Examples
Low temperature: Potato tuber injury
High temperature: Blossom end rot of citrus fruit
Effect of light: Bean scald
Excessive moisture: Blossom end rot of tomato
Low oxygen: Black heart of potato
Air pollution: Ozone injury to onion, tomato and cucumber.
Chemical injury: Ammonia on apple

5. B. Based on Occurrence
Parasitic diseases are often classified according to their occurrence as epidemic, endemic, sporadic
and pandemic diseases.
a. Epidemic/Epiphytotic
The term epidemic is derived from Greek word ("epi-” = “ upon"; + "demos" = “people or
population" = "epidemos” = “upon the population") meaning “among the people" and in true sense applies
to those diseases of human beings which appear very virulently among a large section of population. To
carry the same sense in the case of plant diseases, the term epiphytotic has been coined. An epiphytotic
disease is the one which occurs widely but periodically. It may be present constantly in the locality but
assumes severe proportions only on occasions. This is because the environmental conditions favour rapid
development of the disease periodically. Thus, environmental factors are the major determining factors.
Ex: Downy mildew of Grapevine – Plasmopara viticola, Late blight of Potato – Phytophthora infestans

6. b. Endemic
The word endemic is also derived from Greek (en-= in; + dēmos= people) meaning "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 from year to year in a moderate to severe
form, in a particular country or part of the earth, it is called as endemic disease to that area.
Ex: Club root of cabbage - Plasmodiophora brassicae.
Wart disease of potato - Synchitrium endobioticum.
c. Sporadic
Sporadic disease is the one which occurs at very irregular intervals and locations and in relatively few
instances.
Ex: Angular leaf spot of cucumber - Pseudomonas lachrymans.
d. Pandemic
These occur all over the world across the continents and result in mass mortality of people.
Ex: Late blight of potato - Phytophthora infestans.

7. C. Based on Symptoms
Based on symptom, diseases are classified into 3 types
a) Necrotic disease
b) Hyperplastic / Hypertrophic disease
c) Hypoplastic / Hypotrophic disease
a. Necrotic Diseases
The diseases that involve the death of the tissue due to the derangement of cells are termed as Necrotic
diseases. Necrotic diseases involve decay or rotting.
The different necrotic diseases are:
i) Wilts: They are characterized by destruction or necrosis of vascular tissue resulting in dying of entire plant.
The plants suffer from dehydration and show the symptoms of drought.
Ex: Panama Wilt of banana - Fusarium oxysporium f.sp. cubense.
ii) Spots: The spots are self-limiting necrotic areas or the necrosis is local on leaf lamina, stem or fruit.
Ex: Chillies anthracnose - Collectotrichum caspcii.

8. iii) Blights: Blights are characterized by rapid and extensive necrosis i.e., quick death of cells, tissues or whole
plant or plant parts.
Ex: Late blight of potato - Phytopthora infestans.
iv) Rots: They are characterized by necrosis of parenchyma, collenchyma and pith tissues in which the necrosis
is not localized.
Ex: Bud rot of palms - Phytopthora palmivora ; Soft rot of vegetable - Erwinia caratovora
v) Damping Off: Necrosis of basal portion of stem or crown region of seedlings, resulting in toppling down of
seedlings or death of seedlings in groups.
Ex: Damping off of solanaceous plants - Pythium aphanidermatum.
vi) Canker: This canker is due to necrosis of woody tissue in which the infection is deep-seated and
hypertrophy of cambium tissues.
Ex: Citrus canker - Xanthomonas campestris pv. Citri; Guava fruit canker - Pestalotia psidi.
vii) Scab: It is also a necrosis which is superficial and restricted to epidermal region.
Ex: Citrus scab Elsinoe fawcettii; Apple scab Venturia inequalis.

9. b. Hyperplastic/Hypertrophic Diseases
They are characterized by over-growth of the tissue due to either hyperplasia or hypertrophy.
Hyperplasia: It is increase in number of cells due to excess cell division.
Ex: Crown gall of apple and pears - Agrobacterium tumefaciens
Club root of cabbage - Plasmodiophora brassicae
Hypertrophy: It is abnormal increase in size of cells.
Ex: Tumors: Wound tumor of clover - Wound tumor virus.
Wart: Wart of potato – Synchitrium endobiotium.
Galls: Stem gall of coriander - Protomyces macrosporus.
Woody gall: Woody gall of citrus – Viral disease.
Blisters: White rust of crucifers – Albugo candida.
White pine blister rust - Cronartium ribicola.
Witches' brooms: Potato witches' broom – Phytoplasma

10. c. Hypoplastic / Hypotrophic Diseases
They are characterized by under-development of tissues or stunting or dwarfing of plant due to
hypoplasty or hypotrophy.
Hypoplasia – Decrease in normal growth of the tissue due to reduced cell division
Ex: Citrus Tristeza disease - Citrus Tristeza clostero virus
Chrysanthemum stunt - Chrysanthemum stunt viroid.
Hypotrophy - Decrease in normal growth of the tissue due to size reduced cell
Ex: Aster yellows – Phytoplasma
- Downy mildew of Grapevine – Plasmopara viticola

11. Groups of Plant Pathogens
 Common biotic causes of diseases in plants belongs to Fungi, Bacteria, Viruses, Viroids, Algae,
Fastidious vascular bacteria (RLO’s), Phytoplasma (MLO’s), Spiroplasma, Protozoa, Nematodes and
Phanerogamic parasites.
 The biotic agents are categorized into prokaryotes and eukaryotes based on the nuclear
organization and other physiological processes involved in their growth and development.
 while bacteria, phytoplasma etc., belong to prokaryotes
 Fungi, nematodes, algae and phanerogamic parasites come under eukaryote.

12. Fungi
 Eukaryotic, spore-bearing, achlorophyllous, generally reproduce sexually and asexually, filamentous
branched somatic structures, typically surrounded by cell walss containing chitin or cellulose or both,
saprophytic, pathogenic or symbionts absorptive nutrition.
 Powdery mildew - Erisiphe polygoni ; Blast – Pyricularia oryzae

13. Bacteria:
 Prokaryotic, rigid, unicellular, free of true chlorophyll, generally devoid of any photosynthetic pigment,
most commonly multiplying asexually by simple transverse fission.

14. Fastidious vascular bacteria (Rickettssia like organism, RLO’s):
 Similar to bacteria in most respects but cannot be grown on routine bacteriological media and needs
complex medium for its growth.
 These are typical Gram-negative bacteria. These are fastidious in their nutritional requirements, refusing to
grow on routine bacteriological media. They have a cell wall unlike MLO and spiroplasma.
 MLO is restricted to phloem tissues where as RLB are restricted mostly to xylem or phloem.
 They reproduce by binary fission. Mostly insect vectors transmit them.
 Nematode (Xiphinema index) also helps in transmission of RLB (yellow disease of grapevine). Mechanical
inoculations (as in Pierce's disease of grapevine, almond leaf scorch and alfalfa dwarf) or vegetative
propagation also reproduce disease symptoms.
 Penicillin is effective against RLB.
 The RLB can be divided into three groups.
1. Xylem-limited RLB
2. Phloem-limited RLB
3. Non-tissue restricted RLB

15. Phytoplasma (MLO’s) (Non-helical mollicutes):
 Prokaryotic, pleomorphic, wall-less prokaryotic micro-organisms, infect plants and cannot yet to be
grown in culture.
 They lack cell wall, are bounded by a unit membrane.
 They are pleomorphic.
 They lack cell wall.
 They have fried egg appearance of colony.
 They have both DNA and RNA.
 They cannot be grown on artificial media.
 They are mostly transmitted by leafhoppers.
 They are insensitive to penicillin and sensitive to tetracycline.
e.g. Phyllody of sesame, little leaf of brinjal

16. Spiroplasma (Helical mollicutes):
 Prokaryotic, helical, wall-less micro-organisms, present in phloem of diseased plants, often helical in
culture, thought to be a kind of mycoplasma, can be cultured on artificial medium
 They are insensitive to penicillin and sensitive to erythrocin and tetracycline.
e.g. corn stunt, citrus stubborn

17. Virus/Virion:
 Sub-microscopic, potentially pathogenic obligate parasites, consisting of mainly nuclic acid and
protein, multiplies only intra- cellularlly.
 They have only one type of nucleic acid viz., either RNA or DNA.
 Most of the plant virus is having RNA. E.g. TMV.
 Few viruses contain DNA.
E.g. Cauliflower mosaic virus, Banana bunchy top virus

18. Viroid (Nuked nucleic acid or virus without protein coat):
 Smallest known plant pathogens with low molecular with low molecular weight ribonucleic acid
(ssRNA), without protein coat, replicate themselves, cause disease only in plants.
 They are also called as mini viruses
E.g. Potato spindle tuber, Chrysanthemum stunt, Coconut cadang cadang

19. Protozoa:
 Microscopic, non-photosynthetic, eukaryotic, flagellate, motile, single-celled animals.
 Eg. Club root of cabbage (Plasmodiophora brassicae)

20. Algae (Few algae mainly green algae):
 Eukaryotic, photosynthetic, uni- or multi-cellular organisms, containing chlorophyll, causing plant
diseases.
 Many algae thrive as terrestrial or subterranean algae.
 The size of algae ranges from 1.0mm to many cm in length.
 They contain chlorophyll and are photosynthetic.
 They reproduce by asexual and sexual processes.
 The study of algae is called phycology or algalogy
 F. E. Fritsch - father

21. Phanerogamic parasites:
 These flowering parasitic plants damage the host plants through exhaustion of nutrients and sometimes
through restriction in growth of the plant. Few of them produce toxins.
 Phanerogamic parasites are flowering plants or seed plants, which lead a parasitic life on other living
plants. Some of these parasites possess chlorophyll, which manufacture carbohydrates to a limited extent
and depend on the host for mineral, salts and water. These are generally called as semi or partial
parasites.
Partial stem – Loranthus ; Total stem - Cuscuta
 Some of the parasites, which do not have chlorophyll, depend entirely on the host plants for their food
materials. They are called holo or total parasites.
Partial root – Striga ; Total root - Orabanche

22. Loranthus

23. Cuscuta
Striga
Orabanche

24. Koch Postulates:
Robert Koch (1890), a german scientist propose a three set of rules to demonstrate the
association of a microbe with the disease in a host. Later in 1905 E. F. Smith appended fourth step
to the Koch’s rules and these rules are now universally accepted and popularly known as Koch’s
postulates.
1. Association:
The pathogen must show constant association with the disease. The microorganism must be
found in the host suffering from the disease, but should not be found in healthy host.
Limitations:
Host become asymptomatic carriers to the pathogens due to their immunological status,
physiological status, genetic variability etc.
2. Successful isolation:
The microorganism must be isolated from the diseased host and grown in pure culture
Limitations:
Certain microorganisms are uncultivable/obligate/can’t be grown in pure culture

25. 3. Inoculation:
The microorganism from the artificial culture should be able to reproduce the disease when inoculated on
healthy plant of the same kind. The symptoms produced should be identical with those seen on the plant from which
isolation was mode.
4. Successful re-isolation:
The artificially produced disease should yield the same microorganisms on re-isolation.
Limitations:
Postulates cannot be fulfilled with viruses/human and animal pathogens.