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1. Lecture 3 & 4:
Causes of Plant diseases – biotic and
abiotic- Biotic factors- fungi – bacteria,
virus, viroid, and Candidatus
Phytoplasma

2.  Plant disease- Any abnormal condition that
alters the appearance or function of a plant.
 Broken tulips –Viral infection
Component of plant disease:
1. Pathogen- study of pathogen & susceptible
host called etiology.
All the pathogens are parasite but all the
parasites are not pathogen
2. Host
3. Environment

3.  Disease triangle: Interaction between host,
Pathogen & environment
 4th factor “Time” - Disease tetrahedron
 5th factor “Human” - Pyramid
 Structure given by Stevens &Van der plank

4. Symptom:
 Visible effect of pathogen called symptom
 Morphological sym. – Externally visible
 Histological sym. - microscopic examination /
detection of diseased tissue
 Characteristic series of symptoms or sum total of
symptoms exhibited during the disease
development called Syndrome
 Visible structure of the pathogen produced in or
on the diseased tissues is called sign

5. 4 major basic symptoms:
 Colour change
 Necrosis
 Hypertrophy-increase in size of the individual cell
 Hyperplasia-increase in number of cell
 Hypoplasia - reduced development of the whole
plant, parts of the plant, flowers or fruits.
 Atrophy – the organ or tissue doesn’t develop

6.  Causes of plant diseases:
 Any agent that makes the plant to suffer is called
cause or pathogen
 Till 1850, they were classified mainly on the basis
of the effect. After 1850, diseases were classified
on the basis of their cause rather than effect

7. Classification of plant diseases on the basis of
effect of the diseases
1.Based on type of infection:
 Localized – limited to a definite area of a organ or
only to a particular part of a plant eg., leaf spot,
anthracnose
 Systemic – pathogen spreads from a single
infection point so as to infect all or most of the host
tissuses eg; smut, downy mildew in pearl millet

8. 2.On the basis of natural perpetuation and mode of
spread
 Soil borne – root rot of pulses
 Air borne – Tikka leaf spot of groundnut
 Seed borne – Loose smut of wheat
 Vector borne – RTD by green leaf hoppers
3. On the basis of symptoms
 Rust, Smuts, Rots, Blight
 Leaf spot, Canker, Wilt, DM,PM

9. 4.On the basis of host plants
 Cereal diseases
 Vegetable diseases
 Fruit diseases
 Forest diseases
 Ornamental diseases
5.On the basis of organ they attack
 Root diseases
 Shoot diseases
 Fruit diseases
 Foliage diseases

10. 6. On the basis of occurrence and distribution
 Endemic: when a disease more or less constantly present
throughout the year in a moderate to severe form in a
particular country. Eg., wart disease of potato-Darjeeling
Club root of cabbage- Nilgiris
 Epidemic or epiphytotic: sudden outbreak of a disease
periodically. It may be constantly present in locality but
assume severe form only on occasions. Eg., wheat stem Rust,
Late blight of potato, sugarcane red rot, downy mildew &
brown spot in rice
 A disease endemic in one area may be epidemic in another
area

11.  Sporadic: diseases which occur at very irregular
interval and location in a moderate to severe
form Fg., udbatta disease of rice, wilt, wheat
ergot
 Pandemic: diseases occurring throughout the
continent or sub-continent resulting in mass
mortality eg., late blight of potato & wheat
stem rust

12. 7. Based on pathogen generations
 Simple interest / Monocyclic diseases
Those diseases the increase of which isanalogous
of increase in money due to simple interest i.e
those diseases which have only one generation in
one cropping season eg. Loose smut of wheat
 Represent by sigmoid curve
 Compound interest/polycyclic diseases:
those diseases which have more than one
generation in a cropping season eg: rust, mildew,
late blight of potato
 Represent by starvation curve

13.  Polyetic diseases:
Polycyclic diseases but they complete their
disease cycle in more than one year over a
years Eg., cedar apple rust

14. On the basis of Causes (Pathogen identity)
 1. Animate or biotic causes: Pathogens of
living nature
 (i) Fungi, Bacteria, Phytoplasma, Rickettsia-
like organisms
 Algae, Phanerogams, Protozoa, Nematodes
 Phanerogamic parasites

15.  Inanimate or abiotic causes
 (i) Deficiencies or excess of nutrients
 e.g. ‘Khaira’ disease of rice, white bud in maize due to Zn
deficiency
 Whiptail of cauliflower- Mb defci.
 Coconut pencil point- MN defi.
 Blossom end rot- Ca defici.
 Pahala blight of sugarcane- Mn defi.)
 (ii) Light
 (iii) Moisture – Damping off, Root rot
 (iv) Temperature – High temp. tip burn, heat canker;
Low: Mango malformation

16. Cont…
 (v) Air pollutants (e.g. black tip of mango)
 (vi) Lack of oxygen (e.g. hollow and black heart of potato)
 (vii) Toxicity of pesticides
 (viii) Improper cultural practices
 (ix) Abnormality in soil conditions (acidity,-- Club root;
alkalinity- Scab)

17. Fungi
1. All are eukaryotic
(Possess membrane-bound nuclei (containing chromosomes)
and a range of membrane-bound cytoplasmic organelles (e.g.
mitochondria, vacuoles, endoplasmic reticulum).
2. Most of them are filamentous
3. Some are unicellular E.g. Yeasts
4. Cell surrounded by rigid cell wall, made up of chitin and
glucans (some contain cellulose).
5. Many reproduce both sexually and asexually

18. Fungal diseases
 Powdery mildew
 Downy mildew
 Club root
 Damping off
 Leaf spot
 Leaf blight
 Scab
 Rust
 Smut
 Root rot
 Wilt

19. Bacteria
• Microscopic, unicellular, prokaryotic organism occur in
single or in colonies
• Possess rigid cell wall which is made up of peptidoglycan
and lipopolysaccharides
• Mostly heterotrophic and some autotrophic
• Reproduce mostly by asexual method (binnary fission).
• Sexual reproduction is lacking.
• Motile bacteria possess one or more flagella.
• Bacteria are named by shaped viz., cocci (ball-shaped),
bacillus (rod-shaped) and spirilli (spiral-shaped).

20. Bacterial diseases
 Spot Blights Galls
Canker Wilt

21. Virus
The word ‘Virus’ means slimy liquid, poison, venom or
infectious matter.
Viruses are ultramicroscopic entities (which can pass through
bacterial filters), which are infectious agents and obligate parasitic
pathogens.

22. • They are nucleoproteins (nucleic acid and protein) and
devoid of enzymes and depend on the hosts protein
synthesizing machinery.
• They have only one type of nucleic acid, either DNA
(deoxy ribose nucleic acid or RNA (ribonucleic acid).
• Most of the plant virus is having RNA e. g., TMV
• Few viruses contain DNA. E.g., Cauliflower mosaic
virus, banana bunchy top virus.

23. Diseases
 Mosaic Vein clearing
 Bunchy top ring spot


24. Candidatus Phytoplasma
• They lack cell wall, are bounded by a triple layered membrane and contain
cytoplasm, ribosomes and strands of nuclear material.
•They are pleomorphic.
•A group of very small bacteria like organism intermediate between virus
and bacteria
•They have both DNA and RNA.
•They are mostly transmitted by leafhoppers.
•They are insensitive to penicillin and sensitive to tetracycline.
•E.g. phyllody of sesame, little leaf of brinjal, sugarcane grassy shoot

25. Diseases
 Gingelly phyllody Little leaf

26. VIROIDS
Viroids are small naked, circular low molecular weight ssRNA without coat protein
capable of causing diseases in plants. They are pathogenic RNAs. These 'mini
viruses' are the smallest known agents of infectious diseases. They are subviral in
size.
The term 'viroid' was first introduced by Diener in 1971.
Symptoms: Potato spindle tuber, Coconut cadang, citrus exocortis
Spindle tuber
Cadang cadang

27. Fastidious vascular bacteria
[Rickettsia-like bacteria (RLB) /Rickettsia-like
organism (RLO)]
• They are small bacteria with a cellular ultrastructure of 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
• They reproduce by binary fission.
• Mostly insect vectors transmit them. Nematode (Xiphinema index) also
helps in transmission (yellow disease of grapevine).
•

28. 1. Xylem-limited FVB
The FVB causing these diseases is named as Xylella fastidiosa.
In general xylem-limited Gram-negative bacteria have elongated cells.
They are susceptible to tetracycline but not to penicillin.
Transmission mostly through xylem feeding insects. E.g., Pierce's
disease of grapevine.
2. Phloem-limited FVB
They are mostly rigid rods and Gram-negative and sensitive to
penicillin. Transmission is by leafhoppers, dodder and grafting. e.g.,
Citrus greening is transmitted by citrus psyllid.

29. Spiroplasma
Spiroplasma is helical, wall-less
prokaryotes requiring cholesterol for
growth and cause diseases in plants,
insects and rats.
They are insensitive to penicillin and
sensitive to erythrocin and
tetracycline.
E.g. corn stunt, citrus stubborn.

30. Diseases
 Citrus stuborn corn stunt Spiroplasma

31. Algae
•Algae are eukaryotic, unicellular or multicellular organisms and
mostly occur in aquatic environments
•They contain chlorophyll and are photosynthetic.
•Orange colour is due to the presence of Chematochrome
•The study of algae is called phycology or algology.
Red rust- Cephaleuros virescens

32. Phanerogamic parasites
Phanerogamic parasites are flowering plants or seed plants, which lead
a parasitic life on other living plants.
The phanerogamic parasites invade stem or root of the host 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.
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.

33. Phanerogamic parasites
 They may be total or partial and it may be
root parasite or stem parasites. They lack true
root system and possess haustoria
Cuscuta/dodder
Loranthus Striga Orabanche
Total stem Partial stem
Partial root Total root

34. Iatrogenic diseases
 The effect of plant protection chemicals which
resulted in either the appearance of new diseases
or increases in the diseases already present.
 Eg; zineb controlling downy mildew in grapes but
increase the incidence of powdery mildew
 Distortion in papaya and cotton due to 2,4 D injury

35. Robert Koch (1884-1890
 1. Recognition/ association: The pathogens
must be found associated with the disease in
the diseased plant. The symptom of the disease
should be recorded
 2. Isolation: The pathogen should be isolated,
grown in pure culture in artificial media. The
cultural characteristics of the pathogen should
be noted.

36. Robert Koch (1884-1890) Koch’s Postulates
 3. Inoculation: The pathogen of pure culture must
be inoculated on healthy plant of same
species/variety. It must be able to reproduce
disease symptoms on the inoculated plant
identical to step 1.
 4.Re-isolation: The pathogen must be isolated
form the inoculated plant in culture media. Its
cultural characteristics should be similar to those
noted in step 2 (This step was added by E.F.
Smith).

37. Thank you