1. Causes of Plant Diseases
A. Parasites
i. Biotic agents/ Animate agents/ Infectious/ Parasitic
1. Prokaryotes
a. Bacteria
b. Fastidious Vascular Bacteria
c. Candidatus Phytoplasma (MLO/ Phytoplasma)
d. Spiroplasma
2. Eukaryotes
a. Protozoa
b. Chromista
b. Fungi
c. Algae
d. Parasitic flowering plants or phanerogamic parasites
e. Animals
ii. Mesobiotic agents
1. Viruses
2. Viroids
B. Non-parasitic or Abiotic agents/ Non-infectious/
physiological disorder
2. Bacteria
Bacteria are microscopic, unicellular prokaryotes, which lack
definitely organized nucleus and chlorophyll.
These microorganisms are with a primitive nucleus lacking a
clearly defined membrane. Majority of them are saprophytes
The bacteria are smaller than fungi and measure about
0.5 to 1.0 x 2.0 to 5.0 µm in size. In general, they vary from
1.0 to 5.0 microns in different forms.
Each bacterial cell is surrounded by a definite cell wall or
membrane, which is nitrogenous in chemical nature rather than
carbon compound (cellulose in fungi and higher plants).
3. Bacteria
Bacteria have been defined by Clifton as "extremely minute,
rigid, essentially unicellular organisms, free of true chlorophyll and
generally devoid of any photosynthetic pigments; most commonly
multiplying asexually by simple transverse fission, the resulting cells
being of equal or nearly equal size".
4. • More than 1,600 bacterial species are known. Of which about
200 spp. cause diseases in plants and are mostly rod-shaped
• Either non-motile or motile by means of one or more flagella on
their body arranged in a specific manner.
• They multiply mostly by binary fission.
• Most bacteria attacking plants are Gram-negative except the
genera, Streptomyces and Clavibacter (Corynebacterium)
• These are passive pathogens (cannot directly penetrate into the
host but need natural openings or wounds for their entry).
• First report of plant bacterial disease was made by T. J. Burrill
(Fire blight of apple and pear, Erwinia amylovora).
• Bacteria cause extensive and destructive plant diseases.
e.g. soft rot, leaf spot, blight, cankers, galls, tumours, wilt, etc.
5. Characteristics of bacteria
• Microscopic, unicellular, prokaryotic organism occur in single or
in colonies
• Possess rigid cell wall which is made up of peptidoglycan and
lipopolysaccharides
• Absence of well defined nucleus
• Mostly heterotrophic and some autotrophic
• Reproduce mostly by asexual method (binary fission).
• Sexual reproduction is lacking.
• Motile bacteria possess one or more flagella.
• Ribosomes are scattered in the cytoplasm
• Bacteria are named by shaped viz., cocci (ball-shaped), bacillus
(rod-shaped) and spirilli (spiral-shaped).
6. Fastidious vascular bacteria
[Rickettsia-like bacteria (RLB) /Rickettsia-like organism (RLO)]
• They are smaller 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 are rod shaped bacteria without flagella and are bounded
by a cell membrane and cell wall unlike MLO and
spiroplasma. The size of rod shaped cells varies from
0.2-0.5 µm dia with 1-4 µm long.
• They reproduce by binary fission.
7. • Mostly insect vectors transmit them.
Nematode (Xiphinema index) also helps in
transmission (yellow disease of grapevine).
• Mechanical inoculations (as in Pierce's disease of
grapevine) or vegetative propagation also reproduce
disease symptoms.
• Penicillin is effective against FVB.
• They may be confined to the phloem tissues or xylem
tissues of the host plants.
8. The FVB can be divided into three groups.
1. Xylem-limited FVB
The FVB causing these diseases is named as Xylella fastidiosa.
In general xylem-limited Gram-negative bacteria have elongated
cells of 0.2 to 0.5 into 1.4 m size. 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.
3. Non-tissue restricted FVB (e.g., Yellows of grapevine and
necrosis of grapevine)
9. Candidatus Phytoplasma
• They are very small, filterable, unicellular, normally non-motile,
wall-less prokaryote (mollicutes – mollis means soft or pliable
and cutis means skin) distinct from true bacteria in its
morphology, mode of reproduction, biochemistry, physiology,
phylogeny and taxonomy
•They are pleomorphic (varied shapes).
•They lack rigid cell wall, are bounded by a triple layered membrane
and contain cytoplasm, ribosomes and strands of nuclear
material.
•They are generally confined to sieve elements of the phloem and in
some cases in the cortical parenchymatous tissues
10. •They have fried egg appearance of colony (non-pathogenic type)
•They have both DNA and RNA.
•They do not have flagella, no spores and are Gram-negative.
•They produce symptoms like little leaf, phyllody, spike, yellows,
stunting, witches’ broom etc.
•They are mostly transmitted by leafhoppers.
•Multiplication of phytoplasma occur both in vectors and in
host plants.
•They are insensitive to penicillin and sensitive to tetracycline.
11. Spiroplasma
Spiroplasma are helical/ filamentous, wall-less prokaryotes
lacking rigid cell wall but bounded by unit membrane with
RNA granules (ribosomes) and DNA strand in the nuclear
region.
They are restricted to phloem region, requiring cholesterol for
growth and cause diseases in plants, insects and rats.
They are Gram positive, facultative anaerobic.
They are insensitive to penicillin and sensitive to erythrocin
and tetracycline.
12. Their motility is in a corkscrew pattern.
It can be cultured on artificial media.
Their major site of existence is either
phloem in plants or hemolymph of insect
vectors.
The average size of cells ranges from 2-4
µm long with a width of 120 nm.
E.g. corn stunt, citrus stubborn.
13. Flagellate Protozoa
Protozoa derived from two Greek words
viz., protos and zoan, which means first animal.
They are the smallest among the animal kingdom.
They are mostly single celled and vary in shape.
They produce spores.
Flagellate protozoa attack several crop plants.
e.g., Phloem necrosis of coffee, Hart rot of coconut.
14. Chromista
• The name Chromista means "coloured", and although some
chromists, like mildews (downy mildews) are colourless,
most are photosynthetic.
• It comprises of single celled or multicelled, eukaryotic, walled
microorganisms that produce heterokont, wall-less cells in
their life cycles.
• The members are photosynthetic organelle (plastid) which
contain chlorophyll c and do not store their energy in the
form of starch, like plants or even other algae.
• Chromists have closer phylogenetic relationship with brown
algae and diatoms than true fungi belong to kingdom Fungi
E.g., downy mildews and damping off pathogens.
15. Fungi
Fungus (pl. fungi) is a Latin word which means “mushroom”.
Fungi are microscopic, eukaryotic, heterotrophic,
achlorophyllous, nucleated, uni or multicellular organisms that
may reproduce sexually and asexually and whose filamentous
branched somatic structures are typically surrounded by cell walls
containing chitin or cellulose.
The branch of biology that deals with fungi is called Mycology
(Gr. myke= mushroom + logos= study of)
16. Fungi
Fungi are eukaryotic, achlorophyllous, unicellular or multicellular
heterotrophic organisms which obtain nutrients by absorption and
reproduce by sexual or asexual spores.
17. Characteristics of 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, golgi bodies, etc. ).
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. 6. Their nuclei are typically haploid and hyphal compartments are
often multinucleate
7. All are achlorophyllous
8. All are chemoheterotrophic (chemo-organotrophic)
(They utilise pre-existing organic sources of carbon in their
environment and the energy from chemical reactions to
sythesize the organic compounds they require for growth
and energy)
9. Possess characteristic range of storage compounds
E.g., trehalose, glycogen, sugar alcohols and lipids.
10. May be free-living or may form intimate relationships with
other organisms i.e. free-living, parasitic or mutualistic.
19. Algae
Algae are eukaryotic, unicellular or multicellular organisms and
mostly occur in aquatic environments. Many algae thrive
as terrestrial or subterranean algae.
The size of algae ranges from 1.0 mm to many cm in length.
They are pigmented and vary in colour. They possess chlorophyll
and are photosynthetic.
They reproduce by asexual and sexual processes.
The study of algae is called phycology or algology.
E.g., Red rust of tea, mango, guava, citrus and Sapota –
Cephaleuros parasiticus
20. 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.
22. Character Prokaryotes Eukaryotes
Cellular
organization
Primitive Advanced
Size Smaller sized cells 1-2 x 1-5µm
or less
Larger than prokaryotes (More
than 5 µm in width or dia
Cell wall Peptidoglycans Cellulose
Nucleus and nuclear
membrane
No organized nucleus. Nuclear
membrane and nucleolus are
absent
Possess organized nucleus.
Nuclear membrane and nucleolus
are present
Membrane bound
cell organelles
Membrane bound cell organelles
such as endoplasmic reticulum,
golgi complex, mitochondria,
chloroplasts amd vacuoles are
absent.
Membrane bound cell organelles
such as endoplasmic reticulum,
golgi complex, mitochondria,
chloroplasts amd vacuoles are
present.
Ribosomes Smaller and made of 70S units Larger and made of 80S units
Number of
chromosomes per
nucleus
One More than one
Chromosome
replication by
mitosis
No Yes
Genetic material
(DNA)
DNA is not found in well
organized chromosome. Found in
nucleoids and in extra nuclear
plasmids and neither of which are
membrane bound. DNA is
shorter, circular and not histone
(protein) bound.
DNA is found in well organized
chromosome. DNA is long and
linear, histone bound.
Genetic
recombination
Unidirectional transfer of DNA
forms partial diploids
Fusion of gametes forms diploids
that segregate by meiosis
Cell division Only by fission (amitosis) By mitosis and meiosis
Flagella Single fibrillar type Bundle of nine fused pairs of
microtubule surrounding two
central single microtubules (9+2)
23. Virus
The word ‘Virus’ means slimy liquid, poison, venom or
infectious matter.
Viruses are ultramicroscopic, nucleoprotein entities, which are
infectious agents and obligate parasitic pathogens.
They are less than 200 m in size (pass through bacterial filters)
24. They are nucleoproteins (nucleic acid and protein) and devoid of
enzymes and depend on the hosts protein synthesizing machinery
(ribosomes) for multiplication.
They neither divide nor produce any structures like spores, but
multiply by stimulating host cells to replicate more viruses.
They have either DNA or RNA . Most of the plant virus is having
RNA e. g., TMV. Few viruses contain DNA. E.g., Cauliflower
mosaic virus, banana bunchy top virus.
Viruses are considered as mesobiotic agents (in between living
non-living) as their biological status is not clear.
25. Viruses are living because
• They have ability to assimilate (metabolism) with the release of
energy
• They are able to multiply
• They exhibit response to environment like temperature,
chemicals, etc.,
• They have genetic materials like RNA or DNA
• They have the ability to infect
26. Viruses are non-living because
• Viruses can be crystallized.
• Stanley (1935) considered virus as a molecule and a molecule is
not capable of self-replication
• Viruses are inert outside the living host
• Viruses do not have cell wall or cell membrane of any type
• They do not show functional autonomy
• They do not respire or excrete
• They lack any energy producing system.
27. Viroids
They are ultramicroscopic, small low molecular weight
ribonucleic acids without protein sheath that can infect plant
cells replicate them and cause disease.
They are also called as mini viruses (without protein coat).
E. g., Potato spindle tuber
Chrysanthemum stunt
Coconut cadang cadang
Citrus exocortis
28. Abiotic disorders
a. Unfavourable temperature (Low temperature, high temperature)
b. Unfavourable light
c. Unfavourable soil moisture
d. Relative humidity
e. Unfavourable oxygen relations
f. Atmospheric impurities (Sulphur dioxide, nitrogen dioxide,
ozone, ethylene, etc.)
g. Toxic effects of decomposing organic matter
h. Nutritional disorders
Excessive minerals - internal bark necrosis in red delicious apple;
Deficiency of magnesium (Red leaf in cotton),
Zinc (Khaira disease in rice, Foliocellosis in citrus),
Molybdenum (whip tail of cauliflower),
Calcium (blossom end rot in tomato),
Micronutrients (coconut pencil point disease).
