2. Virus (Latin word) - slimy liquid, poison, venom or infectious matter
Luria et al.(1978) defined viruses as ‘entities’ whose genomes are
elements of nucleic acid that replicate inside living cells (host
ribosomes)
Viruses are sub-microscopic entities (which can pass through bacterial
filters), which are infectious agents and obligate parasitic pathogens.
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 or RNA
Viruses are considered as mesobiotic agents .
3.
4. 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
Viruses are non-living because
Viruses are crystallized. Viruses can be purified and concentrated from
the extract of infected cells and show uniform size, shape and
chemical composition even after crystallization by chemical
treatments.
Viruses do not have cell wall or cell membrane of any type
They do not respire or excrete
5. Bacteria Viruses
Possess cellular organization Do not possess cellular organization
Grow on inanimate media Do not grow on inanimate media
Multiply by binary fission Do not multiply by binary fission
Posses RNA and DNA Do not posses RNA and DNA
Posses ribosomes Do not posses ribosomes
Sensitive to antibiotics Insensitive to antibiotics
Insensitive to interferon Sensitive to interferon
Differences between bacteria and viruses
6. MORPHOLOGY
Shape and size: Plant viruses are smaller than bacteria and are seen
only with electron microscope. Generally viruses are smaller than
bacterial viruses and are measured in nanometers or millimicrons
or millimues (1 nm = 0.001m or 1 x 10-6 mm or 1 x 10-9 m)
Plant viruses have different shapes and sizes.
They are:
1.Elongated - Tobacco mosaic virus (TMV)
a. rod-shaped - Barely stripe mosaic virus
b. flexuous threads- Potato virus x, citrus tristeza virus
2. Rhabdovirus - Potato yellow dwarf virus
3. Spherical - Tobacco necrosis satellite virus
a. Polyhedral - Wound tumour virus
b. Flexible - Tomato spotted wilt virus
11. COMPOSITION AND STRUCTURE
Two Major components - nucleic acid and protein coat or capsid.
Viral nucleic acid: Cellular organism posses both DNA (chromosomal nucleic
acid) and RNA (present in cytoplasm). Viruses are not cellular organisms.
Nucleic acid is always present in the infective particle (virus) and serves as the
genetic apparatus of the virus. It is the infective component of virus.
In any virus only one type of nucleic acid and either RNA (Ribovirus) or DNA
(Deoxyviruses) is present.
A majority of animal and bacterial viruses (bacteriophages) are DNA viruses
whereas majority of plant viruses are RNA viruses.
About 80 plant viruses containing DNA have been found out. Among the plant
viruses cauliflower mosaic virus, Rice Tungro Bacilliform Virus (RTBV),
Banana Bunchy top viruses are DNA viruses.
13. VIRAL PROTEIN
Virus proteins like all proteins consist of different
amino acids. The proteinaceous sheath consists smaller
units called capsomers.
Each unit is composed of amino acids. The sequence of
amino acids within a protein is dictated by the genome of
genetic material,
Virion or virus and nucleocapsid
A complete, infective form of a virus outside a host cell,
with a core of RNA /DNA and a capsid.
In membrane bound viruses, the inner nucleoprotein core
is called nucleocapsids.
15. Envelope virus:
Some viruses (e.g. HIV and many animal viruses)
have viral envelopes covering their protective
protein capsids.
The envelopes are typically derived from portions of the
host cell membranes (phospholipids and proteins), but
include some viral glycoproteins.
They may help viruses avoid the host immune system.
16.
17.
18.
19. Virus infection
Virus enters into the plant cell through wounds made mechanically
or by vectors or naturally by deposition into an ovule by an infected
pollen grain. Once the virus enters inside the host, it becomes naked
by shedding its capsid or coat.
Movement of virus inside the plants
When a virus infects a plant, it moves from one cell to another and
multiples in most, if not all, such cells. Viruses move from cell to
cell through the plasmodesmata connecting adjacent cell. Viruses
multiply in each parenchyma cell they infect.
20. A movement protein is a non structural protein which is encoded by some plant viruses to
allow their movement from one infected cell to neighbouring cells.
21. There are a group of genetic RNAs, whose size is very small
ranging from 240 to 375 ntds long, naked (i.e. not covered or
enclosed by any capsid proteins, infectious and pathogenic agents,
are called VIROIDS.
There are another type of genetic RNAs, incapable of replicating
by themselves and reside in other viruses and depend upon them
for their replication. They can cause diseases; they are called
VIRUSOIDS or SATELLITE RNAs.
E.g.,
Eg: Barley yellow dwarf virus satellite RNA and a helper
Luleovirm: Tobacco ring spot virus satellite RNA and a helper
Nepovirus
22. Prion:
There is another class of infectious agents, causing serious
diseases, which are neither DNA or nor RNA, but
conformationally changed proteins, which can propagate by
inducing conformational changes in other proteins of the same
kind and they are called PRIONS (proteinaceous infectious
particle).
Eg. Mad Cow diseases in animals
23. A conformational change is a change in the shape of
a macromolecule, often induced by environmental factors
Normally protein linear strand to three dimensional folding.
Prion: Misfolded protein
Protein before and after folding
Normal Microscopic "holes" are characteristic in prion-
affected tissue sections, causing the tissue to
develop a "spongy" architecture.
24. Satellites are subviral agents that differ from Viroids
because they depend on the presence of a helper virus for
their propagation.
Satellite viruses are particles that contain nucleic acid
genomes encoding a structural protein that encapsidates
the satellite genome.
Eg: Tobacco necrosis satellite virus (sTNV), which has
a small piece of ssRNA which codes only for a capsid
protein, and depends for its replication on the presence
of TNV .
Satellite RNAs do not encode capsid protein, but are
packaged by a protein encoded in the helper virus
genome.
27. Transmission of plant viruses
The viruses are infectious and capable of being transmitted from
diseased to susceptible healthy plants.
The viruses may be transmitted in the following methods.
• Transmission through seeds or seed materials.
• Transmission by budding or grafting
• Transmission by sap or mechanical transmission
• Transmission by insects
• Transmission by mites
• Transmission by fungi
• Transmission by nematodes
• Transmission by phanerogamic parasitic
28. Transmission through seeds or seed materials
Seed: Urdbean leaf crinkle virus, tobacco ring spot, TMV on tomato,
soyabean mosaic on soyabean
Seed material: Potato viruses, sugarcane mosaic, Citrus Trizteza
Virus, Cassava mosaic and Banana bunchy top.
Budding or grafting
Most of the viruses have been transmitted either by budding or
grafting. Only grafting can transmit viruses like Potato witches’
broom.
Sap or mechanical transmission
Generally mosaic group of viruses are readily transmitted by sap
inoculation. e.g. TMV
29. Transmission through insects
More than 80 per cent of the viral diseases are spread by different types
of insects. The insect, which act as specific carriers in disseminating the
diseases, are called insect vector
Aphids: Aphids are the most important insect vectors of plant viruses
and transmit the great majority of all stylet-borne viruses.
Aphis gossypii Bean mosaic virus, cucumber mosaic virus
A. maydis Sugarcane mosaic virus
Pentalonia nigronervosa Banana bunchy top,
Katte disease of cardamom
Toxoptera citricides Citrus tristeza virus
Myzus persicae PVY
30. Leafhoppers: Leafhoppers are phloem feeders and acquire the virus
from the phloem region. All leafhopper transmitted viruses are
circulatory. Several of these viruses multiply in the vector (propagative)
and some persist through the moult and are transmitted through the egg
stage of the vector.
Rice dwarf – Nephotettix cincticeps
RTV – N. virescens, N. nigropictus
Nilaparvata lugens - Rice grassy stunt, Rice ragged stunt
Cicadulina bipunctata, C. cinai - Maize streak, Ragi mottle streak
32. Grasshoppers Potato virus X, TMV
Lady bird beetle
Epilachna sp. Cowpea mosaic
Epicanta vitata (blister beetle) Bean pod mottle
Transmission by Mites
Aceria cajani PPSMV
Eriophagus insidiosus Peach mosaic
Transmission by fungi
Olpidium brassicae – Tobacco necrosis
Spongospora subterranean – Potato mob top
Synchytrium endobioticum – potato virus
Nematode transmission
Xiphinema, Longidorus, Trichodorus - Both polyhedral & tubular
viruses
33. Transmission by parasitic phanerogams
When other methods of transmissions like grafting sap
inoculation, etc. are not possible this method comes in handy.
The parasitic dodder Cuscuta californica, C. subinclusa,
C. campestris have been made use of.
The virus passes through food material
drawn by the parasite and reaches the
healthy plants and infects them.
e.g., Sugar beet curly top and
cucumber mosaic virus.
34. Virus -vector relationship
Persistent viruses (Circulative)
Semi-persistent virues
Non-persistent viruses
Latent period: The period from acquisition of virus by the vector
till the time when the vector becomes capable of infecting healthy
plant with the virus.
Incubation period: The time between penetration of a host by a
pathogen and the first appearance of disease symptoms
35. Acquisition feeding period (Actual time)
Time for which a virus free vector actually feeds on a virus plant to
acquire the virus
Acquisition access period (Total time)
Time for which a vector is allowed to feed on a source of virus
Inoculation feeding period (Actual time)
Actual period of feeding to inoculate the virus
Inoculation access period (Total time)
Time for which vector after acquiring the virus is allowed to feed
on healthy plant.
36. Non-persistent Viruses
Virus is acquired by the insect just after feeding on the infected plant i.e.
a few minutes or seconds. The virus is transmitted immediately.
Insect transfers the virus from infected to a healthy by inserting its
stylet.
The insect rapidly (usually within 4 hrs) loses the ability to transmit the
virus.
Non-persistent type of transmission is only mechanical process of stylet
contamination.
Most of the non-persistent viruses are transmitted by aphids.
No vector specificity and individual viruses are usually transmitted by
several aphid species.
E.g. The aphids such as Myzus persicae, Aphis craccivora, A. gossypii
transmits several viruses.
37.
38. S. No. Insect vector Disease
2. Aphis craccivora Bean common mosaic
3. A. gossypii Cucumber mosaic
4. ” Papaya ring spot
5. ” Soybean mosaic
6. Myzus persicae Lettuce mosaic
7. ” Potato virus Y
8. ” Turnip mosaic
9. Rhopalosiphum maidis Sugarcane mosaic
Examples of non-persistent virus and vectors.
39. Persistent / circulative viruses
A type of virus which is acquired and transmitted by the vector after
relatively long feeding times and remains transmissible for a
prolonged period while in association with its vector
The leafhoppers and plant hoppers transmit the maximum number of
circulative or persistent viruses.
Circulative viruses usually show a high level of vector specificity
Besides hoppers, various vector groups, particularly aphids and
whiteflies transmit a large number of circulative viruses.
E. g., Pea enation mosaic, lettuce necrotic yellows, bean golden
mosaic
40. Virus Vector Group Vector species
Banana bunchy top Aphid Pentalonia nigronervosa
Groundnut rosette Aphid Aphis craccivora
Rice grassy stunt Plant hopper Nilaparvata lugens
African cassava mosaic Whitefly Bemisia tabaci
Bean golden mosaic Whitefly B. tabaci
Bhendi yellow vein mosaic Whitefly B. tabaci
Tomato spotted wilt Thrips Thrips tabaci
Examples of circulative (persistent) viruses and their vectors
41. Semi-persistent viruses
They are intermediary between non-persistent and persistent
viruses.
Basically these viruses are non-persistent because they do not
circulate within the vector. But their vector retains their ability to
transmit the virus for 3-4 days.
Like persistent viruses semi-persistent viruses are usually associated
with phloem cells. These viruses also show greater vector
specificity than non-persistent viruses.
42. Vector Virus
Myzus persicae (Aphid) Beet yellows
Toxoptera citricidus (leafhopper) Citrus tristeza
Nephotettix virescens (LH) Rice tungro
Bemisia tabaci (whitefly) Lettuce infectious yellow
Example of Semi-persistent viruses and vectors.
43. S.
No
Test Non-circulative Circulative
Non-persistent Semi-persistent Persistent
1 Pre-acquisition fasting Enhances chances of
acquisition
No such effect No such effect
2 Tissue of acquisition Epidermis Epidermis, mesophyll Mesophyll, phloem
3 Duration of acquisition
access
Longer the access,
lesser the chance of
acquisition
Longer the access,
better the chance of
acquisition
Longer the access, better
the chance of acquisition
4 Latent period None None Always a latent period
5 Effect of moulting Stops transmission Stops transmission Transmission not
affected
6 Retention of infectivity Minutes to hours rarely
longer
Hours to days Days to life long
7 Vector specificity Generally low Medium Medium to high
8 Examples Sugarcane mosaic
transmitted by
Rhopalosiphum maidis
Citrus tristeza
transmitted by
Toxoptera citricidus
Bunchy top of banana
transmitted by
Pentalonia nigronervosa
44. Common symptoms of virus and viroid diseases
Systemic infection: The virus is present throughout the plant and the
symptoms produced are called systemic symptoms.
Local infection: The virus causes the formation of small, chlorotic or
necrotic lesions only at the points of entry and the symptoms are
called local lesions.
Latent viruses: Viruses infect hosts without causing development of
visible symptoms on them and the hosts are called symptomless
carriers.
Plants that usually develop symptoms on infection with a certain
virus may remain temporarily symptomless under certain
environmental conditions (e.g. high or low temperature) and such
symptoms are called masked.
46. Mosaic
Intermingled patches of pale green or yellow and dark green patches
on the leaves is called mosaic.
e.g.Tobacco mosaic viruses
Cucumber mosaic virus,
Cassava mosaic virus,
Pigeon pea sterility mosaic virus,
Cowpea aphid-borne mosaic virus,
Bean yellow mosaic virus
In the pale green areas either the chlorophylls are destroyed by
increased chlorophyllase activity or the development of
chloroplasts is inhibited.
47. Tobacco mosaic Tobacco mosaic Virus (TMV) / Nicotiana Virus 1
Symptoms: Development of light and dark green pattern on the lamina.
Dark green areas are usually associated with the veins, which later
develop into irregular crumpled swellings /blisters due to rapid growth.
Dark brown necrotic spots develop under hot weather (mosaic burn).
Virus: Rod shaped ss RNA.
Transmission: Sap, farm equipments and by contact.
52. Cowpea aphid-borne mosaic virus (CAMV) on cowpea
Symptoms: Chlorosis, dark green and light green patches alternated on
leaves; distortion of leaves.
Vector: Aphids - Aphis gossypii, A.craccivora, Myzus persicae.
53. Cassava mosaic virus on cassava (Tapioca)
Symptoms: Mosaic mottling, chlorosis, distortion, twisting of leaves;
Stunting of plants and tuber splitting.
Vector: White fly - Bemisia tabaci
54. Yellow mosaic of greengram and blackgram caused by
Mungbean Yellow Mosaic Virus (MYMV)
Symptoms: Small yellow patches or spots intermingled with green
patches on the leaves initially, later entire leaf changes yellow in colour.
In severe infections discolouration of pods and seeds to yellow.
Vector: Whitefly - Bemisia tabaci
55. Sterility mosaic of pigeonpea - Pigeonpea sterility mosaic virus
Symptoms: Intermingling of light green and dark green patches in the
leaves, reduction in leaf size;
Clustering of small leaves near the tip of the plants, shortening of
internodes, stimulation of auxillary buds giving a bushy appearance.
No flower and pod formation leading to sterility of affected plant.
Plants remain green till harvest.
Vector: Eriophyid mite - Aceria cajani
56. Chlorosis
Chlorosis is yellowing of normally green tissue due to destruction of
chlorophyll or due to failure of chlorophyll formation.
e.g., Banana infectious chlorosis caused by cucumber mosaic virus.
Vector: Aphids - Aphis gossypii and A. maidis
57. Vein clearing :
e.g. Vein clearing or yellow vein mosaic of bhendi - Bhendi Yellow
vein mosaic virus (BYVMV)
Symptoms: Light yellow streaks along with the smaller veins, later all
the veins become yellow giving yellow network of veins.
Chlorosis of interveinal areas, reduction in size of leaves and small
and fibrous fruits.
Virus: ssDNA Vector: Whitefly - Bemisia tabaci
58. Vein banding
Bands of green tissues seen along the veins because of chlorosis of
interveinal tissues. e.g., Potato vein banding virus on tobacco.
Virus: ss RNA Vector: Myzus persicae (Aphid).
Yellow vein banding of
Grapevine caused by
Grapevine fan leaf virus
59. Colour breaking
The colour of the petals may be altered and such flowers present
shades of colour on virus infection.
e.g. Tulip breaking virus. Diseased plants produce variegated flowers.
60. Leaf curl: Curling of leaves is prominently seen. The smooth margin of
the leaves may become dentate.
e.g., Tomato and tobacco leaf curl - Tobacco leaf curl virus. (TLCV)
Symptoms: Leaves curled, twisted and puckered, leafy outgrowth
called enations can be seen on the under surface of leaves, thickening
and greening of veins in the leaf and calyx, mottling and vein clearing,
stunted plant growth, inflorescence greatly condensed and complete or
partial sterility. Vector: Whitefly - Bemisia tabaci
61. Blackgram leaf curl- Tomato Spotted Wilt Virus (TSWV)
Symptoms: Lateral veins show chlorosis near the leaf margin and the
lamina curl downwards slowly. Infected leaves are brittle and sometimes
vein necrosis present on the under surface of the leaves, which extends
up to the petiole.
Virus: ss RNA
Vector: Thrips tabaci,
Frankliniella schultzii
62. Leaf roll: Leaves of virus-infected plants are folded upward giving a
boat shape. e.g., Potato leaf roll virus.
63. Leaf crinkle: In the crinkle, the surface of leaf is not uniform and is with
undulations, brittle and dark green. e.g., Urdbean leaf crinkle virus.
Leaf crinkle of blackgram - urdbean leaf crinkle virus (ULCV)
Symptoms: Crinkling and curling of leaves, stunted and bushy plants
and malformed inflorescence with sterile flowers.
Virus: ssDNA Vector: Whitefly - Bemisia tabaci
64. Shoe string and rat- tail : Partial or complete suppression of leaf blade
development leads to shoe string or rat tail symptoms.
e.g., Strains of Tobacco mosaic virus and Cucumber mosaic virus.
65. Enations: Small leaf like outgrowths usually seen on the veins on the
lower surface of leaves. They may be cup or boat shaped also.
e.g., Tomato leaf curl - Tobacco leaf curl virus, Pea enation mosaic virus.
66. Streak
It is characterized by elongate lesions or areas of discolouration, usually
of limited length on leaves with parallel venations or on stems.
e.g., Banana streak virus, Maize streak virus, wheat streak mosaic virus,
Tobacco streak virus.
Banana streak virus
67. Stripe
It is characterized by elongate lesions or areas of discolouration of
indefinite length on leaves with parallel venations or on stems.
e.g., Barley stripe mosaic virus.
68. Necrosis
Necrosis is death of tissues, organs or whole plant.
Vein necrosis: e.g., Tomato spotted wilt virus (TSWV) in Tobacco,
Muskmelon, vein necrosis virus.
Phloem necrosis: Phloem tissues are killed leading to accumulation of
starch in the leaves. e.g., Potato leaf roll virus
Top necrosis: Sudden death of top leaves. e.g., Strains of Potato virus X
and Y on chillies.
Bud necrosis or Bud blight: Necrosis of tissues in the terminal leaf
buds. e.g., TSWV on groundnut.
Stem necrosis: Necrosis in the form of streaks on the tender stems of
tomato plants leading to drying. e.g., TSWV on tomato.
69. Bud necrosis of groundnut - Tomato spotted wilt virus
Symptoms: Chlorotic spots on leaves or mottling and necrosis of
terminal buds; reduction in leaflet size; distortion of lamina and shoe-
string formation stunting and bushy appearance of plants.
Virus: ss RNA; Vector: Thrips - Frankliniella schultzei, Thrips tabaci
70. Rosette
Severe reduction of internode growth in a vertical axis without
comparable reduction in the sides of the leaves.
e.g., Groundnut rosette virus Vector: Aphids- Aphis craccivora
71. Ring spots
Appearance of chlorotic or necrotic rings mostly on the leaves.
Sometimes they appear on the fruits and stems.
e.g., Tobacco ring spot virus on tomato.
Papaya Ring Spot Virus
72. Wilt
Wilting of plants due to shortage of water or due to excessive
transpiration or due to impeded supply of plant nutrients.
e.g., Tomato spotted wilt virus.
73. Bunchy top
Extreme stunting of the plant with shortened internodes bunching of
small, erect, brittle leaves.
e.g., Bunchy top of banana - Banana bunchy top virus (BBTV)
Virus: ss DNA Vector: Banana aphid- Pentalonia nigronervosa var. typica
74. Scorching
It is found in the margins of leaves or in the entire leaves and gives burnt
up appearance. e.g., Rice Tungro viruses (RTBV and RTSV)
76. Tumour
Tumour may be found on leaves, stem or
fruits.
E.g. Wound tumour virus on sweet clover.
77. Stem pitting
Shallow pits or depressions of different size on the wood of stem.
e.g., Citrus tristeza virus, Apple stem pitting virus
78. 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: Viroids produce stunting, mottling, leaf distortion and
necrosis. Degenerative abnormalities have been found in the
chloroplasts of viroid-infected cells. Viroid infection caused marked
change in the amounts of host proteins.
Transmission: Viroids are transmitted by mechanical means in most of
their hosts. Transmission in the field may be by contaminated tools and
by vegetative means.
79. Coconut Cadang-cadang
It was first described in Philippines
Cadang-cadang is a lethal disease of
coconut characterized by yellow-
bronze colouration of the lower two-
third of fronds in the crown, cessation
of nut production, crown diminution
and death.
Leaflets become brittle. The crown of
the palm dies. Generally diseased
palms die between 8 and 16 years.
Natural transmission by mechanical
damage and pollen is suspected.
No vector has been identified.
80. Potato spindle tuber
Potato spindle tuber viroid (PSTVd) is the first recognized
viroid disease in plants.
Symptoms: Diseased plants are less vigorous, erect, spindly and dwarf.
Leaves are small, erect, and dark green. Leaflets sometimes show
rolling and twisting. The tubers are elongated with a cylindrical middle
and tapering ends.
Transmission: PSTVd is mechanically transmitted by knives
