2. Three basic steps:
Correct pathogen ID
Understanding pathogen biology/ disease
epidemiology
Development and evaluation of a
management strategy
3. Viruses are very small (submicroscopic) infectious
particles (virions) composed of a protein coat and a
nucleic acid core.
They carry genetic information encoded in their nucleic
acid, which typically specifies two or more proteins.
Translation of the genome (to produce proteins) or
transcription and replication (to produce more nucleic
acid) takes place within the host cell and uses some of
the host's biochemical "machinery".
Viruses do not capture or store free energy and are not
functionally active outside their host.
6. Removing crop residues or remnants of virus infected
plants which act as a sources of virus infection
Prevent direct contact of healthy plants with already virus
infected plants or with contaminated hands, tools and
clothes
Removal of perennial or annual weeds generally act as
sources of virus in diseases of legumes and cucurbits
Use virus-free certified propagation material(nuclear
stock)
Virus free seeds should be used
7. Susceptible seedbeds can be covered with tall barrier
crop to prevent the spread of vectors
Closely spaced plants tend to escape infection
Growing susceptible plants in isolation
Alterations of dates of sowing or changed planting
dates
Growing plants in sterilized soil helps in checking soil
borne infection
Leaving fields fallow and crop rotation
8. Hot water treatment of dormant propagative organs
such as tubers or budwood dipped in hot water (35-
54°C) for a few minutes or hours or
Hot treatment for different intervals of actively
growing plants in growth chambers at 35-40 °C for
several weeks helps in eliminating the virus from the
plants
Potato leaf roll virus can be freed from potato tubers,
if they are stored during summers and in temperature
upto 36 °C
9. Used to produce virus-free stock
Apical meristem region normally free from virus are
excised (short pieces 0.1mm to 0.5 mm) and grown in
vitro in tissue culture medium to raise virus-free plants.
The basic ingredients are an appropriate selection of mineral salts
(macro and micronutrients), sucrose , and one or more growth
stimulating factors such as IAA or GA, sometimes in agar.
Culture of single cells or small clumps of cells from virus-
infected plants may sometimes give rise to virus-free
plants
The technique works well for chrysanthemums, carnations
and potatoes.
10. Chemical control of viruses has not been very
successful. Pretreatment of tobacco plants with virazole
however, delayed or prevented systemic infection with
tomato spotted wilt virus.
A chemical carbendazim present in fungicide
BAVISTIN reduces virus-induced symptoms but has no
effect on virus. Such symptomless hosts may be
dangerous reserviors of viruses for other plants.
Chemical treatment by itself has not yet found practical
use. However, chemical treatment in combination with
heat treatment or meristem tip culture may have an
advantage in a few instances.
11. Natural defense mechanism against viral infection by certain plant
extracts has been reported. Plant extracts show two types of
inhibitory response.
Mostly they reduce ( Phytolaca americana, Dianthus caryophyllous) virus activity
when co-inoculated with virus into susceptible plants.
However, extracts from a few non-host plants (Boerhaavia diffusa, Clerodendrum
aculeatum, Mirabilis jalapa, etc.) inhibit virus infection and development of
symptoms by stimulating the production of a virus inhibitory agents(s) (VIA) in
susceptible hosts, when sprayed prior to virus inoculation.
Similar virus inhibitory agents may also be induced, following
inoculation or spraying of susceptible plants with either virus or
chemicals
• The induced inhibitory agents may be low molecular weight
proteins or glycoproteins and show some resemblance to
interferon.
• Foliar sprays of oil or skimmed milk have also been used to prevent
the spread of virus diseases by aphids. The aphids fail to infect
such plants.
12. a) Avoidance of vectors: In virus diseases which are
transmitted by insects, the disease incidence can be
minimized by avoiding the contact of the vectors with host
by:
Growing crops in isolation where vectors are absent or low in
number
Use of tall cover crop protect an under sown crop from insect-
borne viruses, for example, cucurbits are sometimes grown
intermixed with maize.
Barrier crops may be grown around a crop or in alternate rows
between the crops. Barrier crops have been found useful in
controlling non persistently transmitted aphid borne viruses.
Use of reflective mulches
13. b) Direct control of vectors – The vectors can be
eliminated by direct chemical control, using
suitable insecticides.
• Insecticides, may be natural products or synthetic.
• Some of the natural insecticides commonly used to
control insects are derived from tobacco (nicotine),
chrysanthemum (pyrethrum), a legume Derris
(rotenone) and neem.
• Synthetic pesticides commonly used are
Malathion, Rogor, Dimecron etc. may be sprayed
or applied as soil drench or as granules(carbofuron
or phorate) mixed in soil.
14. Sprays with water-oil emulsions also help
in reducing field spread of viruses.
Fungal and nematode vectors transmitting
viruses can be controlled using soil
sterilization with chemicals.
Nematicides like dichloropicrin or dichloropropane
1kg/10m2 or penta-chloronitrobenzene (quintozine)
kill fungi and nematodes in soil.
15. Availability of sources of resistance
Nature of resistance gene
Their inheritance pattern
Method of breeding
Number of resistant varieties
developed in different plant species.
16. Viral Proteins
Host R protein
Avirulence
factor
Recognized by R protein
Host factor Interaction with Host factors
Active defense signaling
Switching host system
for viral infection
(kang et al., 2005.Annu. Rev. Phytopathol.)
Susceptible
Resistant
Avirulence
factor
18. Infection of a susceptible plant with a mild or attenuated
strain of virus sometimes helps in virus control by
protecting such plants against later infection by a more
severe strain of the same virus. Plants may be purposely
infected with a mild strains as a protective measure
against severe disease.
Mild strains of tomato mosaic virus are particularly helpful in
controlling infection by severe strains in tomato plants.
Protection by CTV mild strain is also helpful in protecting citrus
crops against severe strains of Citrus tristeza virus.
The naturally occurring satellite in CMV strains (CMV-S) has
been used as a biological control agent to protect tomato plants
against disease induced by severe strains of CMV.
19.
20. Virus infection
Epidermal
cells
Kang BC et al., Annu. Rev. Phytopathol.(2005)
Mesophyll
cells
Bundle
sheath cells
Phloem
Parenchyma
Companion cells
Phloem
Other host plants
Replication
Plant-to-plant
movement
Cell-to-cell
movement
Systemic
movement
21. Virus infection
Epidermal
cells
Kang BC et al., Annu. Rev. Phytopathol.(2005)
Mesophyll
cells
Bundle
sheath cells
Phloem
Parenchyma
Companion cells
Phloem
Other host plants
Replication
Plant-to-plant
movement
Cell-to-cell
movement
Systemic
movement
22.
23. In 1985, Sanford and Johnston developed the simple
and elegant concept of parasite- or pathogen-derived
resistance (Sanford and Johnston, 1985).
Such disruptions prevent the replication and/or
movement of the virus beyond the initially infected cell.
Interference in the replication cycle, PDR might
modulate the disease symptoms and result in only a
localized infection.
The goal of constructing genetically engineered plants
resistant to virus infection is to express a portion of the viral
genome, either with or without expression of an encoded
protein, that will interfere with some particular aspect of the
multiplication cycle.
24. Successful strategies based on PDR include
coat protein mediated resistance, expression of a coding
region embedded in the replicase (for instance, the 54-kD
protein of TMV),
use of antisense RNAs that are the complement of the
plus- or minus-sense template of the virus, or
use of satRNAs that can presumably overwhelm the viral
RNA replicase and thereby suppress specific events
required for infection.
two basic molecular mechanisms by which PDR is
thought to operate are
In some systems the expression of an unmodified or a modified
viral gene product interferes with the viral infection cycle called
protein based protection
the second mechanism does not involve the expression of a
protein product called as nucleic acid based protection.
27. . PRSV infected papaya fields in 1994.
Transgenic papaya test field
Yellow plants are non-transgenic papaya; plants on right are transgenic
'Rainbow' papaya.
28. Aerial view of transgenic papaya test field showing block
of healthy transgenic 'Rainbow' surrounded by severely
infected non-transgenic 'Sunrise' papaya.
30. (-) (+) (-) (+)
(+)
Viral genome RNA
[single-stranded,
(+)strand]
Synthesis of
(-)strand from viral
(+)strand
via viral replicase
Newly made (-)strand
RNA used as template
to make new (+)strand
Replicating
form
Double-stranded
viral RNA
(+)
Release of new
(+)strandRNA Etc.
New (+)strand
viral RNA
(+) strand RNA
template removed
-replicase = -viral encoded RNA dependent RNA
polymerase + helicase + host proteins
Replicase binds to RNA
and synthesizes a
complementary (-) RNA
strand
31. CROP VIRUS REFERENCE
Potato Potato virus Y Audy et al., 1994
Pea Pea seed Borne Mosaic
Virus
Jones et al., 1998
Rice Rice Yellow Mosaic Virus Pinto et al., 1999
32.
33. viral RNA
Virus assembly
coat protein
virus particles
Viral movement protein
Movement of virus particle
through modified plasmodesmata
Assembly of viral
movementcomplex
Disassembly of viral
movementcomplex
Virus disassembly
Viral RNA replication,
translation, etc
More cell-to-cell
movement
34. CROP VIRUS REFERENCE
Tobacco Tobacco Mosaic virus Copper et al., 1995
Tobbaco Tobacco Etch Virus Cronin et al., 1995
Potato Potato leaf Roll
Potato Virus X
Potato Virus Y
Tacke et al., 1996
37. CROP VIRUS Sat virus REFERENCE
Tobbaco Cuccumber
Mosaic Virus
Carna-5 Harrison et al.,
1987
Arabidopsis Turnip crinkle
Virus
RNA-C Kong et al., 1997
Cymbidium Cymbidium
ringspot virus
DI Koller et al., 1993
38. Citrus exocortis Viriod in
Tomato
(Atkins et al., 1995, journal of
General virology)
Plum Pox virus
(Liu., 2000. Virus Research)
44. I gratefully acknowledge the use of text book
“Matthews PlantVirology” by Roger Hull.
I acknowledge the scientists who spent valuable
time in generating information on various
aspects of plant Virology and displayed the
same on internet for use by students, teachers
and researchers
PN Sharma
