P.N. Sharma

Department of Plant Pathology,

CSK HPKV, Palampur (H.P.)

Three basic steps:

Correct pathogen ID

Understanding pathogen biology/ disease epidemiology

Development and evaluation of a management strategy

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.

Conventional approaches

Biotechnological approaches

(Hull., 2004. Matthews plant Virology)

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

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

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

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.

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.

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.

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

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.

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.

Availability of sources of resistance

Nature of resistance gene

Their inheritance pattern

Method of breeding

Number of resistant varieties

developed in different plant species.

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

R GENE PLANT PATHOGEN RCY1 Arabidopsis CMV

RTM1 Arabidopsis TCV

RTM2 Arabidopsis TEV

Rx1 Potato PVX

Rx2 Potato PVX

HRT Arabidopsis TCV

Sw-5 Tomato TSWV

N Tobacco TMV (Kang 2005 Annu. Rev. Phytopathol)

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.

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

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

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.

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.

Transgenic papaya showing resistance to PRSV compared to infected non transgenic papaya..

. PRSV infected papaya fields in 1994.

Transgenic papaya test field

Yellow plants are non-transgenic papaya; plants on right are transgenic

'Rainbow' papaya.

Aerial view of transgenic papaya test field showing block

of healthy transgenic 'Rainbow' surrounded by severely

infected non-transgenic 'Sunrise' papaya.

CROP virus Tobacco Tobacco Mosaic Virus

Tobbaco Cuccumber Mosaic Virus

Tobbaco Tobacco rattle virus

Potato Potato Virus X

Potato Potato Virus Y

Tomato Tomato Leaf Curl Virus

Tomato Alfalfa Mosaic Virus

Soybean Bean Mottle Virus

Rice Rice Stripe Virus

Citrus Citrus tristeza

Maize Maize Dwarf mosaic Virus

Melon Cuccumber Mosaic virus

Lettuce Lettuce Mosaic Virus

(-) (+) (-) (+) (+)

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 (+)strand RNA 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

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

viral RNA

Virus assembly

coat protein

virus particles

Viral movement protein

Movement of virus particle through modified plasmodesmata

Assembly of viral movement complex

Disassembly of viral movement complex

Virus disassembly

Viral RNA replication, translation, etc

More cell-to-cell movement

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

(Kawchuk et al., 1991. Plant Microbe interaction)

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

Citrus exocortis Viriod in

Tomato

(Atkins et al., 1995, journal of

General virology)

Plum Pox virus

(Liu., 2000. Virus Research)

Alfalfa Mosaic Virus

Tobacco Etch Virus ( Tenlledo et al., 2004. Virus Research)

Banana bunchy Top Virus

I gratefully acknowledge the use of text book “Matthews Plant Virology” 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