Q.1. Describe in brief the symptoms of plant disease produce by viruses. (2005, 08)
Related Question -
Q. Give an account of different symptoms of viral disease.
Q. Describe the external symptoms of plant diseases caused by viruses. (2013)
Ans. External Symptoms of Viral Diseases of Plants: -
(1) Chlorosis: -
These symptoms represent the areas which become weakened in green colour due to either destruction or inhibition of chlorophyll formation subsequent to infection and the cells with chlorotic symptoms contain less chlorophyll thus appearing pale green or yellowish in colour.
(2) Mosaic: -
These symptoms represent irregular intermingling of light green, yellow or white areas with the normal green colour of the plants or fruits forming a mosaic like pattern. Mosaic symptoms were first reported by Mayer (1882) on tobacco plants.
(3) Mottling: -
It is an irregular pattern of indistinct light and dark areas. Mottling is considered equivalent to the mosaic by some virologists.
(4) Vein Chlorosis and Vein Clearing: -
The former represents the chlorosis restricted to veins while the latter represents the translucence of veins rather than being chlorotic or yellow. These two symptoms make the veins appear lighter in colour against dark background of normal green tissue.
(5) Vein-Mosaic: -
These represent the chlorosis along the main veins with the lighter areas irregular in shape and distribution. Sometimes, the vein mosaics extend to the adjacent areas in the form of irregular patterns.
(6) Vein-Banding: -
In these symptoms the chlorotic area occur along the veins in a regular manner resulting in the presence of chlorotic and regular green coloured bands.
(7) Necrosis: -
These are the common viral symptoms occurring in plants representing the death of the cells in localized areas. These areas of dead tissues are generally differentiated by the presence of dark brown border around them. Necrosis may remain localized but sometimes it spreads as long streaks producing systemic necrosis resulting in the death of the whole plant. One strain of TMV develops streaks on the stems and causes complete necrosis of leaves and fruits resulting in the death of the infected tomato plant.
(8) Ring-Spots: -
These represent the localized circular spots formed by concentric rings of chlorotic and normal green tissue. Ring-spots are generally accompanied with necrosis. They may be present singly or in groups developing concentrically on infected parts.
(9) Enations: -
Enations are the outgrowths generally occurring on vein or midrib on the lower surface of the leaves. They may vary in number and shape. They may be small, large, papillate or spine-like in shape. In some cases like ‘Pea enation mosaic virus’ the outgrowths develop between or adjacent to the veins on the lower surface of the leaves and look like leaves, funnels, wings, cups etc.
(10) Leaf-Narrowing: -
In these symptoms the infected leaves generally become narrow due to reduced growth of laminar tissues. The veins and the midrib remain normal in growth. In the case where leaf-narrowing is at extreme as represented by TMV infected tomato, only the midrib and veins are present and the laminar tissue is almost absent.
(11) Leaf-Curling: -
These symptoms represent irregular and extensive wrinkling and furrowing of leaves due to reduced growth of veins in comparison to the growth of laminar tissue resulting in shrunken veins and raised up laminar tissues leading to the curling of the leaves. Example: leaf curl of papaya, leaf of tomato etc.
(12) Leaf-Rolling: -
In these symptoms the downward and upward rolling of leaves takes place and this covers their entire length. Example: leaf roll of potato.
Q.2. Describe different methods of transmission of viruses. (2012)
Realated Question: -
Q. Describe various methods of transmission of plant viruses. (2010)
Ans. All viruses are infectious and transmissible. There can be two broad lines of transmission.
(1) Artificial Methods of Transmission: -
(a) Sap Inoculation: -
Sap is extracted from the virus infected parts of the plant and is purified.It is now applied to the healthy surfaces of the leaf or it can be inoculated as such.
(b) Grafting: -
For the transmission of virus by grafting technique, bud from the diseased plant is taken out and grafted in a healthy plant. At times stem grafting may be made in which case either scionor stalk should be from infected plant. Common example of transmission by grafting is peach yellow virus.
(2) Natural Methods of Transmission: -
(a) By Soil: -
These are also known as soil borne viral diseases and their transmission may be both –natural by means of fungi and nematodes and mechanical by the persisting viruses in plant debris.
(b) By Seed: -
Viruses may persist in the seeds of some plants which during formation of new plant develop disease. The common example is bean mosaic virus.
(c) By Pollen Grains: -
Pollen grains of an infected plant, when for fertilization reach the healthy plant, transmits the disease. After fertilization viruses make house in young developing seeds. Germination of such seeds into new plants develop viral diseases.
(d) By Natural Grafting: -
Parsitic angiosperms like cuscuta make natural grafts place to place on various parts of different plants and when grow on more than one plant including diseased and healthy ones, they transmit the disease.
(e) By Vegetative Propagation of Plants: -
Certain plants reproduce by formation of vegetative propogules which transmit viral disease from parent to young plant.
(f) By Roots: -
Injuries produced in the roots during cultivation also become infected by viruses from soil.
(g) By Contact: -
Certain contagious viral diseases spread by mere contact in between healthy and diseased plants eg, TMV.
(h) By Weeds: -
Occassionally weeds serve as off sea so alternative host which transmit viral diseases during the growth of original host.
(i) By Vectors: -
Vectors constitute one of the most effective and notorious agencies of viral transmission. Two basic categories of such transmission have been recognized-
(i) Simple Mechanical Transmission: -
Rabies and beetles of dogs are known to transmit TYMV(Turnip yellow mosaic virus). When beetles, grasshoppers or ear wigs feed an infected plant, their mouth parts become contaminated with viruses which are later transmitted to healthy plats. Common vector of TYMV is turnip flea beetle.Such insects have no salivary glands.
Another category is that of aphids which are helpful in mechanical transmission of virus. Aphids commonly feed phloem sap of deseased plants and sip the viruses along with. These aphids can transmit viruses only immediately after feeding.
(ii) Virus and Vectors are Intimately Connected: -
Aphids: -
Afew viruses have intimate relationship with aphids, eg. peoenation mosaic virus and barley yellow dwarf virus. Such viruses are found in phloem sap and when they are extracted by aphid they reach in aphid‘s gut. Viruses do not multiply in the gut. These are transmitted after moulding stage of insects. Another category of viruses multiplies in aphid‘s gut. These are called propogative viruses. Common example is potato leaf roll virus in the aphid .
Leaf Hoppers: -
Propogative type of transmission is also seen among grass hoppers. Besides a few examples of stylet borne and circulative transmission is found in case of dwarf disease of rice by Nephteltix apicalis.
Mosquitoes: -
These are common transmitters of animal viruses and methods of transmission are similar to that of aphids and grass hoppers.
Nematodes: -
These are one of the agencies of spreading soil borne disease is well known for the transmission of diseases such as grape fan leaf and tobacco ring spot disease of virus.
Fungi: -
Olphidium brasscae transmits tobacco necrosis virus and lettuce big vein by infecting a wide range of roots. Though it caused a heavy transmission but only few fungi are known for viral transmission.