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The following points highlight the eight modes of transmission of plant viruses. The modes are: 1. Mechanical Transmission 2. Vegetative and Graft Transmission 3. Pollen Transmission 4. Seed Transmission 5. Nematode Transmission 6. Fungal Transmission 7. Insect Vector Transmission 8. Dodder Transmission.
Mode # 1. Mechanical Transmission:
In nature plant viruses are mechanically transmitted from diseased to healthy plants by rubbing leaves together, injecting plant extract, by action of animals, etc. Viral particles remain adhered to plant surfaces, epidermis or hairs. During rubbing the cells are broken and viral particles are liberated in the damaged cells. Transmission through this mechanism occur in such plants which are closely planted.
Similarly viral particles attached on surface of animal body are transmitted when they rub their body first on infected plants and then on healthy plants. Viral particles enter through the injuries made by animals. Similarly birds also transmit viruses by this method, for example TMV.
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Potex virus survives on human clothing and agricultural tools such as cutting knives, sickles, etc. These tools are frequently used in agriculture and horticulture. Hence their use first on infected plants then healthy plants facilitates viral transmission.
Indicator Plants:
The test plants that react promptly and characteristically to particular viruses are called ‘indicator plants’ or differential hosts’. Only susceptible plants act as indicator plant but not the virus resistant plants.
Usually the cotyledons of cucumber and primary leaves of cowpea and beans are inoculated by viruses, because older plants are less susceptible. Virus resistant plants consist of some special inhibitory substance that prevent the transmission of viruses. Table 16.5 shows some of the indicator plants commonly used for inoculation purpose.
Mode # 2. Vegetative and Graft Transmission:
Generally viral particles are present almost in all parts of systematically infected plants. Virus will certainly be transmitted to the progeny, if any part of such mother plants in used for vegetative propagation through rhizomes, bulbs, corns, tubers, cuttings, etc. Therefore, one must not use the infected vegetative part of vegetatively propagating plants such as dahlia, chrysanthemums, carnations, potatoes, etc.
Grafting technique (placing the cut end of one plant onto immediate contact of tissues of other plants to establish a union product in one plant) has been well practiced in India since time immemorial. There is wide variety of grafting techniques such as stem grafting or wedge grafting, tuber grafting (in potatoes), root grafting, etc. Grafting is widely used commercially for propagation of plants.
For example, more than 4,000 varieties of mango have been possible due to grafting. Systemic virus can be transferred from infected portion of a plant to the healthy portion of other one e.g. colour breaking of tulip, apple mosaic. Nicotiana glutinosa plants die when tomato or tobacco plants systematically infected with TMV are grafted with it because TMV produces necrosis of leaves and buds.
Mode # 3. Pollen Transmission:
When pollens consisting for viruses fall on stigma of female plants, they germinate and eventually facilitate the virus to infect the ovules of plants. Such viruses are called pollen-borne viruses. Example of pollen-transmitted viruses are: barely stripe virus, tobacco ring spot virus, bean common mosaic virus, fruit ring spot virus. Dhatura mosaic virus is transmitted through pollen to seeds in 79% offspring.
Mode # 4. Seed Transmission:
Seed transmission of viruses is very rare but many viruses are known to be seed transmitted. However, a very low level (0.1%) of seed transmission had epidemiologically been found out.
Some example of seed transmitted viruses are bean mosaic virus, tomato ring spot virus, tobacco ring spot virus, cowpea mosaic virus, cucumber mosaic virus, mung and urd bean mosaic viruses, etc. Seed transmitted viruses are present in embryo, endosperm or seed coat. After seed germination, virus- infected seedlings are produced.
A substantial yield loss is caused by viruses. For example, soya bean mosaic virus results in 10-20% decline in seed weight with seed discolouration. Since seeds are carried for a long distance within a country or across the country, viruses are transmitted over long distances. Seed-borne viruses get good chance of survival between two seasons and during adverse conditions. Besides, nematodes also transmit the seed-borne viruses.
Mode # 5. Nematode Transmission:
There are some plant parasitic nematodes that feed roots of plants. Such nematodes also act as vector for some viral pathogens. Vectors are the organism that assist in transmission of viruses. Examples of some plant parasitic nematodes are: Longidorus, Paratrichodorus, Trichodorus and Xiphinema. These nematodes transmit viruses for about 10 months but transmission does not involve viral replication inside the vector.
There is virus-vector specificity for transmission by nematodes. Nepoviruses (polyhedral) are transmitted by Xiphenema and Lingidorus, and Tobraviruses (tubular) are transmitted by Paratrichodorus and Trichodorus.
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In soil two types of virus transmission can be observed:
(a) Some of the viruses remain active in debris of roots are leaves. Healthy plants that come in contact of debris are infected, e.g. TMV,
(b) Virus is transmitted to healthy plants by nematodes that inhabit the soil e.g. potato rattle virus.
Mode # 6. Fungal Transmission:
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There are many soil-inhabiting fungi which transmit several viruses, for example Olpidium brassicae, Polymyxa graminis, Synchytrium endobioticum and Spongospora subterranean. These fungi are obligate endoparasite of higher plants.
Their zoospores infect the roots of new hosts, introduced viruses and produce virus specific symptoms. These fungi acquire virus from virus- infected plants which persists in soil for several months or years. O. brassicae transmit tobacco necrosis virus, P. graminis transmit wheat mosaic virus and S. subterranean transmit potato mop top virus.
Mode # 7. Insect Vector Transmission:
Vectors are highly mobile and play an important t role in natural ecology of viruses. In many cases relationship between virus and vector is an intimate biological association, besides mere mechanical transfer.
Most of the viruses are transmitted by vectors. They have slender, needle-like mouthparts to which they pierce the cells and suck cell sap of host plants. Virus is transmitted to healthy plants when the viruliferious insects feed the plant tissue.
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Viruses are divided into three groups, on the basis of length of the period and relationship with insect:
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(a) Non-persistent [where acquisition period i.e. feeding period on diseased plant is for a short period (10-60 second); thereafter rate of transmission decreases because viruses survives for a short period],
(b) Semi-persistent (acquisition period is of 12-24 hours and transmission occurs immediately after acquisition for 1-3 hour.
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Hence, virus survives for a few hours in the vectors), and (c) persistent (viruses survives for a week or month in the vectors; hence, transmission occurs only where the inoculation feeding lasts at least for some hours).
Some of the important insect vectors and viruses transmitted by them are described in this section:
(i) Aphids:
Aphids are the most notorious and important groups of plant vector. They are found in large numbers during spring and winter seasons. They show preference towards feeding of the hosts, for example Aphis craccivora and A. fabae prefers beans; A gossypii prefers cotton, cucurbits, chilli and brinjal; Myzus persicae prefers tobacco; Liaphis erysimi infects crucifers such as mustard.
Examples of viruses transmitted by aphids are: barely yellow dwarf virus, potato virus S, alfalfa mosaic virus, cucumber mosaic virus, potato leaf roll virus, lettuce necrotic yellow virus, and bean mosaic virus, barely mosaic virus, pea mosaic virus, chilli mosaic virus, etc.
(ii) Leafhoppers:
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All the leafhopper-borne viruses are transmitted in a persistent manner, except rice tungro virus and maize chlorotic dwarf leaf virus which are transmitted in semi-persistent manner. Examples of viruses transmitted by leafhoppers are: maize streak virus by Cicardulina mobila, rice dwarf virus by Nephotettix nigripictus, beet curly top virus by Circulifer tennelus, etc.
(iii) Grasshoppers:
Grasshoppers also transmit many viruses such as TMV, turnip yellow mosaic virus, turnip crinkle virus, etc.
(iv) Beetles:
More than seventy four species of beetle are known to transmit virus. Examples of beetle-transmitted viruses are cow pea mosaic virus, turnip yellow mosaic virus, squash mosaic virus, southern bean mosaic virus.
Mode # 8. Dodder Transmission:
Dodders are the trailer or climber parasitic plants which grow forming bridge between two plants. Cuscuta reflexa is the most famous dodder plant that lacks leaves. They belong to the family Convolvulaceae.
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Dodders wind around the host and penetrate its haustoria into host tissue sending up to vascular tissue. Haustoria acquire virus from the infected plants that are eventually transmit to the new hosts. Dodder transmitted viruses are sugar beet curly top virus, tomato bushy stunt virus, tobacco rattle virus, etc..
