The following points highlight the seven important methods by which viruses are transmitted to plants. The methods are: 1. Transmission by Mechanical Means 2. Seed Transmission of Virus 3. Transmission by Vegetative Propagation 4. Transmission by Soil 5. Transmission by Pollen Grains 6. Transmission by Dodder 7. Transmission by Vectors.
Method # 1. Transmission by Mechanical Means:
Transmission by this means consists of the removal of the plant sap from a diseased plant by artificial or natural means and its transfer to a healthy plant by rubbing or injecting. It occurs in those plants in which virus are present in high concentration.
It takes place by following methods:
(a) By contact of healthy and infected leaves brought about by wind.
(b) By rubbing the sap of diseased plants over the surface of the healthy plants.
(c) By agricultural tools.
(d) By grafting infected buds on the healthy plants.
(e) By mechanical contact of infected roots with healthy ones.
Method # 2. Seed Transmission of Virus:
The transmission of plant virus by seeds is not very common.
Seed transmission can occur in two ways:
(a) Virus particles are attached to the surface of the seeds and during germination the seedlings are infected through the wounds caused by soil particles. (Surface seed transmission),
(b) Virus particles enter the reproductive organs in some phase of the generative process. (Internal seed transmission). A few viruses of legumes, wild cucumber, tomatoes and bean spread by this method. Tobacco ring spot virus is transmitted by bean seeds but not by tobacco seeds.
Method # 3. Transmission by Vegetative Propagation:
Viruses are transmitted by the diseased vegetative parts such as tubers, bulbs, roots, buds and scions which are used for propagation e.g., mosaic and leaf curl of raspberry.
Method # 4. Transmission by Soil:
Those viruses which are transmitted through soil, lie in the soil with the plant debris after harvesting the crop and subsequently infect the new crop, growing in the same field for example, Potato mosaic virus.
Method # 5. Transmission by Pollen Grains:
Flowers of healthy plants on pollinating with pollens from virus infected plants produce seed that carries the virus e.g., Bean mosaic virus and Necrotic ring spot virus.
Method # 6. Transmission by Dodder:
Dodder (cuscuta spp.) can transmit virus from plant to plant. For example, sugar beet curly top virus is transmitted by C. subinclusa.
The Cuscuta spp. plant is stem parasite and is commonly known as ‘Amer bel’. It serves as a bridge between an infected plant and healthy plant.
Method # 7. Transmission by Vectors:
No other disease is so much dependent upon vectors for the dissemination of the infectious causal entity as the viruses. About 400 species of insects are known to transmit over 200 different viruses.
Some of the important viruses transmitted by different vectors are as follows:
One species alone Myzus persicae is the vector of 60 different viruses. Depending upon the period for which the insect retains the infective power, the plant virus can be classified into two groups:
(i) Non Persistent:
Also termed as stylet borne viruses. Vector becomes infective only for a few minutes e.g., potato virus Y, Chili mosaic and Pea mosaic.
Also termed as “circulative” viruses. Vectors in this case are able to retain the infectivity for longer duration. In it the viruses undergo a certain “latent period” or “incubation period” before the vector develops infective power. The period varies greatly. The longest period is from 10-19 days e.g., Barley yellow dwarf, ground nut chlorosis and cardamom dwarf etc.
(b) Leaf Hoppers and Plant Hoppers:
Leaf hoppers are also important vectors of viral diseases. Maize streak, Rice dwarf virus are transmitted by Cicadulina mbila and Nephotelix nigricptus respectively.
(c) White Flies:
Viruses of Leaf curl of tobacco, yellow mosaic of Acalypha indica and yellow mosaic of Phaseolus aureus are transmitted by white flies.
Tomato spoiled wilt virus is transmitted by thrip known as Frankilniella insularis.
Fig mosaic and Pigeon pea sterility viruses (PSV) are transmitted by mites.
Viruses, such as squash mosaic, Potato virus X, Turnip yellow mosaic are transmitted by beetles.
Tobacco mosaic virus is transmitted by a grass hopper Melanoplus differentialis. With this Turnip yellow mosaic and Turnip crinkle viruses are also transmitted by grasshoppers.
The nematodes are important carriers of viruses. About 12 important plant pathogenic viruses are transmitted by three nematodes viz. Longidorus, Xiphinema and Trichodorus. Nepo viruses are transmitted by Xiphinema and Longidorus while tobra virus is transmitted by Trichodorus spp. Fanleaf virus of grapes, and Grapevine yellow mosaic virus is transmitted by Xiphinema nidex.
(i) Chytrid Fungi:
The unicellular ‘Water molds’ called chytrids (e.g., Olpidium and Synchytrium) are vectors of several viruses. Olpidium brassicae transmit the tobacco mosaic virus, tobacco stunt virus and Lettuce big vein disease virus. Synchytrium endobioticum transmits potato virus X. Polymyxa graminis transmits soil borne wheat mosaic virus.