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The below mentioned article provides quick notes on plant viruses.
Plant viruses are a persistent threat to production of vegetable crops in India. Although severe outbreaks of viral diseases tend to occur on a sporadic basis, many plant viruses are ubiquitous in nature and can result in severe outbreaks under the right conditions. Virus diseases result in losses through reduction in growth and yield and are responsible for distortion and mottling of fruit, making the product unmarketable.
Among the viruses, Aphid borne viruses, whitefly transmitted and thrip-vectored tospoviruses have become major problem in recent years. These viruses are very common on several vegetable crops in Andhra Pradesh, Bihar, Gujarat, Haryana, Karnataka, Kerala, Maharashtra, Punjab, Tamil Nadu, Uttar pradesh and West Bengal.
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Significant economic losses were occurring in all of these crops for example, 45 to 100% yield loss in watermelon. The ability of Begomoviruses and tospoviruses to cause severe losses on a broad range of crops, places them amongst the most important viruses at present.
Major contributory factors for the emergence and spread of new viruses include evolution of variants of viruses, changes in vector biology, change in the cropping systems, introduction of new crops and climate change. Presumably, changes in pesticide usage have favored shifts in vector species and virus spread. There are many causes for the apparent increase in new and emerging viruses, man plays an important role.
Misplaced confidence in our ability to control vectors with pesticides has resulted in multiple chemical resistances. International movement of plant material and, lowered trade barriers offers opportunities for unknown viruses to be introduced into new areas, which happened recently with the introduction of Iris yellow spot virus (IYSV) in onion near Pune.
Symptoms caused by viruses are highly variable and of little diagnostic value. Mosaic, distortion, necrosis on several plant parts, chlorosis, ring patterns, mottling, and silvering are the most characteristic symptoms. In some instances, the disease can limit the feasibility of crop production.
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In the past diagnosis and identification of viruses relied on disease symptoms and host reactions while these methods are inadequate, inaccurate, and not suitable for large scale testing.
Presently, diagnosis of viruses is by ELISA using mono- and polyclonal antibodies, molecular assays based on viral genomes (use of nucleic acid probes in dot blot assay, PCR) are becoming more and more popular, as these methods can give a result within a few hours to two days after the collection of the sample.
DNA micro array or DNA chip using viral genes is also now available for rapid detection of viral genes and strains of viruses.
Some plant viruses are not seed borne but cause significant yield loses in seed production and the quality of seed is also very much affected. Different vectors such as whiteflies, aphids and thrips transmit these viruses. The management strategies also have to be in accordance with the vector type and host.
Some plant viruses such as cucumber mosaic virus, Bean common mosaic virus, Cowpea aphid borne virus, Squash mosaic virus, tobacco mosaic virus, tomato mosaic virus, Cucumber green mottle mosaic virus are seed transmitted in various crops.
These viruses mainly transmitted through pollen from infected to healthy plant. While making crosses in hybrid seed production collection of pollen from virus infected plant should be avoided to mange the virus more efficiently.
