Management of viral diseases is much more difficult than that of diseases caused by other pathogens as viral diseases have a complex disease cycle, efficient vector transmission and lack of effective chemical (viricide). Most of the procedures that can be used effectively involve measures designed to reduce sources of infections inside and outside the crop, to limit spread by vectors and to minimize the effect of infection on yield. All these approaches are important, but most practical approach is the use of varieties, which resist vectors, symptom development, cell-to-cell movement and virus multiplication. The mechanism of resistance varies from virus to virus and host to host. Moreover, rapid development of resistant breaking strains of viruses and lack of sources of resistance, make breeding of resistant varieties difficult (Hull, 2002; Varma et al., 2002). Genetic engineering brings new hope for overcoming various drawbacks associated with conventional breeding for developing crop varieties with durable resistance by ‘pyramiding’ genetically engineered resistance over intrinsic plant resistance.
Correct identification of the virus or viruses infecting a particular crop is essential for effective control measure to be applied. Disease symptoms alone may be misleading. Developments of reliable and sensitive methods for detection are important. Of major importance is designing a strategy for control of a virus is an understanding of the epidemiology of that virus. The various management strategies include:
Elimination of sources of infection in the field can be decided on the basis of the knowledge of such sources and of the ways in which the virus is spreading from them in to a crop.
Sources of infection for many of the viruses include: weed hosts, ornamental plants (that often harbour infection in mild form), unrelated crops and plants of the same species, remaining from a previous crop (volunteers) (Matthews, 1991). The removal of these sources is practicable to control viruses having narrow host range. However, it difficult for viruses having wide host range like Cucumber mosaic virus, Tobacco streak virus and tospoviruses.
Regular inspection and removal of infected plants is an effective method especially when spread is relatively slow and mainly from within the crop. Roguing and replanting with healthy plants may maintain a relatively productive stand. This practice could be easily adopted for managing viruses infecting crops where secondary spread of viruses are very slow or nil. Eradication can be done manually, by burning or with herbicides (Hull, 2002).
For some mechanically transmitted viruses belonging to tobamo and potexviruses, human activities during cultivation and tending of a crop are a major means by which the virus is spread. These viruses spread easily through cutting knife and other tools used in farm operations. Once these viruses enter the crop, it is very difficult to prevent its spread during cultivation. Control measures consist of treatment of implements and washing of the hands with a 3% solution of trisodium orthophosphate (Broadbent, 1963).
In such cases most successful methods to control is the development of virus-free clones. Once a good virus-free clone is obtained, foundation stock or ‘mother’ line must be maintained virus-free, while other is grown up on a sufficiently large scale under conditions where re-infection with virus is minimal or does not take place. These stocks are then used for commercial planting.
Visual inspection for symptoms for virus disease is usually quite inadequate when selecting virus-free plants. Appropriate indexing methods are essential. Distribution of a virus within the plant may be uneven; hence repeated tests in successive seasons are necessary to ensure freedom from virus. Methods based on biological (inoculation to indicator hosts, graft indexing), physical (electron microscopy, nucleic acid and coat protein molecular weight) serological (enzyme linked immunosorbent assay, dot immunobinding assay, electro-blot immunoassay) and molecular (nucleic acid hybridization, polymerase chain reaction) could be used for the detection and diagnosis of virus infecting different crops (Matthews, 1991; Hull, 2002).
Occasionally, individual plants of a variety or plants in a particular location may be found to be free of the virus. Healthy plants could also be raised through micro-propagation (Hull, 2002). When all the plants are infected, one or more of the special treatment and methods are used to obtain a nucleus of virus- free material. Heat treatment is one such method, which has been a most useful method for freeing plant material from viruses. Temperatures and times of treatment vary (35-55 °C for minutes to hours), hot water treatment was found to be more effective than hot air treatment (Hull, 2002). Another method of obtaining virus- free plants is through meristem tip culture (Walkey 1991). It is the aseptic culture of the apical meristem dome plus the first pair of leaf primordia (about 0.1–0.5 mm long). Culture of meristem tips have proved an effective way of obtaining vegetatively propagated plants free from certain virus. However, the method is more effective when it combined with heat therapy.
Where a virus is transmitted through the seed, such transmission may be an important source of infection because it introduces the virus into crop at a very early stage, allowing infection to spread to other plants. In addition, seed transmission introduces foci of infection throughout the crop (Maury et al., 1998). Use of virus-free seed may provide effective means of control of the disease caused by them.
A wide range of insecticides is available for the control of insect pests on plants. Control of insect vectors to prevent infection by viruses is difficult as relatively few winged individuals may cause substantial spread of virus. Insecticides are less effective especially when virus in non-persistent. Persistent virus is able to infect many plants, so that killing it on the first plant will reduce spread. Disease forecasting data is an important factor in the economic use of insecticides. In a few cases oil sprays have given useful results in field trials against a range of non-persistent viruses.
A tall cover crop may protect under sown crop from insect borne viruses. Growing of barrier crops like sunflower, jowar or bajra around pepper crop at least two months before transplanting the pepper seedlings was found to be effective in reducing virus disease incidence. Many of the viruliferous vectors assumed to land on the barrier crops, feed briefly, and either stay there or fly off. If they then land on the crop, they may have lost any non-persistent virus they were carrying during probes on barrier crop (Matthews, 1991). Mixed cropping with non hosts like maize, use of reflective surfaces (polythene film coated with aluminum) and sticky yellow polythene above the crop height surrounding the plots have also been reported to reduce the incidence of aphid transmitted viruses in paprika (Cohen and Marco, 1973).
There are essentially three approaches that have been used to protect plants: use of a mild strain of the virus (termed as cross-protection), conventional and transgenic resistance.
Cross-protection is a type of induced resistance developing in plants against viruses. Resistance is induced when plants are infected by a mild strain of a virus to protect from infection by a severe strain. The approach has been successfully used in the management of citrus tristeza, cocao swollen shoot, papaya ringspot and passionfruit woodiness diseases (Varma et al., 2002). The approach is well suited for perennial crops which are vegetatively propagated. However, there are some potential concerns in the use of cross-protection. A mild strain for the crop of interest may not be mild in other crops grown in the area, it may interact with other viruses in a synergistic fashion and it may mutate to a more severe strain (Varma et al., 2002).
Since effective chemical control measures are lacking in many virus diseases, breeding of resistant varieties requires high priority. A single dominant gene controls resistance to viruses in most crop-virus combinations. Quite often no resistance could be found for particular crops and viruses In a few cases even if genes for resistance is found, it is difficult to incorporate them into useful cultivars. Occasionally, useful sources of resistance can be identified by making initial selections from plants showing good growth in otherwise severely infected fields. Useful resistance has sometimes been found among a collection of mutants induced by physical or chemical means (Matthews, 1991).
The development of recombinant DNA technology and efficient plant cell transformation techniques have allowed to test a number of biotechnological strategies for increasing virus resistance levels in plants. There are three sources of transgenes for protecting plants against viruses. They include: natural resistance genes, genes derived from viral sequences (pathogen derived resistance) and genes from other sources. Of these, pathogen derived resistance (also called as viral derived resistance) is the most success and widely used to get transgenic plants resistant to viruses. Both coding and non-coding regions of viral genomes have been used for developing virus resistant transgenic plants. Coat protein gene is the most commonly used transgene for developing virus resistant transgenic plants against viruses belonging to different groups followed by replicase protein and movement protein genes. Progress in the development of transgenic resistance has been extensively reviewed (Beachy et al., 1990; Lomonossoff, 1995; Callaway et al., 2001; Varma et al., 2002; Collinge et al., 2010; Wani and Sanghera, 2010).