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Most CMV strains cause systemic infections, which are sometimes symptomless. CMV strains can induce a range of symptoms, depending on the host. The most common symptoms include severe mosaic, mottling, chlorosis, necrosis and distortion in leaves (fern-leaf and shoestring symptoms in tomatoes) and fruits. Occasionally, CMV is accompanied by a small satellite RNA which, depending on the type of satellite, may result in necrosis and cell death in tomatoes (Xu and Roossinck, 2000) or may cause the amelioration of symptoms (Kaper, 1995). It has been suggested that the CMV RNA2 (the polymerase gene) is associated with symptom determination in cowpea (Karasawa et al., 1999). CMV has the potential to give advance signalling (inducing chemical signals) for the imminent infection of the invading cotyledon cells of Cucurbita pepo (Havelda and Maule, 2000).
Control measures for CMV are mainly preventive. Conventional methods of virus control are difficult to apply due to the wide host range of CMV which infects many weeds that can act as virus reservoirs and infect crops in adjacent fields. Seedlings derived from infected seeds can also serve as potential primary inocula. Infected bulbous ornamental plants can act as virus reservoirs. The application of thermotherapy coupled with tissue culture has greatly reduced the level of CMV in these crops. Developing new crop varieties resistant to CMV, either by conventional breeding methods or by gene technology, is gaining momentum.
Since CMV is transmitted by over 80 aphid species, the diverse behaviour of various aphid vectors can greatly reduce the impact of insecticide sprays, which are more effective when the insects are a direct 'pest' rather than a 'vector'. A more sustainable approach will be the prevention of aphids reaching the crops. This can be achieved by planting barrier crops, applying sticky traps, or covering the ground with an aphid deterrent material like aluminium foil boards (Mansour et al., 2000). Spraying the crops with mineral oil is sometimes effective. The use of virus-free seeds together with the eradication of virus reservoirs can be effective in controlling CMV.
Control strategies utilizing transgenic expression of the CMV coat protein, truncated replicase and/or other non-structural virus genes to protect crops from infection by CMV look promising. Broad-spectrum resistance against strains of both subgroups of CMV was induced in tomatoes transformed with the coat protein gene of a CMV strain from subgroup 1 (Gielen et al., 1996). Field trials have shown a 40% yield increase in tomato lines transformed with CMV coat protein gene compared to non-transgenic plants (Fuchs et al., 1996).
Although the wide host range of CMV makes control measures difficult, new resistance strategies using pathogen-derived genes, which can be inheritable, are encouraging.
The mechanism of resistance in the vast majority of the transgenic technologies for virus resistance is based on RNA-silencing. Inverted repeat constructs encoding self-complementary double-stranded RNA has been demonstrated a potential way to obtain RNA-mediated resistance against CMV at high efficiency in Nicotiana benthamiana plants (Chen et al., 2004). A single chimeric gene consisting of linked viral CP segments was constructed to successfully develop resistance in transgenic watermelon against CMV (Lin et al., 2011). Diaz-Pendon et al. (2007) suggested a model in which 2b inhibits the production of RNA-dependent RNA polymerase–dependent viral siRNAs that confer salicylic acid–dependent virus resistance by directing non-cell autonomous antiviral silencing. The 2b encoded by CMV and other cucumoviruses is multifunctional, having roles in local and systemic virus movement, symptom determination, evasion of defense mediated by salicylic acid and in suppression of antiviral RNA silencing (Lewsey et al., 2010). It also perturbs silencing-mediated regulation of host transcripts. It was revealed that the 2b protein acts synergistically with some other CMV product(s) to induce symptoms and that the role of the 2b protein in symptom determination is host species-specific.
CMV has the widest host range of any virus and is one of the most damaging viruses of temperate agricultural crops worldwide (Gallitelli, 2000). It is also emerging as a major virus, especially in the tropics. CMV has been identified as the causal agent of several recent epidemics in crops such as tomatoes in Spain, Italy, Iran and Japan, bananas in Morocco and Central America (Palukaitis et al., 1992), lupins and capsicums in Australia. The virus has devastated high-value vegetable crops in China (Tien and Wu, 1991). There is no specific information currently available on yield-loss estimates in vegetables affected by CMV, but the total devastation that occurred recently in tomatoes in Europe, was at least due to one CMV satellite RNA (Gallitelli, 2000).