Cucumber Mosaic Virus (CMV) is recognized as one of the most economically significant plant viruses worldwide.
It infects over 1,200 plant species across more than a hundred different plant families.
CMV has been isolated as a cause of major yield losses in crops such as cucumber, tomato, pepper, lettuce, banana, melon, spinach, and tobacco.
The virus is primarily transmitted by aphid vectors in a non-persistent manner.
It can also spread through infected seeds, mechanical contact, and weed reservoirs.
CMV infection results in symptoms like growth stunting, mosaic patterns on leaves, fruit deformation, and reduced productivity.
Due to these impacts, CMV poses a serious threat to farmers around the world.
Causal Organism
Cucumber Mosaic Virus (CMV) is a plant virus classified under the genus Cucumovirus in the family Bromoviridae.
The virus is highly versatile, with several strains differing in virulence, host range, and the severity of symptoms they cause.
CMV has an icosahedral structure, giving it a spherical shape with an approximate diameter of 28–30 nanometers.
Unlike many other plant viruses, CMV lacks a lipid envelope, enhancing its stability in plant sap and allowing it to survive for several months in infected debris.
This structural stability enables CMV to be easily transmitted through mechanical contact between plants.
The CMV genome comprises three single-stranded, positive-sense RNA segments:
RNA1 encodes an RNA-dependent RNA polymerase, essential for virus replication.
RNA2 encodes movement proteins that help the virus move within plant cells and throughout the host plant.
RNA3 encodes the capsid (coat) protein and an additional movement protein, crucial for virus stability and aphid transmission.
CMV also produces a subgenomic RNA4, which directs the synthesis of the coat protein necessary for forming new virus particles.
The coat protein plays a major role in enabling aphid-mediated transmission by interacting with the insect’s mouthparts during feeding.
Some CMV strains also carry satellite RNA (satRNA), a genetic element that does not encode any protein.
The presence of satellite RNA can significantly influence the disease’s nature, leading either to milder symptoms or causing highly aggressive disease characterized by severe stunting and necrosis in infected plants.
Symptoms of Cucumber Mosaic Virus (CMV)
Leaf Symptoms
One of the most characteristic symptoms of CMV infection is leaf mosaic, where irregular patches of light and dark green appear on the leaves.
The mosaic pattern is often accompanied by mottling, chlorosis (yellowing), and leaf distortion.
Infected leaves may become wrinkled, curled, or crinkled, reducing their ability to perform photosynthesis efficiently.
In some plants, such as tomatoes, CMV causes the "shoestring effect," where leaves become narrow, thread-like, and severely deformed.
These symptoms weaken the plant, making it more susceptible to secondary infections and environmental stress.
Growth and Developmental Symptoms
Affected plants often exhibit stunted growth, with shortened internodes and reduced leaf size.
In severe cases, infected plants appear dwarfed and bushy, failing to reach their normal height.
The virus interferes with flower development, causing delayed flowering or flower abortion.
This disruption reduces the plant’s ability to set fruit, directly impacting yield.
In crops like tobacco and tomato, CMV causes general weakening of the plant, increasing vulnerability to environmental stressors such as drought or nutrient deficiencies.
Fruit Symptoms
Fruit quality and quantity are significantly affected by CMV infection.
In cucumbers and melons, infected fruits often develop deformations, wart-like growths, and uneven shapes.
Fruit surfaces may show yellowish or white streaks, color-breaking patterns, or necrotic spots, making them unmarketable.
In tomatoes, CMV infection leads to uneven ripening, discoloration, and poor fruit set, reducing the crop’s commercial value.
Some infected fruits may also be smaller in size and tasteless, further decreasing their market appeal.
In crops like peppers and squash, fruit symptoms may include rough texture, cracking, and internal browning, making them unsuitable for sale or consumption.
CMV infection has long-term effects on crop productivity, with reduced photosynthesis, stunted growth, poor fruit formation, and premature plant senescence leading to significant yield losses.
Infected plants become more susceptible to secondary infections by bacteria, fungi, and other viruses, further deteriorating plant health.
Farmers often face economic losses due to reduced marketable yield, poor fruit quality, and increased production costs for managing infected fields.
Disease Cycle of Cucumber Mosaic Virus (CMV)
Virus Acquisition and Transmission by Aphids
The major mode of transmitting CMV is through aphids.
More than 80 species of aphids, including the green peach aphid (Myzus persicae) and the melon aphid (Aphis gossypii), can transmit CMV.
Acquisition occurs when an aphid feeds on an infected plant and picks up the virus.
The virus attaches to the stylet of the aphid but does not circulate within the insect’s body.
CMV transmission is non-persistent, meaning the aphid can immediately transfer the virus to another plant during feeding but loses the virus after several feedings.
This rapid transmission mechanism allows CMV to spread quickly over both short and long distances, especially when aphid populations are large.
Penetration and Primary Infection
After penetrating a new host, the virus enters epidermal cells at the feeding site and begins to infect the host plant.
CMV, with its single-stranded RNA genome, is uncoated within the cell and hijacks the plant’s cellular machinery to produce viral replication proteins.
The virus starts replicating, creating many copies of itself.
At this early stage, CMV remains limited to a small number of cells near the infection site.
Within a few days, the virus starts moving through plant tissues.
Replication and Cell-to-Cell Movement
Inside the plant cells, CMV uses a special enzyme called RNA-dependent RNA polymerase, which is encoded within the virus, to replicate.
After producing sufficient amounts of virus particles, CMV begins spreading cell-to-cell through plasmodesmata.
Plasmodesmata are microscopic channels between plant cells that allow the virus to spread.
CMV produces a movement protein that expands the plasmodesmata, enabling the virus to move freely between cells.
The virus causes the development of the first symptoms, including mosaic patterns, leaf distortion, and chlorosis (yellowing of the infected area).
Systemic Infection
When CMV infects the plant’s vascular system, specifically the phloem, it moves very quickly throughout the entire plant.
The virus spreads to young leaves, stems, flowers, and fruits, leading to severe mosaic symptoms, stunted growth, and fruit deformities.
At this stage, the infection becomes systemic, affecting the whole plant rather than remaining localized.
The infection often results in weakened plants, reduced photosynthesis, and poor fruit set.
Crop yield and quality are enormously affected due to the systemic infection.
Secondary Spread
CMV-infected plants act as virus reservoirs, further spreading the disease.
The spread becomes most significant when aphids continue feeding on infected plants and transmit the virus to new plants.
Once infected, plants can also spread CMV through mechanical means, such as contaminated farming tools, human handling, and direct plant-to-plant contact.
CMV can also be transmitted through seeds in a few crops like lettuce, beans, and cucumbers, where the virus from infected seeds is passed to the next generation of plants.
Survival during the Off-Season
CMV does not have an overwintering stage but survives in weed reservoirs and through seeds and debris of infected plants during the off-season.
The virus can be present in many weeds and alternative host plants, allowing it to survive between growing cycles.
When new crops are planted, aphids feeding on these infected weeds or volunteer plants pick up the virus and introduce it to fresh, healthy plants.
This process restarts the disease cycle.
Disease Management of Cucumber Mosaic Virus (CMV)
CMV Management Challenges:
CMV is difficult to manage due to its broad host range, ease of spread through vectors, and survival in weed reservoirs and seeds.
No direct cure is available for viral infections in plants, so preventive measures are essential.
The best management practices include vector control, cultural practices, and using resistant crop varieties. A combination of these strategies helps reduce virus spread and crop losses.
Aphid Population Control or Management:
Aphids are the primary vector for CMV, so managing aphid populations can help prevent further spread.
Insecticide-based strategies are often ineffective, but reduced aphid populations can slow virus activity.
Integrated pest management (IPM) strategies, such as reflective mulches, row covers, and natural predators (ladybugs and lacewings), help control aphids.
Mineral oils or insecticidal soaps can disrupt aphids' ability to feed and transfer viruses.
Cultivation of Resistant and Tolerant Crop Varieties:
Resistant or tolerant crop varieties are the best long-term strategy for managing CMV.
Some tomato and pepper varieties are bred for resistance or tolerance, minimizing symptoms and yield loss.
Resistance is unavailable for crops like cucumbers and melons, but partially resistant or tolerant varieties can still minimize symptoms and losses.
Elimination of Infected Plants and Weed Management:
CMV can survive in infected plants and weeds, so removing virus sources is critical.
Infected plants should be immediately identified and removed to prevent further spread.
Proper sanitation practices, such as disinfecting tools and avoiding contact with infected plants, help reduce mechanical transmission.
Cultural Methods:
Crop rotation with non-hosts reduces the opportunities for virus reservoirs to persist.
Avoid planting during peak aphid activity to reduce infection rates.
Reflective mulches prevent aphids from settling on plants, thus reducing infection.
In greenhouses, using insect-proof screens and double-door entry systems helps prevent aphid infestations.
Proper fertilization and irrigation improve plant health and enhance resistance to viral infections.
Reducing Mechanical Transmission:
CMV can be transmitted through human handling, contaminated tools, and farm equipment.
Avoid touching infected plants and then handling healthy ones to prevent mechanical transmission.
Disinfect pruning tools and wash hands after working with plants to prevent accidental virus spread.
Biological Control:
Beneficial organisms, like ladybugs, lacewings, and parasitic wasps, can help control aphid populations and reduce CMV spread.
Some studies use induced resistance by triggering plant defense responses using chemicals or beneficial microbes.
Chemical and Organic Treatments:
No chemical treatments exist for CMV itself, but natural compounds may reduce virus spread.
Plant-based oils (e.g., neem oil, mineral oils) may deter aphids from feeding on plants, thus reducing virus transmission.
Antiviral plant extracts or biological treatments can increase plant immunity, but their effectiveness varies by crop and virus strain.
Seed Testing and Certification:
Since CMV can be seed-borne, planting certified virus-free seeds is crucial.
Molecular tests (RT-PCR or ELISA) are used to detect CMV in seeds, preventing its spread.
Farmers should buy seeds from reliable sources, as seeds harvested from diseased plants can unknowingly transmit the virus to future crops.
References
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