Viroids
Viroids are infectious agents composed of short strands of circular RNA without a protein coat. They are smaller than viruses and lack the ability to code for proteins, relying on the host cell’s machinery for replication. Viroids primarily affect plants and can cause a range of diseases. Below is a detailed overview of viroids and the diseases they cause:
Key Characteristics of Viroids:
- Structure: Composed of small, circular RNA (usually 246–399 nucleotides in length).
- Lack of Protein Coat: Unlike viruses, viroids do not have a protein capsule (capsid).
- Replication: They replicate within the host cell’s nucleus or chloroplast using the host’s RNA polymerase.
- Host Specificity: Viroids typically affect plants but have a very narrow host range.
Common Viroid Diseases:
Disease | Viroid | Affected Plants | Symptoms | Transmission | Control Measures |
Potato Spindle Tuber Disease | Potato spindle tuber viroid (PSTVd) | Potatoes, tomatoes, peppers, eggplants, tobacco | Stunting, spindle-shaped tubers, chlorosis, malformed leaves | Mechanical transmission, vegetative propagation, infected seed tubers | Use certified disease-free seed, heat treatment of seed tubers |
Citrus Exocortis | Citrus exocortis viroid (CEVd) | Citrus (oranges, lemons, mandarins, grapefruit) | Gummosis (sap leakage), bark scaling, stunted growth, leaf chlorosis | Seed, budwood, and rootstock transmission | Use virus-free planting material, quarantine infected plants |
Coconut Cadang-Cadang Disease | Coconut cadang-cadang viroid (CCCVd) | Coconut palms | Yellowing, premature fruit drop, stunted growth, necrosis | Spread by mealybugs and infected plant material | Infected plants must be removed, use healthy planting material |
Tomato Apical Necrosis Disease | Tomato apical necrosis viroid (TANVd) | Tomatoes | Necrosis of the apical meristem, stunting, leaf distortion | Mechanical transmission, infected plant material | Use disease-free seeds, sanitation measures |
Avocado Sunblotch | Avocado sunblotch viroid (ASBVd) | Avocados | Sunken spots on fruit, deformity, yellowing of leaves | Seed, budwood transmission | Use certified healthy planting material, remove infected trees |
Chili Vein Banding Disease | Chili vein banding viroid (CVBVd) | Chilies, peppers | Vein banding, mosaic, and chlorotic lesions on leaves | Mechanical transmission, infected seeds and plant parts | Use disease-free seeds, sanitation measures |
Details on Specific Viroid Diseases:
- Potato Spindle Tuber Disease (PSTVD):
- Viroid: Potato spindle tuber viroid (PSTVd)
- Hosts: Potatoes, tomatoes, peppers, and other solanaceous crops.
- Symptoms: This disease causes stunted growth, malformed tubers, and characteristic spindle-shaped tubers that are often small, with yellowing leaves.
- Transmission: Primarily transmitted through mechanical means, including infected seed tubers or plant material.
- Control: Using certified disease-free seed tubers, applying heat treatment for seeds, and avoiding the use of infected plant material.
- Citrus Exocortis:
- Viroid: Citrus exocortis viroid (CEVd)
- Hosts: Citrus plants (e.g., oranges, lemons, mandarins).
- Symptoms: Gummosis (exudation of sap), bark scaling, stunting of plants, and leaf chlorosis.
- Transmission: Seed, budwood, and rootstock are common vectors for transmission.
- Control: Quarantine infected plants, use certified disease-free rootstocks, and implement strict sanitation practices.
- Coconut Cadang-Cadang Disease:
- Viroid: Coconut cadang-cadang viroid (CCCVd)
- Hosts: Coconut palms.
- Symptoms: The disease results in yellowing and premature fruit drop, stunted growth, and necrosis in severe cases.
- Transmission: Spread by mealybugs and infected coconut palms.
- Control: Infected palms should be removed, and only healthy planting material should be used.
- Tomato Apical Necrosis Disease:
- Viroid: Tomato apical necrosis viroid (TANVd)
- Hosts: Tomatoes.
- Symptoms: Characterized by necrosis at the tip of the growing shoot, stunted plant growth, and distorted leaves.
- Transmission: Mechanical transmission and infected plant material are the primary means of spread.
- Control: Use disease-free seeds, disinfect tools, and remove infected plants.
- Avocado Sunblotch:
- Viroid: Avocado sunblotch viroid (ASBVd)
- Hosts: Avocados.
- Symptoms: The disease causes sunken spots and deformed fruit, as well as yellowing leaves.
- Transmission: The viroid is transmitted through seed and budwood.
- Control: Remove infected trees, and use certified healthy planting material.
- Chili Vein Banding Disease:
- Viroid: Chili vein banding viroid (CVBVd)
- Hosts: Chilies and peppers.
- Symptoms: Vein banding, mosaic patterns, and chlorotic lesions on leaves.
- Transmission: It is mechanically transmitted through infected seeds and plant parts.
- Control: Use clean, disease-free seeds and follow proper sanitation measures.
