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:

Details on Specific Viroid Diseases:

1. Potato Spindle Tuber Disease (PSTVD):

1. • 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.

2. Citrus Exocortis:

1. • 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.

3. Coconut Cadang-Cadang Disease:

1. • 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.

4. Tomato Apical Necrosis Disease:

1. • 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.

5. Avocado Sunblotch:

1. • 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.

6. Chili Vein Banding Disease:

1. • 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.