Write the diagnostic symptoms of potato spindle tuber disease, its mode of transmission and management.

Diagnostic Symptoms of Potato Spindle Tuber Viroid (PSTVd)

• Aboveground Symptoms: Initially, symptoms are subtle. Young plants may exhibit stunted growth, chlorosis (yellowing) of leaves, and distorted leaves, resembling nutrient deficiencies. Stems can show thickened areas or "spindle" shape.
• Belowground Symptoms: The most characteristic symptom is the formation of spindle-shaped, cracked, and deformed tubers. These tubers are often unmarketable. The internal flesh may be discolored.
• Variability: Symptom expression varies depending on the potato variety, PSTVd strain, and environmental conditions. Some varieties are more susceptible than others.

Mode of Transmission

• Mechanical Transmission: PSTVd is primarily transmitted through mechanical means, such as contaminated tools, equipment, and hands. This is the most common route of spread.
• Vegetative Propagation: Transmission occurs readily through infected seed potatoes, tubers, and cuttings used for vegetative propagation. This is a major source of initial infestation.
• Insect Vectors: While not considered primary vectors, aphids and other sucking insects can potentially transmit the viroid mechanically by carrying the viroid particles on their bodies.
• Seed Transmission: The viroid can be transmitted through seed tubers even if they show no visible symptoms, making detection challenging.

Management Strategies

Preventative Measures

• Certified Seed Potatoes: Using certified, disease-free seed potatoes is the most critical preventative measure.
• Biosecurity Protocols: Strict biosecurity measures should be implemented, including disinfecting tools, equipment, and footwear before entering and leaving potato fields.
• Crop Rotation: Rotating potato crops with non-solanaceous crops can help reduce the inoculum load in the soil, although PSTVd persistence in the soil is relatively low.
• Quarantine Measures: Implementing strict quarantine regulations for importing potato planting material.
• Monitoring and Surveillance: Regular monitoring of potato fields for symptom expression and implementing testing programs.

Curative Measures

• Roguing: Removing and destroying infected plants promptly can help prevent the spread of the viroid within a field.
• Heat Treatment: Hot water treatment (45-50°C for 30 minutes) of seed potatoes can reduce PSTVd titer, although it may also reduce seed viability.
• Chemical Control: There are no specific chemical controls for PSTVd. Focus remains on preventative measures.
• Breeding for Resistance: Developing potato varieties with resistance or tolerance to PSTVd is a long-term solution, but progress is slow.

Potato Spindle Tuber Viroid poses a significant challenge to potato production globally, demanding vigilant management strategies. The disease’s efficient mechanical transmission and potential for seedborne spread necessitate stringent biosecurity protocols and the use of certified seed potatoes. Continued research focused on developing resistant varieties and improving diagnostic techniques is crucial for mitigating the economic impact of PSTVd and ensuring sustainable potato production. Early detection and rapid response are key to preventing widespread outbreaks.