Blister Blight of Tea

Causal Organism: Exobasidium vexans (a fungal pathogen)

Symptoms:

1. Initial Stage: Small, pale, translucent spots appear on the young leaves, usually on the upper surface. These spots are round, water-soaked, and about 2-5 mm in diameter.
2. Blister Formation: Spots enlarge and form blister-like swellings on the lower surface of the leaves. The upper surface shows a corresponding depression.
3. Advanced Stage: Blisters turn white due to the formation of a whitish fungal layer on the lower surface. Infected areas become necrotic, turning brown and eventually black.
4. Leaf Distortion and Defoliation: Severe infection causes leaf distortion, curling, and premature leaf fall. This affects the quality and yield of tea leaves.
5. Other Plant Parts: Young stems, buds, and tender shoots can also be affected, leading to stunted growth.

Pathogen Details:

• Fungal Structure: The fungus produces basidiospores on the blister surface, which are responsible for spreading the disease. Basidiospores are produced in large numbers during humid and rainy weather.
• Environmental Conditions Favoring Disease: High humidity (>80%) and moderate temperatures (18°C to 25°C). Frequent rainfall and cloudy weather promote sporulation and infection.

Mode of Spread and Survival:

1. Spread: Airborne basidiospores spread the disease to nearby plants. Spores are dispersed by wind, rain splashes, and insects.
2. Survival: The pathogen survives on infected plant debris and can remain viable during unfavorable conditions. Young, tender leaves are more susceptible than mature leaves.

Impact on Tea Cultivation:

• Causes significant yield losses due to defoliation and reduced leaf quality.
• Economic losses occur due to decreased production of tea shoots, impacting tea industries.

Management Practices:

Cultural Practices:

• Pruning and Shade Management: Proper pruning to remove infected leaves and improve air circulation. Manage shade trees to reduce humidity around the tea bushes.
• Sanitation: Collect and destroy fallen infected leaves to reduce the source of inoculum. Maintain field hygiene to prevent disease spread.

Chemical Control:

• Fungicide Sprays: Spray Copper oxychloride (0.25%) or Chlorothalonil (0.2%) at 10-14 day intervals during the rainy season. Hexaconazole (0.02%) and Carbendazim (0.05%) are effective in controlling the disease.
• Preventive Sprays: Start preventive sprays at the onset of the rainy season. Rotate fungicides to avoid the development of resistance in the pathogen.

Biological Control: Trichoderma spp. and Pseudomonas fluorescens can be applied to reduce disease severity. Biological control is still under research and not widely adopted.

Resistant Varieties: Planting resistant tea cultivars like TV-26, TV-27, and UPASI-9 can reduce disease incidence. Breeding programs aim to develop more resistant varieties.

Integrated Disease Management (IDM): Combine cultural, chemical, and biological control methods for effective disease management. Regular monitoring and scouting for early detection of symptoms.