Citrus Canker
Causal Organism
Name: Xanthomonas citri subsp. citri (previously known as Xanthomonas axonopodis pv. citri).
Symptoms
1. Lesions on Leaves:
- Initially appear as small, raised, water-soaked spots.
- Lesions become necrotic, raised, and corky with a brown to yellow halo.
- Frequently appear on the underside of leaves and can coalesce to form larger necrotic areas.
2. Lesions on Fruits:
- Small, round, water-soaked spots that become raised and corky.
- Lesions are surrounded by a characteristic yellow halo.
- Severe infections can lead to premature fruit drop.
3. Lesions on Stems:
- Corky, raised lesions similar to those on leaves and fruits.
- Can cause dieback of twigs and branches in severe cases.
4. Other Symptoms:
- Defoliation.
- Reduced fruit quality and yield.
Etiology
Citrus Canker is caused by the bacterium Xanthomonas citri subsp. citri, which is a gram-negative, rod-shaped bacterium. It is highly infectious and spreads rapidly under favorable conditions.
Disease Cycle
1. Infection:
- Bacteria enter through natural openings (like stomata) or wounds on leaves, stems, and fruit.
- Infection is facilitated by wet conditions, including rain, dew, and high humidity.
2. Colonization:
- Once inside the plant tissue, the bacteria multiply and cause cell death around the infection site, leading to the characteristic lesions.
3. Dissemination:
- Bacteria are spread by wind-driven rain, overhead irrigation, and mechanical means such as pruning tools and contaminated clothing.
- Insect vectors, such as the citrus leaf miner, can exacerbate the spread by creating wounds that facilitate bacterial entry.
4. Overwintering:
- The bacterium can survive in lesions on infected plant tissue and in the soil, providing a source of inoculum for the next growing season.
Management
1. Cultural Practices:
- Use of resistant or tolerant citrus varieties.
- Regular inspection and removal of infected plant material.
- Avoiding overhead irrigation to reduce leaf wetness.
2. Chemical Control:
- Application of copper-based bactericides to protect new growth from infection.
- Copper sprays should be applied during periods of active growth and before and after periods of rainfall.
3. Sanitation:
- Sterilization of pruning tools and equipment.
- Washing hands and clothing after working with infected plants.
4. Regulatory Measures:
- Quarantine measures to prevent the movement of infected plant material.
- Eradication programs in areas where Citrus Canker is not yet established.
5. Biological Control:
- Use of antagonistic bacteria and other biocontrol agents, though still under research and not widely implemented.