Citrus canker is a bacterial disease that affects citrus plants like oranges, lemons, and limes. It is found in almost every country where these plants grow. This disease spreads quickly and harms the quality of the fruit, making it a serious issue for farmers and markets. To manage it, effective practices and strict rules are necessary.
Causal Organism:
The causal organism of citrus canker is the bacterium Xanthomonas citri (also referred to as Xanthomonas axonopodis pv. citri and Xanthomonas axonopodis pv. aurantifolii). It is a Gram-negative, rod-shaped, motile bacterium with a single polar flagellum. It forms yellow, smooth, mucoid colonies and is aerobic.
Symptoms of Citrus Canker
Citrus canker causes raised, brown, corky lesions with yellow halos on leaves, stems, and fruits. Lesions start as small green spots, turn grayish-white, and develop sunken centers, ranging from 2-10mm on leaves and up to 1cm on fruit and twigs. Visible 7-10 days post-infection, they persist on twigs for years, aiding bacterial survival. Severe infections weaken trees, causing premature fruit drop, scabbing, and deformation, though internal fruit quality remains unaffected.
Disease cycle:
The disease development involves the bacterium Xanthomonas, which infects plants through stomata or wounds caused by injuries or feeding activities. Infected twigs with old lesions are primary sources of infection, oozing bacteria that spread via wind, rain, or human activities. Once inside the host, the bacteria multiply in intercellular spaces, dissolve the middle lamella, and establish in the cortex. Lesions become visible on leaves within 7 to 10 days. Warm, rainy weather facilitates bacterial spread, with high temperatures and rainfall favoring disease severity. The bacteria can overwinter in lesions and cause multiple infection cycles, especially on fruits, leading to lesions of various sizes.
Control measures for Citrus Canker Disease:
Control measures and management strategies for citrus canker include a combination of cultural, chemical, and regulatory approaches to effectively manage and prevent the spread of the disease. Here are the key strategies:
Cultural Control Measures:
- Removal and Destruction of Infected Plant Parts:
- Avoid Overhead Irrigation: This reduces the spread of bacteria through water splash.
- Pruning During Dry Season: Prune infected parts during dry periods and apply protective sprays like 1% Bordeaux mixture.
- Windbreaks: Planting windbreaks can help reduce the dispersal of bacteria by wind.
- Resistant Cultivars: Replace susceptible citrus varieties with resistant or moderately resistant cultivars.
Chemical Control Measures:
- Copper-Based Sprays: Apply copper-based fungicides such as Kocide 3000 or copper oxychloride at recommended intervals to reduce bacterial spread.
- Antibiotics: Use antibiotics like streptomycin sulphate or phytomycin for bacterial control.
- Regular Spraying: Implement a regular spraying schedule, especially during periods conducive to disease development.
Regulatory and Quarantine Measures:
- Strict Quarantine: Enforce strict quarantine regulations to prevent the import of infected plant material from non-canker-free regions.
- Eradication Efforts: In canker-free areas, eradicate infected and adjacent trees to prevent the spread of the pathogen.
Integrated Management:
- Leaf Miner Control: Manage citrus leaf miner populations, as their feeding activity creates wounds that facilitate bacterial entry.
- Monitoring and Forecasting: Regularly monitor orchards for symptoms and use disease forecasting to anticipate and manage outbreaks.
By combining these strategies, citrus growers can effectively manage citrus canker and minimize its impact on citrus production.