1. Overview of Citrus Canker
Citrus canker is a severe bacterial disease affecting citrus plants such as oranges, lemons, limes, and grapefruits. It is caused by a bacterium called Xanthomonas citri belonging to the genus Xanthomonas, which leads to lesions on leaves, stems, and fruits, reducing fruit quality and yield.
2. Scientific Classification of the Bacterium Xanthomonas citri
The primary causative agent of citrus canker is Xanthomonas citri pv. citri (Xcc). However, there are other pathovars responsible for different types of citrus canker. The taxonomic classification of the bacterium is as follows:
| Taxonomic Rank | Classification |
|---|---|
| Kingdom | Bacteria |
| Phylum | Proteobacteria |
| Class | Gammaproteobacteria |
| Order | Xanthomonadales |
| Family | Xanthomonadaceae |
| Genus | Xanthomonas |
| Species | Xanthomonas citri |
| Pathovar | Xanthomonas citri pv. citri |
Other synonyms for Xanthomonas citri include:
- Xanthomonas campestris pv. citri
- Xanthomonas axonopodis pv. citri
- Pseudomonas citri (previous classification)
The bacterium is classified as a quarantine pest in many citrus-producing countries due to its economic impact.
3. Morphology and Structure of Xanthomonas citri Bacterium
- Xanthomonas citri is a rod-shaped, Gram-negative bacterium.
- It measures approximately 1.5–2.0 × 0.5–0.75 µm.
- The bacterium has a single polar flagellum, which enables motility.
- It forms yellow colonies in culture due to the production of a characteristic xanthomonadin pigment.
- In laboratory media, when glucose or other sugars are added, colonies become mucoid due to the production of extracellular polysaccharides.
- The bacterium is obligately aerobic (requires oxygen for growth).
- Optimum growth temperature: 28–30°C.
- Maximum temperature tolerance: 35–39°C.
4. Genome and Genetic Features Xanthomonas citri Bacterium
- The genome of Xanthomonas citri pv. citri is approximately 5 megabase pairs (Mb) long.
- The genome consists of genes responsible for virulence, adhesion, and survival under different environmental conditions.
- Pathogenicity-related genes: The bacterium produces effector proteins that help in plant infection by manipulating plant defense responses.
- The hrp (hypersensitive response and pathogenicity) gene cluster is essential for the bacterium to infect citrus plants.
- The bacterium also has genes responsible for the production of exopolysaccharides, which aid in biofilm formation and colonization.
5. Major Pathovars of Xanthomonas citri
The bacterium Xanthomonas citri has several pathovars and strains that cause different forms of citrus canker. The major pathovars include:
| Pathovar | Type of Canker | Geographical Distribution | Primary Host Plants |
|---|---|---|---|
| Xcc (Pathotype A) | Asiatic citrus canker (Canker A) | Asia, USA, Brazil, Australia, India, China, and other tropical citrus-growing regions | Most citrus species (oranges, lemons, limes, grapefruits) |
| XauB (Pathotype B) | Cancrosis-B | Argentina, Paraguay, Uruguay | Primarily affects lemons, minor infections in sour orange, pomelo, and Mexican lime |
| XauC (Pathotype C) | Cancrosis-C | Brazil | Affects only key limes and bitter oranges |
| XauD (Pathotype D) | Previously thought to be a citrus canker strain, later identified as caused by Alternaria limicola | – | – |
| XauE (Pathotype E) | Previously classified as citrus canker but later reclassified as Citrus Bacterial Spot (CBS) | Florida, USA | Affects various citrus species |
These pathovars can be distinguished using:
- DNA sequencing
- PCR-based assays
- Serological tests like ELISA
- Biochemical and molecular techniques
6. Disease Mechanism and Pathogenicity
The infection process occurs in the following stages:
- Entry into the Plant:
- The bacterium enters through natural plant openings (stomata, lenticels) or wounds caused by wind, rain, or insect feeding.
- Rain, wind-driven water droplets, and contaminated tools help in bacterial dissemination.
- Colonization and Multiplication:
- Once inside, the bacterium multiplies in the intercellular spaces of plant tissues.
- It produces exopolysaccharides that help in attachment and colonization.
- Lesion Formation:
- The bacterium induces cell necrosis, leading to the formation of characteristic corky, raised lesions on leaves, stems, and fruits.
- These lesions ooze bacterial exudate, which further spreads the disease.
- Disease Spread:
- The bacterium spreads through wind-driven rain, contaminated tools, human handling, and insects.
- Hurricanes, typhoons, and cyclones significantly increase its spread over long distances.
7. Historical Background
- 1912: First identified in Florida, USA.
- 1915: Officially described by Hasse as Pseudomonas citri.
- 1939: Reclassified as Xanthomonas citri.
- 1978: Moved to Xanthomonas campestris pv. citri.
- 1995: Reclassified again as Xanthomonas axonopodis pv. citri.
- 2000s–Present: The most accepted classification is Xanthomonas citri pv. citri.
The geographical origin of citrus canker is debated:
- Some researchers suggest southern China as its origin.
- Others propose India and Indonesia, based on herbarium records dating back to 1827.
8. Methods of Detection and Identification
Several techniques are used to detect Xanthomonas citri, including:
- Serological Methods:
- ELISA (Enzyme-Linked Immunosorbent Assay)
- Protein electrophoresis
- Molecular Techniques:
- PCR-based assays (Polymerase Chain Reaction)
- DNA sequencing
- Restriction Fragment Length Polymorphism (RFLP) Analysis
- Pathogenicity Tests:
- Inoculating susceptible plants (e.g., grapefruit, Mexican lime) and monitoring lesion formation.
9. Control and Management
Since Xanthomonas citri is a quarantine pathogen, strict measures are taken to prevent its spread:
- Quarantine Regulations:
- Infected plants and nursery stock are destroyed to prevent further spread.
- Cultural Practices:
- Avoid planting citrus in areas with a history of citrus canker.
- Use disease-free planting material.
- Chemical Control:
- Copper-based bactericides are commonly used to manage the disease.
- Biological Control:
- Research is ongoing for the use of bacteriophages (viruses that infect bacteria) and antagonistic bacteria.
- Resistant Varieties:
- Developing genetically resistant citrus varieties through breeding programs.
Conclusion
Citrus canker, caused primarily by Xanthomonas citri pv. citri, is a significant disease affecting citrus crops worldwide. Due to its rapid spread through environmental factors and human activities, it poses a major threat to citrus production. Continued research and strict phytosanitary measures are necessary to manage and control this devastating disease.