Citrus Greening (Huanglongbing)

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Citrus Greening (Huanglongbing)

            Caused by the bacterium Candidatus Liberibacter, primarily Candidatus Liberibacter asiaticus (most common), africanus, and americanus.




  • Domain: Bacteria
  • Phylum: Pseudomonadota (formerly Proteobacteria)
  • Class: Alphaproteobacteria
  • Order: Rhizobiales
  • Family: Rhizobiaceae
  • Genus: Candidatus Liberibacter
  • Species:
    • Candidatus Liberibacter asiaticus (most widespread and virulent species)
    • Candidatus Liberibacter africanus (found mainly in Africa)
    • Candidatus Liberibacter americanus (less common, found in parts of Brazil).

Transmission:

  • The primary vector is the Asian citrus psyllid (Diaphorina citri), which transmits the bacterium while feeding on the sap of citrus trees.
  • Once a tree is infected, the bacterium moves through the phloem, affecting the plant's ability to transport nutrients.

Pathogenicity of Citrus Greening (HLB):

Intracellular Colonization:

  • Candidatus Liberibacter is an obligate intracellular pathogen, meaning it lives and replicates within the host plant’s phloem cells (nutrient-conducting vessels).
  • It has evolved to evade plant defense mechanisms by remaining hidden within the plant’s vascular tissue, making it difficult for the plant to mount an effective immune response.

Systemic Infection:

  • The bacterium moves through the plant’s phloem, colonizing various parts of the tree, including the roots, leaves, and fruits.
  • As a phloem-limited bacterium, it spreads through the sieve tubes, hindering nutrient transport across the plant.

Impact on Root System:

  • The bacterium affects the root system early in the infection, even before above-ground symptoms are visible.
  • Root growth is stunted, and the roots' ability to take up water and nutrients is compromised, further intensifying the decline of the tree.

Symptoms:

  •    Leaf Yellowing: Leaves show blotchy yellowing that resembles nutrient deficiencies but is uneven and asymmetrical.
  •  Stunted Growth: Infected trees show a decline in overall health, with slow, stunted growth.
  • Misshapen Fruits: Fruits are small, misshapen, and often bitter or sour, with some areas remaining green even after ripening.
  • Fruit Drop: Affected trees often experience premature fruit drop, reducing yield.
  •  Root Damage: The disease weakens the root system, leading to poor nutrient uptake.

Management Strategies:

  • Insect Control: Managing populations of the Asian citrus psyllid through chemical, biological, and cultural methods is critical.
  • Removal of Infected Trees: Prompt removal of infected trees helps reduce the spread.
  • Resistant Varieties: Research is ongoing to develop citrus varieties that are resistant to HLB.
  • Quarantine Measures: Movement of citrus plants and materials is often restricted to prevent the spread of both the bacterium and its vector.

 

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