Huanglongbing (HLB) or Citrus Greening Pathogen

Citrus Greening

  • Candidatus Liberibacter spp. are phloem-limited plant pathogenic bacteria.The phloem system of the plant transports sugars, which are the food source of the plant, bidirectionally through the plant. The phloem system of the plants transports the products of photosynthesis (sugars) from sources of photosynthetic activity (leaves) in the plant to sinks (flowers, fruits, roots, seeds).

  • Psyllids are the main means of transmission to plants. An infected psyllid feeds on a healthy tree and injects the bacterium into the phloem. Once a tree is infected with the bacterium, there is no known cure for the disease. This in part is because the bacterium is inside the vascular system of the plant (systemic) and is therefore very difficult to access.

  • The bacterium is Gram-negative. Part of the evidence used to show that the bacterium is Gram-negative was the fact there were two membranes (figure 2), outer and cytoplasmic, and a thin peptidoglycan layer in between the cell walls seen in electron micrographs.

  • At this time, greening has not been cultured, but ongoing research shows promise this will be accomplished. Once the bacterium has been isolated in pure culture plant pathologists will be able to complete Koch's postulates (pathogenicity tests). Completetion of Koch's postulates will allow plant pathologists to definitively say Ca. Liberibacter sp. causes the symptoms of HLB.

Liberibacter cells gram negative  
Figure 1. Electron micrograph of Ca. Liberibacter cells in a sieve tube of sweet orange leaf in Saudi Arabia Figure 2. The peptidoglycane layer of the Gram negative cell wall can be visualized (PG) in between the inner membrane cytoplasmic membrane (CM) and the outer cell wall membrance (OM).  


Photo Credit: Huanglongbing: A Destructive, Newly-Emerging, Century-Old Disease of Citrus, J.M. Bové

Various Species of Ca. Liberibacter

FORM
VECTOR
HEAT TOLERANCE
Asian
Diaphorina citri (psyllid)*
heat tolerant
African
Trioza erytreaae (psyllid)*
heat-sensitive
American
Diaphorina citri ( psyllid)
heat tolerant

*It has been shown experimentally that the two psyllids can transmit both the Asian and African forms, but the preferential vector is shown above.

Various Names of HLB (Huanglongbing) throughout the World

HLB became the official name because it was the first recorded in the literature.

  • South Africa: greening
  • Philippines: mottle leaf
  • India: dieback
  • Indonesia: vein phloem degeneration
  • China: Huanglongbing (yellow shoot or yellow branch disease)
  • Taiwan: likubin

Links

 
• History • Regulations
• Transmission • Pathogen
• Alternate Hosts • Symptoms
• Nutrient Deficiencies Compared to Citrus Greening • Diagnostics
• Management • Photo Gallery
• Links • Contacts
• HLB Scion Survey