Penyakit 'Greening Disease' Limau


Citrus Greening Disease (CGD) / Huanglongbing (HLB) A major threat to the citrus industry
 
What is Citrus Greening Disease (CGD)?
 
Citrus Greening Disease, also known as Huanglongbing (HLB), is one of the most devastating diseases of citrus plants worldwide. It is caused by a fastidious bacterium, Candidatus Liberibacter asiaticus.
The disease has had a severe impact on global citrus production, including in Malaysia. In Sarawak, the citrus industry, once thriving in the 1970s-80s, faced a major crisis following the arrival of CGD in 1988, leading to significant economic losses and threatening the livelihood of local farmers.
 
A close-up of a plantAI-generated content may be incorrect.
Close-up of a tree branch with leavesAI-generated content may be incorrect.
Examples of CGD symptoms: yellowing leaves, misshapen and bitter fruit.
 
How Does CGD Affect Citrus Plants?
The bacterium infects the phloem (the food-conducting tissues) of the plant, disrupting the flow of nutrients.
  • Leaves: Develop blotchy yellowing or mottling, often resembling zinc or magnesium deficiency. Leaves become smaller and can drop prematurely.
  • Fruit: Becomes small, lopsided, and fails to colour properly (remaining green at the stem end). The fruit tastes bitter and salty and is often dropped before harvest.
  • Tree Health:Branches gradually die back. The tree becomes stunted, produces minimal fruit, and eventually dies within a few years of infection.
How is the Disease Spread?



CGD is primarily spread in two ways:
  1. Insect Vector: The Asian citrus psyllid (Diaphorina citri) feeds on the sap of infected trees and carries the bacteria to healthy trees when it feeds on them.
  2. Infected Planting Material: Using buds, shoots, or marcots from infected trees to propagate new plants will guarantee the new plant is also diseased.
 
The Challenge of Detection: PCR vs. The Need for Speed
 
The gold standard for confirming CGD is the Polymerase Chain Reaction (PCR) test performed in a laboratory. This method detects the bacterial DNA with high accuracy but has significant limitations for field use:
  • Time-Consuming: Takes 24 hours – 7 working days from sample collection to result.
  • Expensive: specific kits and reagent required for analysis.
  • Requires Expertise and Equipment: Needs a fully equipped lab and trained technicians.
This delay between sampling and result means farmers cannot take immediate action, allowing the disease to spread further.
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The Sarawak Solution: SwiftScan Rapid Field Test Kit
To overcome the limitations of lab-based testing, researchers from the Agricultural Research Centre (ARC) Semongok, Sarawak Department of Agriculture, developed an innovative, low-cost, and rapid detection kit for use directly in the orchard.
 
The SwiftScan Rapid Field Test Kit



How Does the SwiftScan Kit Work?
The kit is based on a simple colorimetric principle, with the application of a reagent that reacts with the excess starch known to build up in the leaves of citrus trees infected with CGD.
  • After preparing the leaf sample, the provided reagent solution is applied.
  • A visible color change to a dark hue serves as a clear, easy-to-interpret indicator of a potential infection.
  • This allows for on-the-spot diagnosis within 5 minutes without needing a laboratory.
Advantages of the SwiftScan Kit
  • Rapid: Results in 5 minutes vs. 2-5 days for lab results.
  • Cost-Effective: <RM10 per sample compares to RM80 for molecular analysis.
  • Easy to Use: Designed for farmers and field officers with minimal training.
  • Portable:No complex equipment or electricity needed.
  • Proven Effective: Validation against PCR has shown an accuracy reaching 92.7% for mandarin varieties like Limau Madu and Langkat.
Management: How to Protect Your Citrus Trees
Start Clean:
  • Always plant CGD-free planting materials.
Remain Vigilant:
  • Even CGD-free trees are not immune.*They can become infected if the psyllid insect vector is present.
  • Use the SwiftScan kit for regular monitoring and early detection in your orchard.
  • Control psyllid populations with recommended insecticides (http://www.portal.doa.gov.my/sismarp/ )
  • Isolate new plantings away from existing, potentially infected citrus areas.
  • Immediately remove and destroy any tree confirmed to be infected to prevent the disease from spreading to healthy trees.
Together, through early detection and good management, we can secure the future of the citrus industry in Sarawak

(Contributor: Lai Lee San, Johnny Meddy anak Kesmed, Felecia Collick, Jisia Jimbau, Research Officer ARC Semongok)
 
For More Information, Contact:
Agricultural Research Centre (ARC), Semongok
Department of Agriculture Sarawak
Dr. Maclin Dayod (maclind@sarawak.gov.my)
Senior Assistant Director (Crop Research and Development Division)