Citrus Greening: A Species You Wood Not Expect To Be In Quarantine

In the midst of the COVID-19 pandemic, we have been forced to adapt to a completely new lifestyle in quarantine. However, another species – our friendly neighborhood citrus trees that have gone mainly unnoticed – has been experiencing this same dilemma for decades but for a completely different disease: The Citrus Greening Disease

What is the Citrus Greening Disease?

The Citrus Greening Disease, also known as HuangLongBing (HLB), is a bacterial disease. Unlike COVID-19 which is a viral disease, HLB does not impact humans. Instead, it targets species of the citrus genus, like lemon and orange trees. Although there are two strains of the disease originating from Asia and Africa, the Asian strand has spread to the United States through the introduction of the invasive small winged insect, Diaphorina Citri, commonly known as the Asian Citrus Psyllid. 

Since its first reporting in the United States in 2004, the disease has caused millions of dollars of damage to the citrus industry, severely affecting regions in Florida and California. Symptoms of the HLB include leaves bearing a blotchy mottled pattern and deformed fruit with discoloration. 

What caused the Citrus Greening Disease?

The causative bacteria of HLB is from the genus Candidatus Liberibacter (CLas). Candidatus refers to the inability to culture the bacteria, meaning that the bacteria cannot be made to grow in laboratories. This is a trait which makes studying how the bacteria works extremely difficult for scientists. Due to its difficulties with culturing, scientists mainly relied on a DNA probe that looks for RNA molecules specific to the bacteria in order to identify the bacteria. New research has innovated ways to culture CLas in laboratories, but there are still long ways to go until the molecular mechanisms of the bacteria are well understood. 

CLas Bacteria in Citrus Greening Disease

CLas is a bacteria that relies on a vector, an animal or object that transmits a pathogen to another organism. In the case of CLas, the vector is small psyllid insects including the Asian Citrus Psyllid that carry the bacteria in their hemolymph and salivary glands. These psyllids eat the stems and leaves of citrus trees, providing the opportunity for the CLas bacteria to find the phloem inside the citrus trees. Phloem is the tissue inside plants like citrus trees that transport necessary nutrients to different parts of the plant. However, CLas disrupt the function of phloem, cutting off the transportation of sugar, water, and important nutrients. This leads to the detrimental symptoms of HLB like the green discoloration of fruit and leaves. 

Treatment Options for Citrus Greening Disease

Treatment options for infected trees are limited and widely considered ineffective. Antibiotics oxytetracycline and β-lactams have been used for controlling the disease, but it has not been proven to be effective in helping trees fully recover. In addition, recent studies have shown that the antibiotics caused resistance in other neighboring harmful pathogens and in some cases, oxytetracycline was found to even promote CLas activity. 

Controlling Citrus Greening Disease Through Prevention

Currently, the best way to control the HLB is through prevention. The use of pesticides such as Neonicotinoids can help kill the vector insect and reduce the spread of the bacteria. A newly found class of plant hormones, Brassinosteroids, can also help increase plant immunity to the HLB. However, quarantine has been shown to be the most effective preventative measure of the HLB. For example, state legislation in Florida and California provide guidelines and transportation bans for citrus trees. Preventing the introduction of the vector in the first place is thus the surest way to save the citrus trees from infection.

Take-Home Insights on the HLB Citrus Greening Disease

The HLB provides valuable insight into the current COVID-19 pandemic. Despite the differences, they are both highly infectious diseases that do not yet have an effective cure. Until further research and treatment testing, the only sure solution for both the citrus trees and us is quarantine. So, if there is one thing that the Citrus Greening Disease teaches us, it is to stay at home. 

Works Cited

Alferez, Fernandez, et al. “Update on Brassinosteroids for HLB Management.” Citrus Industry Magazine. June 19, 2019. <>

Barnett, Melanie J., David E. Solow-Cordero, and Sharon R. Long. “A high-throughput system to identify inhibitors of Candidatus Liberibacter asiaticus transcription regulators.” Proceedings of the National Academy of Sciences 116.36 (2019): 18009-18014. <>

Bove, J.M. “Huanglongbing: A Destructive, Newly-emerging, Century·old Disease Of Citrus.” Journal of Plant Pathology (2006). <>

Fujiwara, Kazuki, Toru Iwanami, and Takashi Fujikawa. “Alterations of Candidatus Liberibacter asiaticus-associated microbiota decrease survival of Ca. L. asiaticus in in vitro assays.” Frontiers in microbiology 9 (2018): 3089. <>

Graca, JV da. “Citrus greening disease.” Annual review of phytopathology 29.1 (1991): 109-136. <>

Washington State University. “Growing citrus disease bacteria in the lab.” ScienceDaily. ScienceDaily, 12 September 2019. <>. 

“Citrus huanglongbing (greening) disease (citrus greening).” Invasive Species Compendium. <>

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11 replies »

  1. I have been curious about why local stores such as Home Depot, Lowe’s stopped selling citrus trees since last winter. It is an interesting coincidence that both we and the citrus tree are experiencing quarantine at the same time. This article gave me a clear reason with scientific evidence and research. It is very interesting that it says not only what caused this disease, but also treatment options and prevention. It is a well organized article and Take-Home insights is very cute!


  2. Very well written and researched – a reminder of how fragile the ecosystem is and how easily it can be seriously compromised. Good job!


  3. A well researched article with good insight into the ecosystem. Excellent takeaway making the research relevant to our current environment.


  4. George An excellent article; it clearly explains the hard science in a manner that makes it understandable . . . and interesting. It is surprising that there does not seem to be much public awareness of this problem. I hope she writes again. Many people don’t seem to grasp the difference between a bacterium and a virus, nor what “herd immunity” means. Perhaps Elena can explain it to us.


  5. You made an excellent observation. The effective prevention method of COVID 19 and citrus tree greening disease is similar. The way you look at the world is extraordinary. You certainly have a trait of a good scientist. I am looking forward to your future. Thank you.


  6. Seems like a real good technical summary of HLB and its impact in the U,S, Comparing with the COVID-19 situation was good, and it might be interesting to have a few more aspects of that comparison throughout, rather than just at the beginning and the end.


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