Citrus prices in the United States have increased over the past year and are expected to continue rising as citrus greening disease keeps killing citrus trees throughout the southern United States . Citrus greening, also called Huanglongbing or Dragon Yellows, is believed to be native to parts of Asia and Africa and was detected in Brazil and Florida in 2004 and 2005, respectively. Since arriving in Florida, it has now been found in orange groves in California in the last few years. This disease is devastating in its new hosts, where it is estimated to have caused 30-40% loss of yield and about $4 billion of loss from 2006-2011 in Florida [5,7].
Fig. 1 Orange tree infected by citrus greening disease (Source: By Mmacbeth)
Infected trees develop symptoms slowly and there are currently no cures for this disease. The most distinct symptom is called blotchy mottle, which is a patchy yellow color on the leaves that can be confused with nutrient problems due to their similar appearance (Fig. 2) . As the disease progresses, the entire tree may become yellow and has reduced fruit production, often yielding fruit that are green, bitter to taste, and unsuitable for eating or juicing (Fig. 3) .
Fig 2. Blotchy mottle on citrus leaves of a tree infected with citrus greening disease (Source: U.S. Department of Agriculture)
Fig. 3 Oranges that will stay green due to citrus greening disease (Source: U.S. Department of Agriculture)
Phytopathologists, scientists who study plant diseases, are working hard to prevent the spread of this disease and to find treatments for infected trees. Their first step was to determine the cause of this disease, which turned out to be two organisms working together.
Citrus greening is believed to be caused by three different species of bacteria called Candidatus Liberibacter asiaticus, Candidatus Liberibacter africanus, and Candidatus Liberibacter americanus . In the United States, we believe only Candidatus Liberibacter asiaticus is present. It lives inside the Asian citrus psyllids (Diaphorina citri), small insects sometimes referred to as jumping plant lice (Figure 4) [2,6]. The Asian citrus psyllids were introduced to Florida in the early 2000s and have become an invasive species–an organism that is not native to an area but successfully colonizes it–and scientists believe this is how citrus greening started in the United States .
Fig. 4 Citrus psyllid larvae feeding on citrus (Source: USGS Bee Inventory and Monitoring Lab)
The citrus psyllids feed on the citrus tree by sucking out plant nutrients and secreting a sticky sugar mass on the outside of the plants which attracts fungi to grow . Plants infected with high populations of psyllids may have reduced growth or fruit yield. Additionally, as the psyllids feed on the citrus tree they transfer the Candidatus Liberibacter bacteria into the tree, causing citrus greening. Currently, there doesn’t seem to be any negative affect of the bacteria on the psyllids.
Controlling this Disease
Two strategies for controlling a disease are to stop its spread or decrease the susceptibility of its host. The first way to reduce citrus greening is to reduce the psyllids. Citrus growers are encouraged to buy disease-free trees and to remove any infected limbs or trees to prevent the spread of the insects to new trees or groves. Insecticides, applied at the right time of year, can control psyllids, with varying success . Another strategy growers have been using is increasing nutrients to the trees to overcome the disease symptoms and extend the life of the infected tree . Phytopathologists are also looking for disease-resistant citrus plants that they could breed with our Florida citrus trees to create disease resistant varieties that still produce the fruits we as consumers desire (such as good taste or lots of juice). However, no disease resistant varieties have been identified so far .
Since the plants are infected for a long time before showing symptoms, scientists think they may be able to optimize the plant’s immune response to make it better at defending itself against citrus greening and potentially keep it from becoming so sick that the fruit are affected . If phytopathologists understand how the plant’s immune system responds to the disease and key factors associated with the pathogen being able to establish an infection, then we may be able to manipulate the genes controlling these interactions to stop the infection .
As devastating as this disease is, many resources are going towards preventing the spread of this disease to new citrus groves and finding ways to prevent citrus plants from being killed by this pathogen. Phytopathologists are using similar strategies to understand and stop other plant pathogens that can cause our grocery bills to increase, like coffee rust, black sigatoka on bananas, or wheat rust. So next time you see an orange, think of the phytopathologist working hard to keep them in our grocery stores!
 White, M.C. 2015. Get Ready For Higher Orange Juice Prices. Time Magazine. (http://time.com/money/4108947/orange-juice-price-increase/))
 da Graca, J.V., Douhan, G.W., Halbert, S.E., Keremane, M.L., Lee, R.F., Vidalakis, G., Zhao, H. 2016. Huanglongbing: An overview of a complex pathosystem ravaging the world’s citrus. Journal of Integrative Plant Biology. 58: 373-387.
 Kabashima, J.N., Paine, T.D., Daane, K. M., Dreistadt, S.H., 2014. How to Manage Pests in Gardens and Landscapes: Psyllids. University of California IPM. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7423.html
 University of Florida Extension. 2016. Citrus Greening (Huanglongbing). UF/IFAS Citrus Extension. http://www.crec.ifas.ufl.edu/extension/greening/index.shtml
 Singerman, A. and Useche, P. 2015. Impact of Citrus Greening on Citrus Operations in Florida. University of Florida IFAS Citrus Extension. https://edis.ifas.ufl.edu/fe983
 Brlansky, R.H., and Rogers, M.E. 2007. Citrus Huanglongbing: Understanding the Vector-Pathogen Interaction for Disease Management. Online. APSnet Features. doi: 10.1094/APSnetFeature-2007-1207. http://www.apsnet.org/publications/apsnetfeatures/Pages/Huanglongbing.aspx
 Hodges, A.W. and Spreen, T.H. 2012. Economic Impacts of Citrus Greening (HLB) in Florida, 2006/07–2010/11. University of Florida IFAS Citrus Extension. http://ufdc.ufl.edu/IR00005615/00001
More From Thats Life [Science]
- Biology Superpowers: X-Ray Vision
- How to Expand Your Senses by Reading a Blog Post
- What's up with bat echolocation?
- Seeing is Believing - How Can We Visualize Tiny Colorless Bacteria?
- Saving water is no longer a matter of how short our showers are · Water balance in a man-made world
- Double Digestion in Rabbits · Why Does Mopsy Eat Her Own Poop?
- Should I say sex or gender? Pt. 2
- Should I say sex or gender? Pt. 1
- How To Catch Hard-to-Catch Fish?
- Finding the Perfect Partner
- Is your gut trying to kill your resolve? · Mind over microbe
- Why Do Mothers Mother?
- GMO! The Places You'll Go!
- New-Fangled Paleontology · Really Old Fossils, Really Strong Predators, and Cool New Tech
- A Brief History of Evolutionary Thought, part III
- Saving face: transplanting our most distinctive features
- A Brief History of Evolutionary Thought, part II
- DNA as a solution for data storage · DNA - Nature’s Hard Drive
- A Crash Course in the Coolness of Mitochondria · Mitochondria: The Underrated Organelles
- A Pollinator’s Job Description and Why We Should All Care About Them · Pollination 101
- You May Say I’m Biased, But I’m Not the Only One
- The evolution of one of the greatest medical discoveries in history. · The Path of Least Resistance: Our Relationship with Antibiotics
- Mother Nature’s History Book · Estimating the Age of Life Long-Gone
- Proprioception as a vital sense · Know Your Limb-its
- Man’s Best Artificially-Selected Friend · Your Dog is a GMO Wolf
- Better Safe Than Sorry: The Pesticide Industry is Getting a Revamp
- Sometimes scientists have to get creative in order to effectively do science – especially on a budget. · The Bizarre Shopping List of a Determined Scientist
- Insects Get Sick Too: The Study of Insect Pathology
- Our teeny tiny friends and their huge potential · Employee of the Month - Hire a Microbe to Do Your Work
- A Brief History of Evolutionary Thought - Part I
- The Effects of Custom Build Paradise · Artificial Islands
- To B(PA) Or To Not B(PA): Regulating Endocrine Disruptors
- Bioluminescence truly looks like it is nothing short of sorcery, and although this naturally occurring phenomenon is well studied and explained, that does not take away from its beauty. · Fireflies of the Ocean: Lighting up the Dark with Science
- Part II - Cases of altruism in the animal kingdom · Charity cases in nature - when are animals more likely to be altruistic?
- Part I - Why true altruism is a rare behavior in the animal kingdom · Being selfish means staying alive
- Penguins and other strange things we study from space
- Pseudoreplication and the Art of Biological Statistics
- What is wrong with my tomatoes?...And other plant disease questions · Why did orange prices increase?
- How fecal microbiota transplants can improve lives and possibly save them · The Wonders of Fecal Transplants
- The scientific facts behind the safety and efficacy of childhood vaccines · Calling the Shots - Discussing Vaccines
- 3D Printing for Fun and Science
- What is wrong with my tomatoes?...And other plant disease questions · What is Phytopathology?
- Medical Mysteries Still Surrounding Zika Virus
- More ›