Citrus greening and the future of Florida’s oranges

Anh Nguyen
2 December 2013

Above: Image © istockphoto.com/Wileigh

Whether you prefer them peeled or juiced, most oranges consumed in North America come from Florida. Unfortunately, since 2005, the amount of ripe and healthy fruit available from that state has declined substantially because of a disease called citrus greening. It has affected all 32 Florida counties where citrus fruits are grown, and harvests have shrunk by 30 to 40 per cent for most growers. Meanwhile, the wholesale price of Florida oranges has almost tripled.

Did You Know? Citrus greening may not cause symptoms immediately. The onset of the disease can occur between two and five years after infection.Citrus greening can be caused by three separate strains of bacteria: an Asian strain, an American strain, and an African strain. None of them seem to survive outside of the host insect, so studying the disease has been difficult. However, researchers do know that the Asian and American strains are carried and spread by a tiny insect known as the Asian citrus psyllid, and that the African strain spread by the African citrus psyllid.

The Asian strain of bacteria is the most widespread, affecting parts of Asia, Africa, and Brazil. It is also the strain responsible for the outbreak of citrus greening in Florida. While harmless to humans, the bacteria affect all types of citrus and they are especially harmful to oranges.

Infected trees grow shrunken, unripe, and bitter fruit. The trees themselves whither and die within three to five years of infection. These symptoms occur when the bacteria settle in the phloem, the layer inside the bark where trees absorb water and nutrients from the surrounding environment. As the bacteria spread, the trees lose this ability and can no longer grow properly.

Huanglongbing (citrus greening) on mandarin oranges. Click image to enlarge (Pfly)

Citrus greening doesn’t just affect the quantity and quality of fruit available to consumers. There have also been significant losses of jobs and revenues. Some fruit growers have given up entirely, selling or abandoning their groves in frustration.

Orange trees have no known natural defense against citrus greening and there is no known cure for the disease. The use of insecticides and antibiotics cannot keep up with the spread of the bacteria, allowing both the insect carrier and the disease-causing bacteria themselves to develop resistance.

However, orange lovers shouldn’t abandon hope just yet! Coca-Cola has spent $2 billion buying 25,000 acres of orange groves in Florida. This has given the industry a boost by providing new jobs and a guaranteed buyer for the fruit. Money has also been invested into research for a cure.

In particular, genetic engineering has emerged as a possible solution to the citrus greening problem. Scientists are looking for traits present in other plant or animal species that could provide some kind of defense against the carrier, the Asian citrus psyllid. For example, if a particular gene could make orange trees inhabitable or unpalatable to the citrus psyllid, it could be introduced into the genome of a new genetically-engineered variety of oranges. The same techniques could be used to make orange trees tolerant of different climatic conditions, allowing them to be farmed outside of extra-sunny places like Florida. Or the orange trees could be made resistant to the insect carriers or to the bacteria themselves, thereby preventing the disease from developing.

Did You Know? Citrus greening is also called Huanglongbing (HLB), which means “yellow dragon disease” in Chinese. It was first described by Chinese scientist Ling Kung Hsiang.Some in the industry are hesitant to alter the DNA of oranges, especially since oranges are often marketed as “natural”. However, existing methods, such as pesticides, have so far failed to halt the spread of the disease. And since there appear to be no naturally-occurring plants that are resistant to citrus greening, doing nothing will probably result in more trees being infected and eventually dying. Meanwhile, orange growers in California are keeping a close eye on citrus greening. It hasn’t affected that state yet but could become a problem in the future.

Do you think genetic engineering is a good approach to fighting citrus greening? Can you think of any other possible ways to combat this disease?

References

General information

A Race to Save the Orange by Altering Its DNA (Amy Harmon, New York Times) Citrus Bacterial Canker Disease and Huanglongbing (Citrus Greening) (University of California) Citrus Disease With No Cure Is Ravaging Florida Groves (Lizette Alvarez, New York Times) Citrus greening (US Department of Agriculture) Citrus Greening Forces Florida Growers To Trust A Controversial Savior (Joe Satran, Huffington Post)

Industry publications

The Coca-Cola Company Commits $2 Billion to Support Planting of 25,000 Acres of New Orange Groves in Florida (Coca-Cola)

Scholarly publications

Lin KH, Lin KTsun. 1956. The Citrus Huang Lung Bin (Greening) Disease in China. Translation of Chinese document originally published in Chinese in Acta Phytopathologica Sinica. 2:1-11,14-38. http://swfrec.ifas.ufl.edu/hlb/database/pdf/00000094.pdf

Anh Nguyen

For the past five years, I have been a lab technician/research assistant in several labs at UBC. I was lucky to have a hybrid job - half the time I'd be collecting samples and doing bench work; the other half I'd be doing bioinformatics.  What's that, you say?  In short, I look at DNA sequence data and try to find reoccurring patterns in order to answer questions like, "How often does this gene occur in our samples?", or, "Is this gene present in all or some of the species we're looking at?". Now I'm looking for something new to do with my bachelor's degree in combined computer science and biology... who knows what that will be? In the meantime, I'm working on my writing skills through volunteering with CurioCity, since writing has always been fun for me. And writing for a younger audience and making science sound cool... well, that's much easier said than done!



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