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Treasured Gold: Citrus Diversity


An hour west of Los Angeles, the University of California, Riverside safeguards a golden treasure. Neat rows of green trees carry bright, vibrant medallions in many shades of yellow and orange. The Citrus Variety Collection, curated by Tracy Kahn, is a 22.3 acre orchard whose riches are not precious metals, but delicious, tangy fruit. The collection includes more than 1000 different varieties or types of citrus, offering an abundance of genetic diversity. So why is the collection so valuable? The wealth of genetic information housed here may contain unique genes resistant to a devastating disease threatening citrus production worldwide.

Image Credit: chriscom via Flickr.

Genetic diversity can be thought of as the number of different traits a species has. For example, characteristics such as color, taste, and smell distinguish blood oranges from navel oranges. In addition genetic diversity can provide organisms with different tools to fight diseases.

Today any citrus fruits you find in the grocery store are descended from only four ancient species: citrons, mandarins, papedas, and pomelos. By the luck of nature, these plants were all sexually compatible and highly prone to natural genetic alteration, meaning successful combinations survived to reproduce and create even a greater diversity of citrus. Whether you enjoy an orange or grapefruit with breakfast, both contain the same genetic material from these ancient fruits, just remixed in a way that made the plants more versatile.

While many varieties of citrus exist in the wild, large scale agriculture has turned to using single, reproducible clones for their crops. This technique, while useful to maintain product uniformity, has dramatically reduced the amount of genetic diversity within citrus populations. This means that if one tree in a plot is vulnerable to a disease, the others around it will be too as they all share the same genes.

Here is where Huanglongbing (HLB), or citrus greening disease rears its ugly head. Spread by a small insect called the Asian Citrus Psyllid, the bacteria that causes HLB easily infects commercial citrus varieties. The disease first destroys the root system, restricting nutrients to the top of the tree. Eventually the foliage may show visible signs of infection including yellowing of the leaves and green, misshapen fruits. Each year the dying tree will produce less and less, and the fruit often does not meet food quality standards. Because these tiny insects can easily hop from tree to tree, entire groves can be affected before preventative measures can be enacted.

Currently, scientists have documented HLB in every country that produces citrus. And the disease not only affects crop production, but people’s livelihoods as well. In Florida, HLB has caused growers to completely abandon groves resulting in a loss of nearly ⅕ of local jobs in a $9 billion industry.

Yet, hope may be hiding in the UC Riverside Citrus Variety Collection. Varieties with natural resistance to citrus greening disease may already exist here. Additionally, the collection’s mission “to provide a resource of citrus genetic diversity for research” enables researchers to investigate conventional breeding and genetic modification techniques to create disease resistant trees. However these solutions will take time to develop. For now, growers can reduce further exposure by using insecticides, monitoring and treating sick trees with antibiotics, and removing infected trees. Longer term solutions, including disease resistant rootstock/scion (foliage graft) combinations and genetically modified varieties, can provide the next generation of trees. In order to eradicate citrus greening disease, researchers and growers will have to work together in a coordinated effort.

Image Credit: Rafael Castillo via Flickr. 

The Citrus Variety Collection at UC Riverside is truly a treasure. Researchers can use the many varieties to prevent diseases now and in the future. If you cherish a glass of orange juice with your breakfast each morning, consider the people who depend on citrus to make their living, from the growers and pickers to all of those in associated industries. Support genetic research at your local universities and, if you’re feeling generous, contribute to the Citrus Variety Collection so it can continue to be a bounty not only for researchers and producers, but consumers and everyone in between.

Thank you for contributions from Dr. Kevin Folta.

About the Author

image002Hailing from the deserts of Arizona, Mackenzie Carter is an enthusiastic masters student in the College of Veterinary Medicine. She is currently studying tissue engineering to model disease states in bone, namely panosteitis in canines. Mackenzie loves hands on projects, from ceramics to solar powered robots. In her free time she explores her passions: cephalopods, tea, and swing dancing. You can connect with Mackenzie via email at mackenziecarter@uga.edu.


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