Citrus greening: What is it and why is it so hard to stop?

If you’ve noticed the price of citrus creeping up, there’s a hidden reason behind it: a silent killer has been sweeping through the U.S.’s citrus groves for decades, and it’s a threat to more than just your morning OJ.

Citrus greening, also known as Huanglongbing, is a plant disease that has decimated citrus production in the U.S., particularly in Florida, California and Texas, the top three citrus-producing states. It affects almost all citrus species, including orange, grapefruit, lemon and lime trees. It has caused reduced yields, financial losses for producers and higher citrus prices for consumers.

“Citrus greening is not harmful to humans, but it has been a plague to citrus producers across the country,” said Kranthi Mandadi, Ph.D., professor in the Texas A&M Department of Plant Pathology and Microbiology and researcher at the Texas A&M AgriLife Research and Extension Center at Weslaco. “Citrus production in Florida, for instance, has dropped by about 90% in the past 20 years because of citrus greening. Some think of it as a cancer for citrus trees.”

Mandadi and collaborators within Texas A&M AgriLife Research and beyond are working toward new solutions they hope will eventually control and prevent citrus greening, but it has proven to be an exceptionally challenging disease to study and manage.

Why is this disease so persistent, and what makes it such a fierce opponent for researchers and growers alike? Mandadi explained the basics of citrus greening and the unique challenges it presents.

What causes citrus greening?

The first issue in managing citrus greening is that it’s notoriously difficult to detect early. Mandadi said the disease can go unnoticed for months, even years.

“Infected trees are asymptomatic at first,” he said. “Then they slowly produce fewer fruits, which become smaller, irregularly shaped and bitter. The tree drops them earlier than normal and eventually gets weaker and easily prone to damage by other diseases and environmental stresses like hurricanes and freezes.”

The disease is caused by a bacterium called Candidatus Liberibacter asiaticus. Once inside the tree, Mandadi said, it acts like a blood clot, disrupting the flow of nutrients and starving its host.

Citrus greening is fatal to the trees. Mandadi said that, given the inevitability, some producers cut the trees down before they die and replant, incurring some losses.

How does citrus greening spread?

Some bacterial infections are spread among plants by insect vectors. For citrus greening, that vector is Asian citrus psyllid — a small, winged insect that feeds on the sap of citrus trees.

While feeding on an infected tree, the insect draws in the bacteria and then spreads it to healthy ones when it feeds again.

Mandadi said environmental conditions also factor into the spread and severity of citrus greening. Warm, humid climates, like Florida’s, are an ideal environment for insects carrying the bacteria to thrive. He said perhaps the drier climate in Texas and the robust early mitigation efforts and ongoing vector control strategies followed by producers in Texas may have prevented citrus greening from rising to the severity seen in Florida.

Why is citrus greening so hard to treat?

Beyond the difficulty in detecting citrus greening infections in groves, the bacteria causing it has been nearly impossible to cultivate in a lab for researchers to study — until recently.

In 2020, Mandadi and a team of AgriLife Research scientists developed “hairy root” technology, which allows scientists to grow the pathogen in a controlled setting.

Previously, scientists considered the bacterium unculturable. It could only grow inside the tree’s vascular system, preventing researchers from making considerable progress in testing therapeutic options in the lab.

Mandadi said hairy root technology overcomes the fastidious growth requirements of the bacteria by creating a root-like tissue that mimics the internal anatomy of the tree to support its growth. More importantly, it has allowed him and collaborating researchers worldwide to screen potential treatments quicker and more effectively in the hairy root system.

“AgriLife Research is working with leading plant pathologists to develop a cure for citrus greening, and we’ve made significant progress with our hairy root technology to discover new antimicrobial peptides and chemicals that can lower the disease and improve yields,” Mandadi said. “By continuing to push forward in our work, we hope to safeguard the future of citrus — so that it remains a staple for generations to come.”

Laura Muntean is media relations coordinator for Texas A&M AgriLife Extension Service.

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