Under threat from a crippling, deadly bacteria, the Arizona Department of Agriculture is escalating its war against a disease crippling the nation’s citrus crops by expanding quarantine zones throughout Arizona and enlisting the help of some of the University of Arizona’s brightest minds.

The bacteria, also known as Huanglongbing, functions as an infectious disease and is spread from citrus tree to tree by the Asian Citrus Psyllid, or ACP, an unassuming, mottled brown, winged insect roughly the size of a grain of rice that carries the bacteria within its gut.

Dr. Glenn C. Wright, who works for the University of Arizona’s School of Plant Sciences, says it is easiest to think of citrus greening disease as a virus, like malaria, since they work in similar ways.

“The vector feeds on a tree, it sucks out a little bit of the plant sap, and it goes on to the next tree to have a little bit more,” says Wright. “If the bacteria is inside the insect, it transfers the bacteria to the tree whenever it feeds. When another, possibly bacteria-free, psyllid comes by to feed on that tree it ingests the bacteria and becomes infected.”

Ten years ago, the disease first appeared in Florida, the nation’s largest citrus producer, and within a year it tore through the state’s farms faster than Paul Bunyan and Babe the Blue Ox could clear a pine forest. The disease, which cut Florida’s orange production from 200 million boxes a year to roughly 100 million, stunts the growth of citrus trees and leaves their fruit small, misshapen, and inedible, until they eventually die. So far, there is no cure or serious preventative measure for it.

The insect is steadily spreading throughout the western and southern portions of Arizona, and so far researchers have found that none of the insects within the state are infected with the bacteria.

However, as long as the bacteria’s vector remains present in the state and nearby states like Florida and Texas struggle to contain the bacteria, Arizona’s $37 million citrus industry remains at risk.

John Caravetta, the associate director of the Arizona Department of Agriculture, says that people need to respect the established quarantines and not move any part of a citrus tree in or out of them without special permission.

“If we get movement of leaves or other plant parts from infected areas to Arizona, we have the potential to introduce infected psyllids into the state,” says Caravetta.

The state’s quarantine zones are only a temporary measure against the inevitable spread the psyllid, and eventually the bacteria. Without a cure, or similar preventive measures, the situation will remain troublesome for farmers and hobby citrus growers alike.

Two UA professors, Dr. Judith Brown and Dr. Zhongguo Xiong, are hoping that the projects they have been working on will slow the spread of the greening disease and eventually eradicate it.

Brown, a professor in the School of Plant Sciences who specializes in plant pathology, has been researching the psyllid and its relationship with the bacteria since 2012. Recently, most of her time has been spent looking at where the bacteria resides inside the ACP and which of its genes are manipulated by the bacteria when it’s living inside the psyllid.

In the psyllid, the bacteria acts like a parasite, suppressing the minuscule insect’s immune system, allowing it to grow and make more of itself. Brown’s research is aimed at stopping the parasitic bacteria at this point of its lifecycle, preventing it from leaving the ACP and infecting Arizona’s precious citrus trees.

“We’re trying to be proactive, we know if we shut it down before it even multiplies inside, then maybe we can stop it,” says Brown. “The way we are proposing to do that is to use double-stranded RNA, which is a new technology that is becoming an appealing bio-pesticide.”

RNA helps higher organisms control the expression of genes, like the ones that produce proteins needed for life. Using a method called gene silencing, double-stranded RNA would be introduced to the ACP so it could shut down the production of a specific protein that the bacteria uses to survive and multiply while inside the insect.

The bio-pesticide could be fed to trees or sprayed on its leaves to introduce it to the psyllid and because the RNA breaks down naturally over time, its impact on the environment is minimal unlike traditional pesticides.

Brown is working on identifying which genes will kill off the bacteria when triggered by the RNA. Their goal is to have a candidate identified and ready for testing in citrus plants by the end of 2015 or sooner. Testing is expected to continue for two more years while they finalize a candidate before sending the bio-pesticide to market.

“Our grant to do this work runs out in 2016,” Brown says “But we’re hoping that we can keep on going.”

Xiong, also of professor in the School of Plant Sciences, is working on two projects that may slow and eventually halt the spread of citrus greening.

One of his projects is an early and sensitive detection method for the disease that is more reliable than current approaches and will be an immediate step toward dealing with the bacteria. He is also looking into ways to physically or chemically treat the bacteria that would trigger certain factors in it, similar to the way stress can trigger the herpes virus, stopping or killing the bacteria.

There are already some field trials going on in Florida using this type of therapeutic pesticide, but Xiong says it is still a few years away from production.

Despite the serious and growing threat of the ACP and greening disease to Arizona’s citrus industry, Xiong says “Researchers are working on practical, immediate solutions, but we are also working to understand the bacteria’s biology so we can find better ways to control it.”

Gareth Farrell is a reporter for Arizona Sonora News, a service from the School of Journalism with the University of Arizona. Contact him at [email protected]. 

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