Scientists find breakthrough in deadly citrus disease

The mystery of "citrus greening," the common name for a citrus disease that is wiping out crops worldwide, may have been recently unraveled, giving hope to citrus growers.

Huanglongbing (HLB) is hard to say, but even harder to swallow. This citrus disease, also known as "citrus greening," is caused by the microbe Candidatus liberibacter, and is spread by a pest named the Asian citrus psyllid. Infection begins when the psyllid munches on the leaves of sick plants, then eats off healthy trees.

HLB changes delicious citrus fruit into oddly shaped, bitterly metallic tasting, green ick. Not exactly the healthy breakfast drink and snack we've come to know and love.

Driven by the potential loss of this important, vitamin-rich food source, Carolyn Slupsky, associate professor with the University of California-Davis Department of Nutrition and the Department of Food Science and Technology, has taken this citrus threat head on. And the results of her team's research could help eradicate the plant pathogen.

According to the California Department of Food and Agriculture website, "All citrus and closely-related species are susceptible hosts for both the ACP insect and the HLB disease. There is no cure once a tree becomes infected. The diseased tree will decline in health and eventually die."

This disease has cut a deadly swath through groves in Asia, Brazil and the Dominican Republic. Florida's orange juice crops have been culled by one-third. And, more recently, the disease has been detected in California — spelling trouble for much of the world's fresh citrus crops.

The disease is tricky — it can live in a tree for years before manifesting, and it can show up unevenly in a plant. In other words, one leaf may test positive for infection, while another will not. This makes culling of infected trees nearly impossible for commercial and backyard growers.

Slupsky, along with Andrew Breksa, research chemist with the U.S. Department of Agriculture's Agricultural Research Service, used nuclear magnetic resonance spectroscopy (a tool that looks at cells' nuclei) to examine the juice from three groups of trees: healthy, diseased with no symptoms (asymptomatic), and those with symptoms.

"We found major differences in the chemical fingerprint among healthy, asymptomatic and symptomatic fruits," Slupsky said.

Turns out, the amino acid composition was different for each group, making identification easy.

And although the research teams' work could make early detection of diseased trees possible, proving to be a boon for citrus growers, their additional discovery was even more fortuitous. It is possible that Slupsky and colleagues might have unraveled the way that HLB attacks trees.

 "The pathogen responsible for HLB seems to cause havoc with a tree's ability to defend itself from infection" by interrupting a tree's production of key amino acids that work as the tree's immune system, Slupsky revealed.

"It could be that the pathogen is outsmarting the tree by undermining its defenses," Slupsky said. "That's a spectacular discovery, because when we understand the mechanisms behind the attack, we have a chance at blocking them. Maybe we can find ways to enhance a tree's natural immunity."

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