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Fighting back the Yellow Dragon

Citrus greening is currently one of the most economically devastating citrus diseases worldwide, affecting over 40 countries in Asia, Africa, and the Americas. Also known as Citrus Huanglongbing, (literally, “Yellow Dragon Disease”), or HLB for short, the disease is caused by a bacterium, and symptoms include yellow shoots, splotchy leaves, stunted tree growth, and irregularly-shaped and bitter fruit.

HLB has made US headlines, particularly in Florida, a state whose citrus industry, valued at $10 billion, is so important to the economy that oranges are featured on its license plates. But since the disease hit in 2005, crops have been decimated, and some question whether Florida’s industry will survive.

“Currently, there is no cure. Usually, the only thing to do is destroy the tree to stop the disease from spreading,” says Dr. Jonathan Walton, Professor at the PRL.

Digital drawing of a dragon menacing a castle sitting lonely on an island.
Metaphorical fighting of the dragon
By Baltasar Vischi, CC BY 2.0

Infected citrus plants are difficult to maintain and regenerate, which makes studying the disease a challenge. Many treatments have been tried to control HLB, including penicillin and antibiotic treatments.

Now, a team of international scientists from Cuba, Japan, and the US, including Walton, are suggesting a plant steroid might help with managing the disease.

Image of an infected mandarin, cut in half and showing brown seeds.
Brown seeds in infected mandarin
By J.M. Bové, CC BY-NC 3.0
Image of splotchy leaves on an infected sweet orange tree.
More devastating than real dragons
By H.D. Catling, CC BY-NC 3.0

Countering with plant steroids

“Plant steroids are relatively a new subject of study,” Walton says. “We used to think steroids were only important for regulating the growth of animals.”

Animal steroids are a class of hormones that help with metabolism, inflammation, immunity, and normal development. Recently, however, scientists have discovered that plants also use steroids, called brassinosteroids, to control their growth and development.

“Brassinosteroids have been found to activate genes involved in defense against plant diseases, so the researchers in this study were curious as to whether they could be used to control HLB.”

The experiment involved spraying brassinosteroids on citrus plants, both under greenhouse conditions and in planted fields. The treated plants showed a dramatic improvement in health, with up to 160-fold reductions in their bacterial levels.

Improvements were more pronounced in the greenhouse, however. “It’s because there are a lot of field factors beyond our power: weather, soil, humidity, and so on. These are easier to control in a greenhouse.”

“But the important point is that brassinosteroids did contribute towards activating plant defenses that play some role in resistance against HLB. That was the pleasant surprise.”

Image of Dr. Jonathan Walton
Dr. Walton
By Harley J Seeley Photography

“The flipside is that these experiments didn’t eradicate the disease. When spraying stopped, bacterial numbers went back up. Maybe this brassinosteroid is limited in its ability to spread in the plant or is too unstable. We will have to experiment with different spraying schedules or longer treatment periods.”

That is why the researchers suggest that brassinosteroids might one day be a tool to be used within an integrated disease management approach that includes other control methods.

“Will this approach to controlling HLB be implemented some day? We don’t know yet. These things ultimately boil down to cost, and sometimes, the price of curing a disease is too expensive to justify. Looking ahead, researchers will have to demonstrate the economic benefits of spraying brassinosteroids.”

The findings have been published in the PLoS One journal.

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