This year, for the first time, the Environmental Protection Agency is allowing citrus growers to spray their trees with large quantities of two antibiotics to combat citrus greening, a bacterial infection that is devastating Florida’s citrus crops, as Nature has reported. Because the antibiotics are also used to treat human bacterial infections, public health advocates worry that their use on crops could promote the development of antibiotic-resistant human pathogens. In a new wrinkle, a study reports that one of the drugs, oxytetracycline, doesn’t protect trees against citrus greening.

The study, conducted by researchers at the University of Florida and published August 1 in the journal Phytopathology, finds that orange trees infected with the citrus-greening bacteria and sprayed with oxytetracycline at doses recommended by the manufacturer for six months did not fare any better against the disease than infected trees sprayed with water, The New...

“Under our test conditions, the concentration is just not enough to suppress the pathogen,” Nian Wang, a microbiologist at the University of Florida’s Citrus Research and Education Center and a lead author of the paper, tells the Times.

Citrus greening, which has reduced citrus production in Florida by 70 percent since 2005, according to the Times, is caused by the bacterium Candidatus Liberibacter asiaticus and spread by an insect, the Asian citrus psyllid. Fruit from infected trees is “green, misshapen, and bitter,” and unfit for sale, according to the United States Department of Agriculture.

Although spraying didn’t work, injecting the drug into tree trunks decreased the levels of the citrus-greening bacteria, the study found. However, that drug delivery method has not been approved by the Environmental Protection Agency (EPA) and moreover, would be too expensive to be feasible, the Times reports.

Taw Richardson, chief executive of AgroSource, the company that manufactures sprays containing the antibiotics, tells the Times that the company’s own research found that the drug worked and criticizes the study for not using the recommended combination of products to ensure the drug enters tree leaves. 

“The good news is that oxytetracycline is definitely inhibiting the pathogen,” James Adaskaveg, a University of California, Riverside, plant pathologist, tells the Times. “They just need to figure out how to deliver it in the most effective way.”

But environmentalist and public health advocates tell the Times that the results of the new study indicate that the data, provided by AgroSource, upon which the EPA based its decision to approve oxytetracyline in the first place, were flawed.

Ashley P. Taylor is a New York–based freelance reporter. Follow her on Twitter @crenshawseeds and read her work at ashleyptaylor.com.

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