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‘Green menace’ strikes California citrus

A disease that has devastated the citrus industries in Florida, China, Brazil and elsewhere now threatens California’s US$2-billion-a-year crop.

Last month, a lemon-pomelo hybrid tree in a Los Angeles-area yard tested positive for huanglongbing, or citrus greening, according to the Los Angeles Times. The bacterial disease is spread by the Asian citrus psyllid. California had been the last major citrus-growing region in the world to remain free of huanglongbing, which translates roughly to ‘yellow-dragon disease’.

The disease probably originated from imported bud wood, which is used to make citrus hybrids such as the lemon-pomelo tree in which the bacteria was identified. Los Angeles citrus-tree owners will be the ones most immediately threatened by the disease, according to the New York Times. But agriculture officials and growers in the San Joaquin Valley, the heart of California’s citrus industry, worry that the bacteria will eventually make the 200-kilometre trek from Los Angeles.

Considered one of the most dangerous plant pathogens in the world, the bacteria spreads from tree to tree via the psyllid, draining nutrients from the plant and resulting in diminutive green fruits, thinning branches and, eventually, the death of the tree. It has devastated the citrus industry in many Asian countries, killed hundreds of thousands of trees in Brazil and swept through Florida since its arrival on US soil in 2005.

The bacteria that cause huanglongbing is named Candidatus Liberibacter (the Candidatus means the species designation is provisional, because the bacteria have yet to be cultured). Until 2008, huanglongbing was listed alongside anthrax and Ebola as a potential agent of bioterror on the the US government’s list of select agents.

Nature examined Ca. Liberibacter’s status as a select agent in a 2008 feature (see The Green Menace). The designation resulted in strict guidelines for labs working with huanglongbing-infected plants, such as background checks by the Federal Bureau of Investigation for workers and strict record-keeping rules. Scientists and officials in Florida complained that the status hampered the response to the outbreak in that state. The US Department of Agriculture defended the select-agent status and said that mechanisms were in place to consider its removal from the list.

Other scientists Nature interviewed for the feature questioned whether any plant pathogen should be listed as a select agent, because plant pathogens are unlikely to be successfully deployed as agents of bioterror.

The US government eventually removed Ca. Liberibacter from the select-agent list because the bacteria were already endemic in the United States. But eight other plant pathogens remain select agents.

Eric Triplett, a microbiologist who studies huanglongbing at the University of Florida in Gainesville, says that the rule-change made life easier in his lab. His team no longer works with infected plants in a special room sealed by two locked doors. More scientists are also studying huanglongbing and working to spare citrus crops from it. “There are lots of people working on the problem right now, and that wasn’t easy to do when the select agent rules were in place,” Triplett says.

Image of huanglongbing infected citrus via Wikimedia Commons


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