CORNELL (US)—The U.S. citrus industry is bracing for what could be a devastating blow. A recently introduced disease known as citrus greening is threatening domestic production, and researchers around the world are searching for a solution.
The disease causes juice from infected fruit to “taste like jet fuel mixed with Vicks VapoRub,” according to a USDA entomologist.
While the pathogen responsible is believed to be a variety of a bacterium called Candidatus Liberibacter, scientists have been unable to conclusively determine the cause of citrus greening because the bacterium cannot be routinely cultured independent from its host, a small insect known as a psyllid.
To get around these issues, scientists at Cornell University’s College of Agriculture and Life Sciences are using an advanced method for sequence analysis—known as metagenomics—to identify the Ca. Liberibacter DNA from a genetically mixed population of environmental samples.
Researchers are also looking to similar bacteria that cause zebra chip disease—named for the dark striations it creates in chips made from infected potatoes—as an alternative approach for understanding Ca . Liberibacter’s basic biology.
“There are a number of groups who are basically competing to try to culture it and get the complete genomic sequence,” says David Schneider, a USDA-Agricultural Research Service (ARS) scientist and adjunct associate professor of plant pathology at Cornell. Still, he adds, this class of organisms is not well understood, so the mechanisms of virulence are not known.
“Without that basic knowledge,” says Schneider, “it’s hard to proceed.”
Once a psyllid delivers the bacterium into a citrus plant, infected trees may not show symptoms for years. Before dying early, trees with citrus greening will produce misshapen, undersized green fruit without economic value.
Meanwhile, the psyllid continues to spread as far as Texas and Louisiana, and most recently to southern California. The disease was first identified in China—where it is know as Huanglongbing—during the 1920s and was likely introduced to the U.S. via international transport of small ornamental plants during the 1990s.
Lindeberg says the genome analysis methods developed at Cornell are transferable to many systems. “I think the potential for improved diagnostics will be of particular interest in developing countries.”
In the fight against citrus greening, orchard management problems are proving to be a substantial obstacle. Private citrus growers facing bankruptcy have abandoned tens of thousands of acres of land in recent years. Lindeberg notes these “feral orchards” act as petri dishes for Ca. Liberibacter. “If a hurricane goes through, it basically blasts the orchard all over the state.”
While the abandoned orchards are privately owned and thus difficult to manage, Cornell scientists, in partnership with other national universities, hope to produce cultivars resistant to citrus greening that may stop the spread of the disease before it is too late.
“We are very much at the start of a true collaboration period,” says Lindeberg, who is attending the annual meeting of the American Phytopathological Society in August for a special session on citrus greening with guest speakers from around the world.
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