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Insect-to-Insect Disease Transmission Squeezes U.S. Citrus Crop

A fatal bacterial tree disease spread among flies could deal a devastating blow to the citrus industry
orange tree



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If orange juice is a part of your breakfast, you might be in trouble. A debilitating disease has citrus growers questioning the future of the orange industry in Florida and other parts of the U.S., and new revelations about how it spreads is making prospects look even worse. According to recent research, the insects that transmit the disease don't just spread it from tree to tree—they spread it amongst themselves as well.

In 2006 Florida citrus growers found the bacterium Candidatus Liberibacter asiaticus (Las) in their trees. Las causes a disease known as huonglongbing in Asia and citrus greening in the U.S.—and no matter what it is called, it spells big trouble for citrus crops. Infected trees produce bitter, lopsided fruit that often fails to ripen. (Las causes the biggest problem in oranges, but it can infect all citrus plants.) There is no cure, and slowly the whole tree turns yellow and dies. "This is the most important disease affecting citrus production in the world," says Lukasz Stelinski, an entomologist at the University of Florida.

For several years, citrus growers thought that removing infected plants could stop the disease. The ailment is transmitted by a flying insect called the Asian citrus psyllid. It drills into plants leaves and stems, and sucks out food through its strawlike mouth, or proboscis, transmitting the Las bacteria from tree to tree during feeding, similar to the way infections are spread via shared needles among humans. Until now, it was thought that the malady could only be picked up from an infected tree, but Stelinski's recent work, published in PLoS ONE, showed that the insects not only passed the bacteria to new plants, but that they could also pass Las to one another during mating. So psyllids that pick up the bacteria from one plant can transfer it to other psyllids who have never themselves come in contact with an infected tree. Therefore, although a grower can remove infected trees, a neighbor's sickly saplings could serve as another Las source. This is the first time scientists have documented a bacteria being transmitted sexually between insects, Stelinski says.

When citrus greening was first discovered in Florida in 2006, "it was total doom and gloom," says David Hall, and entomologist at the U.S. Department of Agriculture. Many growers left the business, sold their land and gave up. Controlling the psyllid requires using lots of pesticides, which is expensive, toxic and does not solve the problem if infected psyllids are getting in from nearby groves. Removing sick trees was difficult and costly. Everyone thought: "There's no way that the citrus industry as we know it today would exist," Hall says.

The future of citrus production remains unclear, Hall says, but for now, the citrus industry is spending much of its resources and money to find a solution for citrus greening. Each year it commits about $16 million to research aimed at fighting the disease. From genetically engineering citrus trees and psyllids to finding parasites that attack the bugs, "everything that you might dream of that might lead to a solution is being investigated," he notes. Some growers are simply trying to buy time by feeding infected trees extra nutrients to extend their productive lives. Eventually that cost will creep into supermarkets, Hall says. "All of that gets charged back to the consumer."

"I don't think many Americans realize what a threat this disease is posing to citrus production in the U.S.," Stelinski says. If an effective management strategy is not discovered soon, the industry will have a hard time maintaining productive yields and some growers might opt to sell their land. The doom-and-gloom situation Hall described is still a very real possibility, he adds. "I'm not sure you can actually say that isn't going to happen."

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