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Insect is Turning Florida's Oranges Green

Scientists try to block the bug's genes--or harden the trees



FLICKR/ROSSJL

Slowly but surely, Florida's oranges are going sour.

Since its discovery in 2005, a grave disorder known as huanglongbing (HLB), or citrus greening disease, has ravaged the Florida citrus industry. Borne by the Asian citrus psyllid, a small, invasive insect related to the aphid, HLB has spread to every county in the state, causing the citrus trees it infects to sprout misshapen, sour fruit. Once a tree is infected, there is no cure.

Shedding acreage thanks to real estate development and previous bouts with contagion, the citrus industry is doing all it can to fight the disease, pouring millions of dollars into research that it hopes will develop trees resistant to HLB or decimate the psyllid's ability to reproduce or process tree flesh. A gathering of such researchers convenes today at the National Academies, where scientists from across the country will share their efforts -- many based in genetics -- to stem the greening plague.

Almost $10 million will go to funding greening research this year, which "reflects on how serious they believe the threat is to the industry's future," said Jim Graham, a professor at the University of Florida working on creating genetically modified citrus trees.

"In the very south of Florida, it's been devastating," Graham said. In less than two years, the disease can go from infecting 1 percent to 100 percent of a grove if no control measures are taken. The disease may take root in a tree for several years until visible symptoms are seen -- and those symptoms can easily be confused with nutritional defects.

"Some say the survival of the industry may depend on finding [scientific] solutions," Graham said.

These research investments may already be paying dividends, as late last month, the first potential greening-resistant trees were planted in a small testing grove by Southern Gardens, one of Florida's largest citrus producers. Southern Gardens developed the trees, which are little more than seedlings now, with researchers at Texas A&M University.

While the genetically modified trees have shown promising results in the lab, the firm wants "to see if the lab study carries over to actual commercial groves," said Judy Sanchez, a spokeswoman for U.S. Sugar, which owns Southern Gardens. The company, which has three groves in southwest Florida infected with HLB, received permission for the field trial from the Agriculture Department.

There are no species of citrus tree that are immune to greening, which has been found in Asia since the late 1800s. If such a strain existed, building tolerance would be a simple exercise in breeding. Instead, researchers at Texas A&M and elsewhere are focused on developing transgenic trees that have DNA spliced in to create antimicrobial peptides, chains of amino acids that attack the bacterium that causes HLB.

There is no notion how effective these trees will be yet, Graham said. There is some precedent for such splices, he added, as similar methods have been used in apples to combat a bacterial disease called fire blight.

Focus on psyllids

The greening disease is spread by the Asian psyllid, a jumpy, brown insect that was first detected in Palm Beach County in 1998. From there, it quickly spread throughout the state, relying for its food not on commercial citrus groves but rather on an ornamental citrus plant known as orange jasmine or jessamine. These plants, used to decorate homes, have also proved to be potent hosts for HLB.

Many control strategies for greening have focused on the psyllid. Groves have been flooded with insecticide, with limited success, since resistance develops. There have also been attempts to douse trees with pheromones that disrupt the mating process. Growers in Vietnam have also found that cultivating guava among citrus trees reduces greening. These conventional control methods, along with aggressive identification and culling of infected crops, have had some success in slowing the spread of the disease.

To develop more effective ways of controlling psyllids, geneticists have begun mapping the insect's genome. And already, comparisons of the psyllid against fully mapped insects like the fruit fly and mosquito are allowing scientists to rapidly gain knowledge of the psyllid's physiological processes, according to Robert Shatters, a scientist at the U.S. Horticultural Research Laboratory.

Shatters and his fellow researchers are looking at ways to block some aspect of the psyllid's feeding or digesting process, he said. Through analysis, his lab is determining what genes are used for taking in food, he said, and if you "block those, then the plant is no longer a suitable host."

Other geneticists are interested in "lethal genes," Shatters said, which "mess up development." It could be a gene that, harmless when inserted into a male psyllid, will render any female heirs of that modified insect infertile or dead. Researchers can already do this in lab situations, he said, by blocking genes with RNA interference, a recently adapted biological process that allows nearly any gene to be easily silenced.

There are challenges ahead, Shatters said. Breeding a large population of modified psyllids would be difficult, since they only feed on citrus. In his rosiest projections, within four years, his lab could be testing its modified psyllids in the field.

Shatters and others suspect a multitiered effort will be needed to stop the disease. A resistant tree like those possibly developed by Texas A&M would seem to be the easiest solution, he said.

"But we do know with biology there is the dance of evolution," he said, warning that the psyllids could adapt to modified citrus trees.

Such solutions cannot come quickly enough for Florida's embattled citrus industry, which has been stressed since the 1990s.

"Developers were gobbling up land as well as disease gobbling up land," said Mark Fagan, a spokesman for Florida's citrus programs. "Yes, there has been a loss of acreage."

Many growers have had their groves for generations and do not want to give them up for real estate development, Fagan said, but the devastating effect of HLB could finish many groves off.

While other diseases, like canker, may leave lesions on oranges, they do not change taste, allowing fruit to be juiced, Fagan said. Not so with HLB.

"Fruit with HLB, [it] can't be juiced or eaten," he said. "It has a horrible taste. The tree is just completely useless."

Reprinted from Greenwire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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