American Scientist

The epidemic known as citrus greening is the greatest threat the U.S. citrus industry has ever encountered and a dramatic example of the disruptive effects of invasive species. The Asian citrus psyllid (Diaphorina citri), seen for the first time in Florida in 2005, transmits one of three species of Liberibacter from tree to tree. The bacteria then starve the host of nutrients so that its fruits are deformed, taste bitter, and drop early. As infected trees and psyllids have been inadvertently transported around the world, the disease has caused havoc in citrus industries worldwide, including the leading orange-producing regions, Brazil and Florida.

Citrus greening, which infects all types of citrus trees, has no known cure; it ultimately kills the tree. One major orange grower in Florida, Southern Gardens, has lost a quarter of its grove to the disease. A March 2014 report from the USDA forecast a 15 percent decline in Florida’s orange production from last year, largely due to citrus greening. By 2011, the disease had cost Florida’s economy $4.5 billion, according to a 2012 report by extension scientists Alan W. Hodges and Thomas H. Spreen—no small coin for the state’s $9 billion industry to swallow. More broadly, invasive pests and plant pathogens collectively cost agricultural industries about $66 billion annually according to a 2005 study in Ecological Economics.

Meanwhile, citrus greening continues to spread. It has recently been found in California, Texas, South Carolina, Georgia, and Louisiana, and the psyllid is now present in all citrus-producing states. The disease kept advancing undetected in part because the best-available test for confirming Liberibacter infection has a high false-negative rate. Part of the problem is that the bacteria are not evenly distributed through the tree, so some samples can appear clean even if Liberibacter is present. The best measurements come from parts of the plant that actively display symptoms of disease, but the infection can live in the tree—and spread to others—for years before manifesting. Citrus-producing states require that confirmed infected trees be removed, but many infected trees were (and probably still are) flying under the radar.

California’s citrus industry, second largest in the nation, is taking lessons from the hardships in Florida. “Early detection, early response, that’s our mantra,” says MaryLou Polek, the vice president of science and technology for the California Citrus Research Board, which oversees funding for research. Citrus greening arrived there later than in Florida, and is not as entrenched. In 2008 California found its first Asian citrus psyllid in San Diego. Although it was eradicated there, it was later found in Los Angeles County. Attempts to eradicate the insect failed to cover a large enough area fast enough, so it now infests 11 counties in southern California. In 2012, a Liberibacter-infected tree was found near Los Angeles. California growers are now organizing to treat large areas at a time with a variety of conventional and organic pesticides to stop further spread of the psyllid.

To aid the containment effort, the Citrus Research Board has funded more than $3 million in research on early detection methods this year, and has increased funding on such research every year since 2006. By looking for biological and chemical signals made by the bacterium or infected trees, researchers have identified multiple promising techniques. Wenbo Ma’s lab at University of California, Riverside, has found that Liberibacter secretes proteins that are transported throughout the tree, and these proteins can be economically detected with a common serological technique called enzyme-linked immunosorbent assay. At the same university, Hailing Jin has shown that host trees produce certain small RNA molecules uniquely associated with citrus greening. Carolyn Slupsky and her team at University of California, Davis, are identifying other metabolic compounds that are distinctive to infected trees and are developing techniques to determine how long a tree has been infected.

These and other early detection techniques are now being put to the test in a longitudinal study that compares the biochemical blueprint of uninfected trees (half of which are exposed to disease-free psyllids) with that of infected trees (half infected by grafting and the other by psyllids). “If we can find the pathogen early, there may be only a handful of trees that test positive. If we do nothing, we’ll end up like Florida, where hundreds of thousands of trees in the state are infected with the bacterium,” says Polek.

The Citrus Research Board is collaborating with the USDA to validate the potential testing methods. A major limitation in California is that scientists cannot easily target trees in private backyards. So far the board has identified just a handful of orange trees around L.A. homes that appeared infected, tested them, and (with the homeowners’ consent) replaced them with other ornamentals.

On a larger scale, the Citrus Research Board has partnered with the USDA, California Department of Food and Agriculture, and the University of California to discover a biological control for the Asian citrus psyllid. Biological controls rarely can stop an invasive pest alone; for example, the ladybird beetles used as biological control of hemlock woolly adelgid in forests in eastern North America have had some successes, but only in combination with other control measures. A parasitic wasp that preys on the psyllid in its native range in Asia, Tamarixia radiata, has already gone through the host-specificity and environmental-risk trials that are legally required to release a foreign organism in the United States. Researchers have been experimentally releasing the wasps in southern California since 2011. “The bioparasite is not a silver bullet. It’s not a control measure in its own right,” Polek says. “But we’re hoping that it will be successful in backyard trees where we cannot control pesticide treatments and will bolster organic farming operations.”

The long-term solution to the citrus greening crisis has to involve a combination of blocking transmission from insects and promoting disease resistance in the tree. These issues and solutions necessarily arise in dealing with almost any invasive disease—management hinges on restraining transmission and susceptibility. The citrus industry sees no other choice but to embrace genetic modification of citrus trees to make them less susceptible to Liberibacter. As detailed in a July 2013 New York Times article, growers feel stuck between a rock and a hard place: Consumers want economical, U.S.-sourced citrus fruit and juice, but they don’t want foods labeled as genetically modified. Regardless, a genetically engineered tree takes 7 to 10 years to grow and mature—time the industry does not have.

Growers in Florida have therefore turned to stopgap solutions to salvage what remains of their groves. They are using more pesticides than ever to try to keep the psyllids off their trees. A company called Brillyant Organics offers a fertilizer cocktail containing both nutrients and alleged disease-fighting microbes. Yongping Duan of USDA’s Agricultural Research Service is developing a method called thermal therapy that kills the bacteria in the tree canopy by housing the tree in a tent and heating it to about 40 degrees Celsius. California and other states hope to stop the disease now so they don’t have to rely on such reactive strategies later.

Recognizing the scope of this problem, Congress appropriated $20 million in the Fiscal Year 2014 Omnibus Spending Bill and approved $125 million over five years through the Farm Bill to accelerate research on citrus greening. The resulting research effort—using state-of-the-art genomic, proteomic, and metabolomic techniques—should have far-reaching impacts. It will help citrus growers understand a highly complex disease system. It will also further biologists’ understanding of plant disease epidemics in general, to limit or prevent future outbreaks that threaten the food supply.