In April, researchers at the University of Florida (UF) had made noted progress in stopping the spread of the bacteria that causes citrus greening, a devastating disease that has infected millions of trees in Florida over the last decade and threatens the $9 billion U.S. citrus industry. Their research was published in the online open-access journal PLOS Pathogens.
Originated in China, citrus greening (also known as huanglongbing – Chinese for “yellow shoot disease”) is considered the most devastating of all citrus diseases. Infected trees produce misshapen, discolored fruit with a bitter taste and die within a few years of infection. The bacteria, Candidatus Liberibacter asiaticus, travels on the Asian citrus psyllid, which has developed resistance to many common pesticides. Much of the nine percent decline in U.S. citrus production in 2013 is attributable to citrus greening.
Public awareness of the disease, the threat it poses to the U.S. citrus industry and consumers’ access to orange juice were heightened by a July 2013 article in the The New York Times detailing the problem and examining the potential solutions, including genetically modifying orange trees to develop resistance to the bacteria. The article reported on an effort at Texas A&M University, where scientists are adding a gene from spinach to orange trees. The gene produces a protein that fights off bacteria. While this method shows promise, it needs significantly more testing before it can be brought to market, and has also spurred much conversation within the industry about concerns that opposition to genetically modified foods (GMO) might make consumers wary of GMO orange juice and other products.
Recognizing the dire nature of the threat, the U.S. Department of Agriculture (USDA) has allocated $31.5 million for research and outreach to combat the disease. Early grants include projects testing antimicrobial treatments, the use of high-heat treatments to kill the bacteria, and the establishment of model groves to determine whether particular nutrient management practices can prove effective in staving off the effects of HLB. Another project is facilitating the release of parasitic wasps that are natural predators of the psyllids. The USDA also established the Huanglongbing Multi-Agency Coordination Group and will allocate millions more toward efforts to eradicate the disease.
The UF research team is led by Claudio Gonzalez and Graciela Lorca, associate professors in the Department of Microbiology and Cell Science, part of UF’s Institute of Food and Agricultural Sciences. Three biochemical treatments were tested. One treatment, benzbromarone, targeted a protein known as LdtR and disrupted the greening bacterium’s ability to survive and spread. Benzbromarone’s most common use is the treatment of gout in humans. The experiments, done on tree shoots in a greenhouse, were effective at stopping the bacteria in 80 percent of the shoots.
The team plans to conduct further field research, which may yield different results. Government regulations and the need for field-testing mean it will be at least five years before a commercial product could be brought to market. Testing will determine if the chemical treatment has any effect on the fruit’s flavor, on other aspects of tree health, or on human health.
Meanwhile, a research team in California released hundreds of thousands of parasitic wasps, native to Pakistan, killing the psyllids that carry the bacteria that causes citrus greening. Another study in Florida found that exposing infected seedlings to high temperatures could reduce or even eliminate citrus greening from the plant.
By looking at a wide range of possible solutions to the problem, growers are holding out hope that a number of solutions will yield results that can save the industry from what some call the most devastating threat to citrus worldwide.
Winton Pitcoff is a freelance writer based in western Massachusetts.