CLEMSON — South Carolina is known more for peaches than oranges when it comes to fruit, but researchers in the state could play a key role in saving the nation’s citrus industry from an insect-borne disease that has devastated crops in Florida and threatens other states from California to Texas.

Feng Luo, an associate professor in the School of Computing at Clemson University, is leading a $4.3-million study aimed at protecting citrus crops from Huanglongbing (HLB), a bacterial disease that has affected at least 10 states, including South Carolina.

Feng Luo meets with his team in McAdams Hall at Clemson University.

Feng Luo meets with his team in McAdams Hall at Clemson University.
Image Credit: Clemson University

Once the disease takes hold, the promise of a big, sweet orange ends up small, bitter and lopsided. All citrus trees can be affected, including lemon, lime and grapefruit.

The disease has no known cure and has infected about 75 percent of Florida’s citrus crop, which is a $9-billion industry for the state.

“Florida is taking the brunt in the United States, but this should concern anyone who enjoys eating oranges or drinking orange juice,” Luo said. “The worse it gets, the more expensive citrus products will become and the more jobs will be lost. We need to stop the spread of this disaster. A collaborative approach will be necessary to arrive at a solution.”

Scientists involved in the grant include Yong-Ping Duan of the U.S. Department of Agriculture’s Agricultural Research Service in Florida; Fred Gmitter, Zhanao Deng and Liliana M. Cano, all of the University of Florida; Marylou Polek of the USDA’s Agricultural Research Service in California; and Olufemi Alabi of Texas A&M University.

Luo will use his expertise in bioinformatics to help a team of researchers from across the country figure out which citrus trees are most resistant to the disease and to develop new varieties.

“The project aims is to select naturally occurring mutants from commercial citrus cultivars under the current high HLB pressure in Florida citrus groves,” he said. “It will allow HLB-tolerant/resistant cultivars to be developed and deployed at a much quicker pace without regulatory constraints.”

Funding lasts five years and comes from the U.S. Department of Agriculture’s National Institute of Food and Agriculture. The Clemson award was among $13.6-million in grants recently announced by the USDA to combat Huanglongbing.

Here’s a rundown of what researchers are up against and what they propose to do about it:

The threat: Huanglongbing means “yellow dragon disease” in Chinese, a nod to its discovery in China in 1919. It has occurred in about 40 countries around the world and is also known as citrus greening and HLB.

Huanglongbing, or citrus greening, is spread by the Asian citrus psyllid, a winged insect no longer than a grain of rice.

Huanglongbing, or citrus greening, is spread by the Asian citrus psyllid, a winged insect no longer than a grain of rice.
Image Credit: USDA

The disease is spread by the Asian citrus psyllid, a winged insect no longer than a grain of rice. The nymphs feed on new shoots and leaves, removing sap from the plant and injecting a pathogen.

The leaves curl and twist, as the bacteria hinders growth of new shoots and roots. Symptoms include asymmetrical, blotchy yellowing or mottling on leaves with patches of green on one side and yellow on the other.

Twigs and small branches die. New shoots grow with smaller leaves that sometimes are erect and resemble rabbit ears.

It usually takes about three to five years for a tree to die from Huanglongbing.

The USDA has warned that it could destroy the domestic citrus industry “in our lifetimes.”

In nine years, the number of orange trees in Florida declined from about 80 million to about 60 million, according to the USDA. Grapefruit trees have experienced an even greater drop, going from a peak of about 14 million trees to about 5 million, the department reported.

The Asian citrus psyllid has been detected in 15 U.S. states or territories, leading to full or partial quarantine. Those locations include Alabama, American Samoa, Arizona, California, Florida, Georgia, Guam, Hawaii, Louisiana, Mississippi, Northern Mariana Islands, Puerto Rico, South Carolina, Texas and the U.S. Virgin Islands, according to the USDA.

Experts do not believe the disease has the potential to spread to the peach and apple trees that are important to the agricultural economy of South and North Carolina.

“This bacteria and its insect vector are restricted only to citrus and its relatives,” Gmitter said.

Research strategy: Some trees are showing more resistance to Huanglongbing than others.

Luo will analyze data provided by collaborators to help understand the genetics underlying the disease-resistant trees. He will use the techniques of transcriptome profiling and comparative genomics.

Symptoms of Huanglongbing include asymmetrical, blotchy yellowing or mottling on leaves with patches of green on one side and yellow on the other.

Symptoms of Huanglongbing include asymmetrical, blotchy yellowing or mottling on leaves with patches of green on one side and yellow on the other.
Image Credit: USDA

As an expert in bioinformatics, Luo is well qualified to the lead the research. Bioinformatics is the science of collecting and analyzing complex biological data such as genetic codes. Luo has done previous research with the USDA, which led to the work targeting Huanglongbing.

Clemson’s world-class supercomputer will help enable the project. The university’s Palmetto Cluster ranks as the world’s 155th most powerful supercomputer, according to the most recent Top500 List.

Commendations from top officials: Praise for Luo and his team came from across the Clemson University College of Engineering, Computing and Applied Sciences.

Eileen Kraemer, the C. Tycho Howle Director of the School of Computing, said the research reflects the high quality of research done in the school.

“We’re honored that Dr. Luo has been chosen to lead this project,” she said. “He is a highly regarded expert in bioinformatics and highly deserving of the award. Congratulations to Dr. Luo and his team.”

Douglas Hirt, associate dean for research and graduate studies, said the size of the grant underscores its urgency.

“This is absolutely critical research for the citrus industry and anyone who enjoys citrus products,” he said. “With Dr. Luo and the Palmetto Cluster, Clemson University is well-positioned to play a leadership role in tackling this global problem.”

Anand Gramopadhye, dean of the college, said Luo and his team are doing critical research that could have immediate real-world impact.

Huanglongbing affects citrus trees, including the one pictured.

Huanglongbing affects citrus trees, including the one pictured.
Image Credit: USDA

“The award is a testament to Dr. Luo’s expertise in bioinformatics and the world-class facilities available to Clemson researchers,” Gramopadhye said. “I congratulate Dr. Luo and his team.”

Quotes from collaborators:

Fred Gmitter: “I have been breeding citrus for three decades, and the challenges presented by HLB are far greater than any other objective we have tackled.  Having worked with citrus trees in the ‘natural selection laboratory’ that the state of Florida has become, it has been very interesting and encouraging to see some dramatic differences in sensitivity among the different kinds of citrus that we have developed in the UF breeding program, as well as in groves throughout Florida.  The HLB problem is indeed complex, but we can see glimmers of hope for long-term breeding solutions as we evaluate the diversity of citrus exposed to this disease.  I am looking forward to working with this team because we will be able to answer some fundamental questions regarding these inherent differences. But more importantly, we also will use that information and understanding, in a strategic plan for delivering superior varieties to the U.S. citrus industries across the time course of our project.”

Marylou Polek: “I am very excited to be working with this team of world-class scientists on this project! California has not yet experienced the devastation caused by this disease  and I am both optimistic and determined that the products resulting from this research will be in the hands of citrus growers throughout the United States in time to save the fresh fruit industry in California, Arizona and Texas, and allow the Florida citrus industry to recover and regain its viability.”

END