Citrus industry fights 'greening' as it waits for disease-resistant trees

By Sarah Gonzalez

© Copyright Agri-Pulse Communications, Inc.



WASHINGTON, Feb. 26, 2014 - The U.S. citrus industry may not have time to wait for the availability of a genetically engineered fruit-bearing tree resistant to the growing affliction of “citrus greening,” researchers said last week at the USDA's annual Outlook Forum in Arlington, Va.

The disease, which causes fruit to turn green after ripening, is threatening to destroy Florida's $9 billion citrus industry if no viable solutions are found, said Marylou Polek, vice president of science and technology for the Citrus Research Board in California. She said the state industry may have just two years to find an effective way to combat greening.

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The disease, which is technically known as Huanglongbing (HLB), or yellow dragon disease, is so devastating that 30 percent of the root mass is lost before above-ground symptoms are observed, Polek explained. “It can take several years to notice on a mature tree,” she said.

HLB, first detected in Florida in 2005, spread to most of the state's citrus counties by 2008. Now, about 75 percent of Florida's 69 million citrus trees may be infected and greening poses a threat to the entire U.S. citrus industry. The three species of bacteria that cause HLB probably originated in China in the early 1900s, according to USDA. Affected trees have stunted growth and produce small, bitter tasting, irregularly-shaped fruit that remains green at the bottom.

Ed Stover, a subtropical insect specialist with USDA's Agricultural Research Service (ARS) in Florida, said that genetically engineered trees have had significant levels of resistance to the disease in field trials. One experiment involved the transfer of spinach genes into citrus trees. That research is being conducted by Erik Mirkov, professor of plant virology at Texas A&M.

The 2014 Farm Bill authorizes $125 million to fund this and similar research related to HLB over five years.

Stover said there is still a “substantial amount of work that needs to be done” before any GE solution would be deregulated. He said the availability of a GE solution with 10 years is “a reasonable guess.”

In the meantime, “We have to survive with short-term solutions” to keep this industry sustainable, said Prakash Hebbar, a pest management specialist for USDA's Animal and Plant Health Inspection Service who also works with Florida's Citrus Health Response Program.

ARS plant pathologist Yongping Duan in Fort Pierce, Florida, found that heating HLB-infected trees in the sun by encasing them in plastic tents can stall the negative effects of the disease. This  technique, known as thermotherapy, may work in field trials, but there is no current method for large-scale implementation, Stover said, especially on the 500,000 acres of citrus affected in Florida. “We think this method could be effective for a year, but how do you do this over 500,000 acres?” he said.

Another technique using antimicrobial injections can eliminate the HLB bacterium in the greenhouse and greatly reduce disease levels in field trees, Stover said.  However, researchers are trying to find spray applications that can be used practically over large areas.

Providing a major advancement for citrus greening research, ARS scientists in Fort Pierce, Florida, recently published the full genomic sequence of HLB bacterium, “Candidatus Liberibacter asiaticus.” The bacterium is transmitted by plant-feeding insects called psyllids.

“We may be getting insight into the evolution of this new disease,” Stover said.

Although several citrus genomes are sequenced, a lot more data need to be collected to fully understand and take advantage of the interaction between the HLB bacterium and citrus, he added.

“There is considerable HLB resistance in the citrus gene pool-this is good news,” Stover explained. He studied a certain citrus hybrid that was found to have the greatest resistance to HLB in the gene pool. Unfortunately, he said the fruit from the hybrid tastes horrible. However, Stover said there are field experiments being conducted on 85 different genotypes related to citrus.

Another significant challenge is the number of different strains of the HLB disease, just within Florida. Different strains may mean different management techniques. Also, the process for testing certain hybrids for resistance is lengthy, taking around eight months after initial inoculation, he said. 

Stover said that while Florida's citrus industry is “pretty much committed to living with this disease,” in California and Texas, the focus is mostly on psyllid monitoring and management, as well as control techniques in residential areas.

“Other states are very fortunate that HLB was found in Florida and all of this research was mobilized,” he said.  Still, the disease continuing to spread. Earlier this month, the Texas Department of Agriculture established its third quarantine zone for greening over a five-mile radius in LaBlanca. The other quarantine zones are in San Juan and Mission.

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