Citrus-growing regions face different pressures

by | Nov 21, 2016

cilia-orangetreeCitrus growers are uniting to save their groves from citrus greening disease and to fund research into solutions, but growers in California face different challenges than those in Florida, report BTI and USDA researchers.

John Ramsey, a USDA Agricultural Research Service (ARS) postdoctoral associate in the Cilia lab   and Surya Saha, a senior bioinformatics analyst in the Mueller lab, recently returned from presenting their work at major citrus greening conferences. In California, where citrus greening disease has only been detected to a few backyard trees, growers are desperate for accurate, cost-effective early detection methods. But in Florida, where every grove is heavily infected, the industry is open to all kinds of solutions, including replanting with resistant trees.

Ramsey was invited to present his research at the California Citrus Conference, a meeting of growers and citrus industry workers organized by the Citrus Research Board and California Citrus Mutual. Researchers in the Cilia lab are using proteomics approaches that study all of the proteins within the cells of the citrus tree, the CLas bacterium that causes the disease, and the Asian citrus psyllid that acts as the carrier. Citrus greening disease is a scourge for growers because the infection is fatal and infected trees produce bitter, green unmarketable fruits.

“We’re using our basic proteomics research to identify targets of certain genes, proteins or pathways within the insect, which some anti-citrus greening agent or inhibitor could be designed against,” said Ramsey. “We have generated a shortlist of insect proteins that are important for pathogen transmission, and we’re now using their structure to intelligently design inhibitors of those proteins and their interactions with the pathogen.”

Because growers in California still have healthy groves to protect, they are most interested in better ways to detect the infection in their trees and to prevent its spread. The California Department of Food and Agriculture (CDFA) has detected CLas in a handful of backyard fruit trees, but so far, the bacterium appears to be limited to small pockets in Southern California.

Traditionally, the CDFA has diagnosed citrus greening disease in trees and psyllids using a molecular biology technique called qPCR, which detects the presence of DNA from the CLas bacterium. However, not all leaves in an infected tree will contain the bacterium. New approaches presented at the meeting include the use of drones to fly through groves, looking for evidence of infected leaves, and the detection by trained service dogs of volatile scent compounds given off by the trees, which can give an accurate indicator of infection. Whichever method they ultimately choose, it should be sensitive, affordable and rapid to ensure consistent and frequent monitoring.

Growers are also working as a community to slow the spread of the Asian citrus psyllid, through careful pesticide spraying and by only transporting citrus to nearby packing houses, rather than shipping insect hitchhikers across the state.

Saha presented his work in collaboration with the Cilia lab at the California Asian Citrus Psyllid and Huanglongbing Research and Extension Summit at the University of California, Riverside. In the Mueller lab, Saha and colleagues are creating bioinformatics tools through the website They are creating a database to hold information generated by genomic, proteomic and gene expression studies of the citrus, psyllid and CLas. The researchers are in the process of rolling out the improved second draft of the Asian citrus psyllid genome and an “expression atlas” is also in the works with the Cilia lab, which will compare which genes are turned on and off in the insect when it feeds on different hosts in the presence and absence of the pathogen CLas.

“We are trying to provide an infrastructure for knowledge mining of –omics data from this tri-trophic disease system,” said Saha. With so much information available, these bioinformatics tools enable researchers to formulate better questions, which can then be explored in a lab environment.

Ultimately, they hope to model the three-way system. “We can develop resources that will allow people to look at facets of the pathogen, of the psyllid and of citrus, together within one system,” said Saha.

Meanwhile in Florida, citrus greening disease is already devastating groves statewide, making the Florida citrus industry desperate for a solution. But once a viable treatment option is developed in the lab, growers will still face a necessary delay as they wait for regulatory approvals, especially if the solution is a genetically engineered citrus variety.

University of Florida Professor Fred Gmitter is developing citrus varieties that still have strong citrus yields despite infections. Planting improved citrus varieties is an option for the Florida citrus industry, which is losing its trees, but this solution is much less appealing to growers in California, who simply want to protect the groves that have been passed down in their families for generations.

“The experience of driving from the airport in Fresno and going to the meeting in Visalia and seeing citrus as far as the eye can see really brought home what we’re trying to protect,” said Ramsey.

The BTI research presented at these meetings was funded by the USDA (2015-70016-23028, 6034-22320-001-00 and 8062-22000-021-00) and the California Citrus Research Board (5300-155, 5300-163).

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