Citrus greening: Is the end in sight?

Natural Soil Bacterium May Provide Treatment to Citrus Scourge

Scientists with the Agricultural Research Service (ARS) may have found a potential remedy for the untreatable disease that has devastated the American citrus industry since it arrived in Florida in 1998.

Huanglongbing (HLB), also known as citrus greening, is a disease that infects citrus trees in a way that renders the fruit useless and slowly kills the tree. Since 2005, HLB has spread throughout Florida, killing countless trees and devastating orchards, reducing citrus production by 75%, and more than doubling production costs. The disease has now made its way west through Louisiana, Texas, and into California.

It is spread by the Asian citrus psyllid (ACP), a tiny sap-sucking insect that carries Candidatus liberibacter asiaticus (CLas), the bacterium that causes HLB. Psyllids transmit the disease by injecting a CLas-loaded salivary toxin into the tree as they feed. After that, there’s no hope for the tree.

Until now. Researchers at the ARS Crop Improvement and Genetics Research (CIGR) unit in Albany, CA, have discovered a way to augment the tree’s natural resistance to pathogens, including HLB. According to James Thomson, a geneticist at CIGR, by incorporating receptors that can recognize pathogens, they are able to activate a plant’s own innate immune responses.

Leaves with symptoms of citrus greening disease. (Photo by Tim Gottwald, d2259-1)

The challenge in developing this approach was to identify the appropriate HLB-recognition genes, incorporate enough of them to be effective, and design a pathway to introduce them into trees.

One way to deliver the genes is to use agrobacteria. ″Agrobacteria is a microbe that originated in soil, but has been turned into a plant engineering tool,″ Thomson explained. ″Essentially you clone the DNA of interest [in this case, from plants that have a natural resistance the pathogen of concern] and add it to the agrobacteria, then the agrobacteria adds that specific bit of DNA to the genome.″

Getting the ″loaded″ DNA into targeted trees is the next step. ″This is all done in the lab through tissue culture,″ he said. ″A bit of the original plant is cut into little pieces and mixed, temporarily, with the agrobacteria. The plant pieces are then cleaned of agrobacteria and encouraged to regrow into a whole plant.″

Asian citrus psyllids feed on a citrus stem. (Photo by Peggy Greb, d2004-2)

Previously, all attempts to ″cure″ a diseased tree have failed; the only real ways to deal with HLB were to remove affected trees from orchards and to kill the ACPs that were spreading HLB. Those efforts ranged from spraying pesticides to cloaking trees in tents and steaming them to eradicate the bugs.

Now that they have a more effective solution in hand, the researchers’ plan is to transfer their knowledge and technology to tree nurseries, where growers will be able to purchase the trees they need, when they need them, Thomson said. The result would have at least a three-fold economic benefit: growers will get disease-free trees to farm, nurseries will see increased profits, and consumers will have a larger (and potentially cheaper) supply of citrus products.

″I see [this] HLB-fighting technology being deployed in the next several years,″ Thomson said. ″There are already a series of genes that can detect and fight HLB; the biggest problem is how to distribute the solution. This project has made strides in identifying genes from other plants that activate defense responses in the presence of HLB.″