Citrus Greening: Is the End in Sight?

Researchers Boost Citrus Tree Immune System

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 was first detected in Florida in 2005.

Huanglongbing (HLB), also known as citrus greening, is a disease that infects citrus trees in a way that degrades the fruit 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 to Louisiana, Texas, and California.

Citrus greening 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 as they feed. Once the disease is established, there’s no hope for the tree.

However, researchers at the ARS Crop Improvement and Genetics Research (CIGR) unit in Albany, CA, have discovered a way to augment the tree’s natural immune response to pathogens so that it recognizes HLB. According to James Thomson, a geneticist at CIGR, transgenic plants that produce receptor proteins that can recognize pathogens are able to activate a plant’s own immune response when exposed to Clas.

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 from other plant species and incorporate the right ones into transgenic plants to see a response to Clas. James and his collaborators used the potato as a model system and eventually identified three receptors that they thought might respond to Clas in citrus. They are currently testing their results in citrus plants.

Previously, the best ways to deal with HLB were to remove affected trees from orchards and kill the ACPs that were spreading HLB. Those efforts ranged from spraying pesticides to cloaking trees in tents. Transgenic trees with a boosted immune system that responds to Clas could potentially help citrus growers fight citrus greening.

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

“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.” – by Scott Elliott, ARS Office of Communications