Orange growers feel the squeeze of new plague; WSU researcher contributes expertise to solution

PULLMAN, Wash. – A pandemic is destroying orange groves in Florida. The disease, called citrus greening, is also spreading to citrus groves in Texas and California, threatening a more than $3 billion per year industry. If left unaddressed, the entire U.S. citrus industry could be wiped out and, as Florida Sen. Bill Nelson said, “We’ll end up paying $5 for an orange – and it’ll have to be one imported from someplace else.”

Citrus affected by citrus greening disease. (Photo courtesy of the U.S. Department of Agriculture)
Citrus affected by citrus greening disease. (Photo courtesy of the U.S. Department of Agriculture)

Citrus greening disease is spread by bacteria that block trees’ nutrient and water channels and prevent fruit from ripening.
“It’s like choking the tree from the inside out,” said David Gang, a Washington State University molecular biologist and biochemist who is collaborating with a large, interdisciplinary team to combat the disease.

Insect spreads disease

The bacteria are hosted and spread by an insect related to aphids and whiteflies called the Asian citrus psyllid (pronounced sill-id). The disease is thought to have spread from China in the early 2000s. Citrus greening has already destroyed the citrus industry in Jamaica.

The invasive psyllids pierce the citrus trees with a needle-like mouthpiece, similar to the way a malaria-transmitting mosquito infects its victims. As it feeds on the tree’s water and nutrients, the psyllid injects the disease-causing bacteria, which then spread through the rest of the plant.

To combat this aggressive disease, the U.S. Department of Agriculture has funded a multifaceted, multi-institutional initiative involving more than 40 researchers located in several states. Scientists are looking at the disease’s ecological consequences, the biology of citrus trees, the insects and the mechanism by which the insect transmits the bacteria.
Pesticides have been of some use in controlling the psyllids but researchers are concerned the insects will develop resistance. And biocontrols – siccing good bugs to prey on the bad ones – have proven ineffective because the psyllids simply outbreed their predators.

That’s where David Gang enters the scene.

Altering the insect

David Gang
David Gang

Gang’s lab in the WSU Institute of Biological Chemistry focuses on using new technologies like genomics and proteomics to study plant defense mechanisms, particularly the chemical compounds that help plants survive and combat pathogens and pests. In the USDA-funded project, Gang and his colleagues isolate and sequence the genes being expressed in the psyllids as they feed on citrus plants.

“Getting good quality data from this kind of approach is actually quite challenging to figure out,” Gang said. “But once you learn how to do it, you can do it relatively routinely; because of that, we were invited to be part of this project.”
Once Gang and his colleagues obtain gene expression data, they make it available in a database for their collaborators to use.

“We hope this data can be used to develop a ‘nupsyllid’ (as in ‘new-psyllid’) that will be unable to transmit or harbor the citrus greening bacteria,” Gang said.

The researchers are also turning to genetic engineering as a last resort weapon against citrus greening: “We can shut off genes that are involved in transmission of the bacterium,” said Gang. Since citrus plants have no inherent defense and in view of the fact that consumers reject genetically engineered food, the research team is focused on modifying the disease-transmitting pest.

Because citrus greening disease infects and weakens its insect host, Gang said that “nupsyllids” are expected to outcompete and eventually replace the disease-spreading psyllids.

Potential damage devastating

Insuring the economic and horticultural health and sustainability of the U.S. citrus industry is the goal of the USDA-funded five-year project.

“The investment the USDA is putting into this project is really very small compared to the economic damage already caused by this disease and is trivial compared to the potential damage that could be caused down the road,” Gang said. “It’s one of those things where we don’t really have a choice. If we don’t do something, all of the citrus trees in the U.S. will likely be dead within 10-20 years.”

That’s why he and his colleagues are seeking a workable genetic solution to the citrus greening challenge, Gang said: “We’re kind of proud of the fact that it’s difficult to do and we’re good at it.”

-Chelsea Pickett, science writing intern