UF researchers are taking a three-pronged approach to fighting a potentially devastating citrus disease By Kevin Bouffard
Scientists at the University of Florida have a long history of tackling threats to Florida’s signature crop — from freezes to canker.
But when citrus greening — known to scientists as huanglongbing or HLB — was detected in Homestead in 2005, researchers at UF’s world-renowned Citrus Research and Education Center (CREC) in Lake Alfred knew they were in for the fight of their lives.
Greening has devastated citrus crops wherever it has appeared, working its way around the globe from Asia to Africa to South America. The disease slowly weakens and kills all types of citrus trees, and since it can take years before the first symptoms appear, greening has ample time to spread.
Last year UF economists found that between 2006 and 2011 greening had cost the state $4.5 billion in lost economic output, and 8,257 jobs.
“Citrus greening represents a devastating burden on this state’s economy and we’re working around the clock to help,” says Jack Payne, UF’s senior vice president for agriculture and natural resources.
Effective measures against greening cannot come quickly enough for Florida growers, who are just now beginning to see the damage the disease can do to their groves.
Last fall, Hamlin and other early-season oranges harvested from October to March began falling in unprecedented numbers before harvest.
“Everybody is still shocked with how many Hamlins … fell from the tree,” says Ellis Hunt Jr., a Lake Wales grower and member of the Florida Citrus Commission.
The fate of Valencia oranges, harvested from March to June, does not look much better. In February, the U.S. Department of Agriculture changed its projection of premature Valencia drop to “well above average” from “about average” a month earlier.
“It’s the highest drop rate we’ve seen without a weather event,” says Vic Story Jr., a second-generation citrus grower based in Lake Wales, who owns or manages about 5,000 grove acres in Polk and Hardee counties.
“The greening is looking worse all the time. We’re worried more about future years than this year,” adds Jay Clark, a Wauchula grower and member of the Florida Citrus Commission. “What looks alarming right now is these 3- to 5-year-old trees. It looks questionable whether we can grow a young tree to maturity.”
So it’s no wonder that since the appearance of greening in Florida, researchers with UF’s Institute of Food and Agricultural Sciences have been directing almost all of their significant intellectual and financial resources at the disease.
With financial support from the Citrus Research and Development Foundation — a UF direct support organization established with the proceeds from a tax on every box of citrus produced that generates about $5 million annually — scientists at UF and other institutions have pursued a three-pronged approach to combating greening.
First, control the Asian citrus psyllid, the ant-sized insect that transmits the greening bacteria from tree to tree. That offers growers the best hope for immediate results.
Second, understand the bacterium itself, with the goal of neutralizing impact on psyllids or trees.
Third, breed new citrus trees that will tolerate or resist infection.
The psyllid was first detected in two South Florida counties in June 1998. Although at the time it did not carry the greening bacteria, it made establishment of greening more likely if the disease were introduced.
In an attempt to get ahead of the problem, UF entomology Professor Marjorie Hoy and Ru Nguyen, an entomologist with the Florida Department of Agriculture and Consumer Services, went looking for a natural enemy of the psyllid. They found it in a parasitic wasp from Taiwan and Vietnam called Tamarixia radiata. Adult wasps eat some young psyllids and lay their eggs in others. Then the wasp larvae eat the psyllids’ guts from within.
After making sure the wasps would harm only the psyllids, the state began releasing them in 1999.
“Tamarixia radiata is now widely established throughout Florida, feeding on the psyllids and reducing their population by as much as 80 percent in some locations between August and November,” Hoy says.
But the psyllid is a prolific breeder, with each female laying up to 800 eggs and some trees infested with 40,000 bugs, so even an 80-percent kill rate by the wasps is not enough.
That’s why UF entomologist Michael Rogers and his colleagues are seeking a better understanding of the relationship between the psyllid, the bacteria and the tree.
Rogers says he never envisioned he’d be gluing tiny gold wires to the back of insects just 4 millimeters long so he could track their movements.
But when citrus greening was confirmed in Florida, “We had to shift all our focus to the psyllid,” says Rogers, who estimates that 90 percent of his research since 2005 has involved the psyllid.
The psyllid gives off different signals when it feeds, acquires the greening bacteria or infects the plant, so Rogers and his colleagues have been recording these signals and using them to fine- tune pesticide programs for young citrus trees.
Another challenge of controlling the psyllid is that the insects can travel as much as 1.25 miles in 10 days so any pesticide spraying program has been far ranging and comprehensive.
But some land owners have trees on their property but are not cultivating them and so do minimal or no pesticide spraying. These so-called “bad neighbors” negate the efforts of other growers who are spraying.
Based on Rogers’ research showing that low-volume aerial pesticide spraying was much less expensive than ground spraying, the state convinced growers, including “bad neighbors,” to work together to establish 38 Citrus Health Management Areas (CHMAs) to coordinate the timing and application of pesticides within a county or region. CHMAs covering 80 percent of Florida’s commercial citrus acreage resulted in a 50-percent reduction in the state’s psyllid population in 2012 compared to the previous year, Rogers says.
Lukasz Stelinski, also an entomologist at the CREC, says another challenge is preventing the psyllids from becoming resistant to pesticides.
“Currently, insecticides are our best tools,” Stelinski says. “We don’t want current levels of resistance to escalate further.”
Stelinski and his colleagues compared mortality levels from insecticide exposure in psyllids collected from groves in 2009 and 2010 to psyllids raised in the lab in isolation and found that the insects were becoming resistant.
Stelinski says the results are proof that resistance management strategies are imperative. These strategies include using different types of insecticides in a rotation and never applying the same chemical twice in a row.
Stelinski also leads research that found that the bacterium responsible for citrus greening causes infected trees to emit a fragrant chemical called methyl salicylate. When psyllids catch a faint whiff of methyl salicylate they interpret it to mean that other psyllids have found a good place to feed. But once they start to feed on an infected tree, they discover that it doesn’t taste quite right.
Unfortunately, even a short feeding session is enough for the insects to suck up the greening bacterium, along with the plant sap they consume, and carry it to the healthy trees.
The good news is that methyl salicylate is inexpensive and widely available, so it could be used to battle greening. For example, it could be put in traps to monitor groves for the presence of psyllids. Or, it could be released in small amounts throughout a grove to camouflage infected trees’ scent.
In addition to pesticides, citrus researchers have just begun an effort to genetically engineer psyllids so they cannot host or transmit the greening bacteria. The scientists hope to produce a psyllid biologically incapable of carrying or passing on the greening bacterium, says CREC Director Jacqueline Burns. The insect could then be reared in laboratories and released to mate with wild psyllids, ensuring that the no-transmission trait would spread.
While researchers like Rogers and Stelinski are focused on the psyllid, their colleagues are trying to gain a better understanding of the greening bacterium.
One of the first challenges was to determine whether greening was caused by a single species of bacteria or multiple bacteria and/or viral pathogens working together.
Using three types of next-generation genetic analyses, UF researchers examined inner bark from Florida citrus trees infected with citrus greening.
While the team conclusively found the genetic fingerprint of the bacteria commonly suspected to be behind the disease, Candidatus Liberibacter asiaticus, the analysis showed no other DNA of suspect viral or bacterial pathogens.
“This research tells us that our work, much of which has been focused on Liberibacter, is dead on target,” says Burns. “And it gives us confidence to move on with research that helps target this pathogen.”
Along with potential treatments, the genetic analysis could help lead to fast, inexpensive testing methods that can be early indicators of disease.
The research is important because the disease has been especially difficult to analyze, says Eric Triplett, chair of UF’s Department of Microbiology and Cell Sciences and lead researcher on the study.
Normally, researchers would capture a sample of bacteria, grow it in a petri dish, then insert it into a healthy tree to see if it causes the disease. But scientists have not yet found a way to grow greening bacteria in the lab, which is why its scientific name is Candidatus, meaning it has never been cultured.
UF researchers have also created a mathematical model that predicts how citrus greening is transmitted within an infected tree. The model revealed that once a tree is infected, pesticides may not be enough to halt the disease’s progression. It also showed that removing diseased new growth is not a solution because many shoots may already be infected without exhibiting any symptoms.
Plant pathologist Ariena van Bruggen of UF’s Emerging Pathogens Institute (EPI), who supervised the model’s creation, says having the model is “just a first step,” albeit an important one.
“This model gives us hints about experiments we can do, where to look, where to focus our efforts to solve the problem,” says EPI Director J. Glenn Morris Jr.
Other researchers are looking for genetic material that can neutralize the greening bacteria’s action within the citrus tree. The search began after William Dawson, an eminent scholar in plant pathology at the CREC, developed a benign citrus tristeza virus, or CTV, capable of introducing new genetic material into trees.
Dawson and his colleagues are now testing hundreds of antimicrobial peptides, a class of amino acids found in all living organisms, that can be introduced into a citrus tree and undo the harmful effects of the citrus greening bacteria.
“We have a good gun, but we don’t have a good bullet,” Dawson says, adding that if one of the peptides works it would offer an immediate solution.
Dawson’s vector “has already accelerated a lot of research,” says Harold Browning, director of the Citrus Research and Development Foundation. “Many research programs are using it as a gun.”
Dawson’s technology is promising enough that Southern Gardens Citrus Processing Corp., a subsidiary of U.S. Sugar, recently entered into a licensing agreement with the University of Florida. In return for financing the approval costs for the technology with federal regulators — which could take three to five years and cost $20 million to $30 million — Southern Gardens, one of the state’s largest growers with 1.8 million trees on 16,500 acres, would get an exclusive right to the technology and get a share of the royalties when others use it.
A Florida citrus grove may look very different to future generations if growers adopt the Advanced Citrus Production System (ACPS) that IFAS researchers are prototyping on 14 acres in Auburndale with John Strang of Gapway Grove Corp.
ACPS is a collaborative research project among several CREC scientists led by Arnold Schumann, UF associate professor of soil and water science. It seeks to re-engineer a citrus grove from the ground up, employing “open hydroponics,” a drip irrigation controlled by soil moisture monitors; intensive “fertigation,” or introducing nutrients through the irrigation system; and high-density plantings with “dwarf” citrus trees that achieve a mature height at around 6 feet, a third as tall as a standard grove.
These smaller trees enabled researchers working at Gapway Groves to plant 363 Hamlin orange trees per acre, about twice the density of today’s standard groves. Planted in 2008, the Gapway Grove trees produced up to 580 boxes of Hamlins per acre in their fourth year, comparable to yields in a mature standard grove, but with a fraction of the water, fertilizer and pesticides. A standard grove takes up to five years to produce a marketable crop and doesn’t achieve maximum production for another several years.
“Every year saved in the citrus production cycle represents a year of production costs and resources (water, fertilizer, pesticides, etc.) saved,” according to a CREC report on the Gapway project. “Moreover, every year saved due to accelerated production helps to offset losses caused by encroaching diseases and potentially earns early returns on the investment.”