The USDA Horticultural Research Lab in Ft. Pierce has marshaled its resources in recent years to help in the battle against citrus greening (HLB). While the disease was first officially identified in Florida in 2005, Dr. Tim Gottwald, plant pathology research leader at the lab, has been studying the disease for many years in countries where it is present including India, China, South Africa, and Brazil.
When greening was confirmed in Florida, Gottwald predicted, based on his previous knowledge, infected trees could die within three to four years and that the disease would spread across the state in about the same timeframe.
According to Dr. Calvin Arnold, laboratory director in Ft. Pierce, Gottwald’s predictions have unfortunately turned out to be true. He says because of the threat posed by citrus greening, the research facility redirected resources toward greening study.
“When HLB was confirmed, we looked very closely at our research with encouragement from growers and industry,” he says. “We began moving resources from programs, especially from CVT and diaprepes root weevil to HLB research. Since then our HLB research has expanded rapidly and we remain heavily committed to the program.”
Approximately 60% of all the research conducted at the Ft. Pierce facility is focused on citrus and 75% of the citrus research is focused on greening. There are about 40 active projects at the lab focused on the disease, each with multiple experiments under way in the lab, growth chambers, or in the field. According to Arnold, roughly half of the scientists at the lab are working on greening research.
Signs Of Promise
While growers should take hope from all the work being done by researchers at USDA, Arnold notes a couple of projects show signs of promise. In 2006, Andrew Fire and Craig Mello shared the Nobel Prize in Physiology or Medicine for their work on RNA interference or RNAi. Since then, much of modern medical research has focused on using RNAi techniques that promise major breakthrough in the treatment of human illness.
Researchers at USDA are using the same technology in hopes of targeting the psyllid. Wayne Hunter is conducting the RNAi research which aims to disrupt psyllids. He says the approach is very attractive because it is a natural approach that would only target the psyllid, leaving beneficial insects in the groves to do their work.
While the research is highly technical, RNAi basically disrupts the genetic code of the psyllid. Ideally, the compound could be sprayed on trees and translocate throughout, providing residual control of psyllids for a few months.
While this is emerging technology, it has been done before. Hunter was involved with the development of an RNAi product used in honeybee colonies. Based on RNAi technology, the product called Remebee is formulated to silence Israeli acute paralysis virus. It is delivered in feed and sustains colony health in the presence of the virus. The product is currently commercially available to bee growers, giving rise to hope that RNAi techniques can be applied to psyllid control.
Map It Out
To deploy the RNAi technology, scientists will need to map out the full DNA genome of the citrus psyllid, which they are closing in on now.
“We are within a couple of months of identifying the complete genetic sequence of the psyllid,” says Arnold. “Dr. Yong-Ping Duan led a team of researchers here in Ft. Pierce to map out the genome of the HLB bacteria a couple of years ago. These are huge breakthroughs in the science of fighting greening.”
With the psyllid genome mapped, researchers can target specific genes in the psyllid to help control the pest. “If it is a good gene, we can make it more active,” says Arnold. “If it is a bad gene, we can silence it.”
There are many antibiotics available used in human medicine and in other crops. Some of those may be effective against the bacteria (Candidatus Liberibacter), which causes greening.
Currently, the Citrus Research and Development Foundation is hosting a competition for research projects that are seeking antibiotics that might have an impact on greening.
“We are very excited about the potential of antibiotics in fighting HLB,” says Arnold. “Of course, we know a lot about penicillin and other common antibiotics, but there are new types that might be effective against the bacteria. The key is finding one that will move readily through the tree.”
There are a number of antibiotics — when placed in direct contact with the HLB bacteria — that will kill it. The challenge is finding an antibiotic that will move through the phloem tissue of trees and access the HLB bacteria in all areas where it is present.
Loud And Clear
“We realize the industry has its back against the wall in this fight with HLB,” says Arnold. “So we are very committed in the research community to find solutions. I have been involved in ag research for many years and have never been involved in a project where so many scientists are working together and are so passionate. We’ve heard the growers loud and clear: this is not business as usual.”
Arnold says short-term breakthroughs to target the psyllid and nutritional programs will help growers hang in there until more permanent milestones will be marked in the area of genetics and disease resistance. “This is a very resilient industry,” he says. “We’ve survived freezes, hurricanes, and canker eradication. The industry will survive HLB. It will be different, but we will survive.”