A world of scientists takes aim at citrus disease

Growers are strongly encouraged to attend a workshop to be held March 6 to update them on the latest research into citrus greening. The workshop is sponsored by the Citrus Research and Development Foundation and Florida Citrus Mutual. Learn more here.
When more than 500 scientists from 22 countries step away from their labs to gather in Orlando, they must have something important to say. And they did, at the 2013 International Conference on HLB, also known as citrus greening disease.

One of the most serious diseases of citrus, HLB has been detected in all of Florida’s 32 commercial citrus-producing counties. The Asian strain of HLB is caused by the bacterium Candidatus Liberibacter asiaticus and was found in Florida in 2005. The carrier of the disease is an insect called the Asian citrus psyllid, which attacks the vascular system of trees, slowly killing them usually within two years.  Before the tree dies, it may produce bitter-tasting fruit that is unmarketable. The disease, also known as (Huanglongbing or yellow dragon disease), could eventually destroy the state’s $9 billion citrus industry. So far there is no cure, only multiple approaches to management of the disease.

This photo showing blotchy model foliage was taken by Dr. Tim Gottwald, a USDA scientist, in 2005. The leaves are from a pummelo tree in a dooryard near Florida City in South Florida.

USDA Animal and Plant Health Inspection Service, USDA-Agricultural Research Service, the Florida Department of Agriculture and Consumer Services, the University of Florida Institute of Food and Agricultural Sciences and other agencies and research entities have been working together to put an end to citrus greening.

The Orlando meeting included scientists from those organizations as well as Brazil, Argentina, Italy, Mexico and many more countries. All of the citrus-growing regions of the United States were represented.

The five-day event was packed with presentations on detection and discovery of the disease, epidemiology, biology and genomics, transmission, management and other topics. The management session alone featured 27 separate presentations.  The event was hosted by Florida Citrus Mutual.

“A critical part of the research process is getting scientists together in one place to discuss results, data and innovations, and that is exactly why we created the HLB Conference,” said Mike Sparks, executive VP/CEO of Florida Citrus Mutual.

Focusing on the management aspect of the conference, presentations took a variety of approaches.  Many involved genetic modification, which involves more hurdles with acceptance and regulations than with the scientific research itself.

An adult citrus psyllid found in Reunion Island, near Madagascar, in the 1970s. Photo credit: From Dr. Tim Gottwald. Photographer: Bernard Albert

In a posted summary of their presentation (presentation 1.5) on navigating legal and regulatory processes for the deregulation of genetically-enhanced HLB-resistant citrus, researchers said, “… it is widely accepted that genetic modification using a biotechnology approach is likely to be the only way to achieve acceptable levels of resistance in commercial varieties in the near term. Progress has been made by many groups to produce and screen plants with a wide variety of genes and approaches, and more than one group is starting the process to collect the data necessary for deregulation. However, the deregulation process is daunting and full of hurdles, and the science may actually be the easiest and the cheapest part of the project.”

Most strategies presented at the conference rely on specific combinations of pesticides, timing of application, and delivery methods for that pesticide or pesticide combination. Here are highlights of a few others:

Presentation 6.3 involved using the citrus tristeza virus to fight the bacteria that causes the disease. Scientists from USDA Agricultural Research Service in Fort Pierce contributed their time and expertise  to its development.  Center Director Dr. Calvin Arnold told the story in a somewhat non-technical way to Southeast Ag Network in February, saying that this potentially harmful virus can be introduced as a way of transporting an antimicrobial peptide that can combat the bacteria that causes citrus greening.

“You would attach the antimicrobial peptide onto the virus and then the virus would transport it throughout the tree. As you know, the bacteria are found throughout the tree. This virus would transmit the peptide though the tree to destroy the bacteria,” Arnold said.

Fruit infected with citrus greening found in China during the 1980s. - Photo credit: Dr. Tim Gottwald

Another presentation, “Effect of Mineral Oil on Host Selection and Control of Diaphorina citri Kuwayama (Hemiptera: Psillidae) on Citrus,’ (presentation 6.8) concluded that mineral oil has a repellent effect on some psyllids, which prefer oil-free plants.

Presentation 6.9 introduced a fungus to the fight. “Morphological characterization of Hirsutella citriformis Speare Mexican isolates and evaluation against Diaphorina citri Kuwayama (Hemiptera: Psyllidae)” inoculated psyllids by contact with spores from a type of fungus. Researchers found that of two regional tests, after 27 days 90 percent and 70 percent of psyllids had died.

A device called the Flicker, another approach focused on detecting and monitoring, might be used to quickly survey large blocks of trees. USDA researchers reported that this mechanical device can be attached to a vehicle and pulled through a grove. “Vertical baffles extending from the device are kept in contact with the outer edge of tree canopies while driving a long a row,” the researchers summarized. “Adult psyllids are flicked (hence the name) by these baffles onto large, sticky traps positioned below the baffles.” Among 51 blocks of trees studied, the flicker caught an average of 20 psyllids and up to 160 psyllids per 1,000 feet of trees. The researchers said that although it may be similar to other means of detection among high populations of psyllids, the value of the device would be its efficiency when populations are small.

Another management technique presented at the conference was a program that would produce citrus psyllids with malformed wings, thus reducing the insect’s mobility and ability to carry the disease. It was presented as an environmentally friendly management strategy.

An electron micrograph of Liberibacters was from infected leaf tissue taken by Monique Garnier in the early 1980s. -Photo credit: Dr. Tim Gottwald

Other ideas presented would use essential oils or hormones to kill psyllids at the nymph stage and manage the insect population by controlling weed growth in groves.

A grower meeting has been scheduled for March 6. It will feature summaries of the most promising detection, monitoring and management programs presented at the five-day event. Attendance is strongly encouraged because of the urgent nature of this threat. Learn more here. “It’s an opportunity to get the latest information distilled into understandable terms,” said Dan Botts, FFVA vice president of industry resources.

The grower workshop is sponsored by the Citrus Research and Development Foundation and Florida Citrus Mutual.

“The continued increase in infection in Florida citrus is cause for alarm, and aggressive efforts of the industry to organize research may provide the only answers,” said Dr. Harold Browning, chief operations officer of the Foundation, a non-profit corporation created in 2009 to “advance disease and production research and product development activities to insure the survival and competitiveness of Florida’s citrus growers through innovation.”

“Progress in many areas of research offer promise to slow spread, reverse the health of the infected tree inventory, and allow growers to eventually replant with disease-resistant trees,” Browning said.

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