Citrus Greening Solutions

$3.3 billion. That’s what the National Agricultural Statistics Service rates the value of the citrus industry in the United States. Yet danger and some of the industry’s greatest challenges lurk in citrus groves across the country — devastating pests and diseases.

The Department of Homeland Security estimates invasive species annually cause $136 billion in overall lost agricultural revenue in the United States.

The Asian citrus psyllid, which creates a disease-causing bacteria known as Huanglongbing (HLB) or citrus greening, is one of the citrus industry’s most destructive insects. It has infected commercial and residential citrus trees across the country from Florida to Texas to California. The disease clogs an infected tree’s vascular system, preventing fruit from maturing and eventually killing the tree.

First identified in China in 1919, by 1937 it had spread to the Philippines and South Africa.

Dog identifies pests in citrus trees

One segment of APHIS’ comprehensive agricultural quarantine and inspection program is based out of APHIS’ National Detector Dog Training Center, an 18-acre facility in Georgia. Sniffer dog Zsemir alerts to a young tree in Florida.

Originally thought to be caused by a mineral deficiency, in 1956 greening was identified as a graft transmissible infectious disease. Not until 1967, however, did researchers recognize that greening was transmitted by the psyllid.

Even before it was detected in Brazil in 2004, HLB had already moved into North America. The psyllid officially arrived in Florida in 2009, but the first case of citrus greening wasn’t discovered until 2005. It’s now estimated that 75 percent of all Florida citrus trees are infected.

Out West, the psyllid was detected in California’s San Diego County in 2008, but the first infected tree was identified in Los Angeles County in 2012, the same year the disease was identified in Texas.

The government and citrus growers are working to find solutions to this potentially catastrophic issue. The USDA, its state counterparts and individual growers are investing over $100 million annually to combat the impact of the psyllid and greening in the United States. Citrus greening has already caused $7.8 billion in losses to the citrus industry.

HLB grows only in branches, leaves and insects, said Wenbo Ma, associate professor for the University of California, Riverside’s Department of Plant Pathology and Microbiology. Ma is working on solving the issue of HLB detection.

There currently is no cure for citrus greening. And because trees don’t show signs of infection for three to five years, surrounding trees can become infected as well. But there could be light at the end of the tunnel. In 2013, then U.S. Department of Agriculture Secretary Tom Vilsack established a unified emergency response framework to address HLB.

The USDA’s Animal and Plant Health Inspection Service (APHIS), the Agricultural Research Service (ARS), the National Institute of Food and Agriculture, state departments of agriculture and the citrus industries in Florida, California, Arizona and Texas formed a Multi-Agency Coordination Group, (HLB MAC group). Its purpose is to create stopgap measures while giving researchers time to find long-term workable solutions.

Researchers have been working on numerous projects aimed at identifying and drastically reducing citrus greening’s destructive effects. These efforts focus on early detection, diagnosis and biological and genetic control.

Dogs as Detectors

One segment of APHIS’ comprehensive agricultural quarantine and inspection program is based out of APHIS’ National Detector Dog Training Center, an 18-acre facility in Georgia. Recognized internationally as a leader in agricultural canine scent detection, NDDTC dogs have been trained to find a variety of pests including Asian longhorn beetles and Mediterranean fruit fly larvae. They have also worked in the Chesapeake Bay wetlands to help eradicate nutria, an invasive semi-aquatic rodent.

One of APHIS’ most exciting and innovative projects is the Canine Early HLB Detection program. In this pilot program, 20 dogs — ages 1 to 3 years — were trained for early HLB detection through a USDA APHIS HLB Mac grant. Dr. Timothy Gottwald, USDA-ARS, has worked with the USDA canine detection program for the past 17 years, in collaboration with Coast to Coast K9 Teams, a Florida-based commercial K9 training company.

Working with the USDA-ARS research team, Coast to Coast developed a canine training protocol, which resulted in the creation of the International Canine Association for Agricultural Disease Detection — a citrus agricultural disease detection organization which offers canine and handler training and certification. The company plans to deploy trained and certified canines both domestically and internationally.

The initial part of the canine detection program was to isolate and identify the volatile compounds (VOCs) associated with HLB. Once researchers accomplished this, the canines were trained to recognize them. The dogs were subsequently field tested in citrus groves, packinghouses and with residential trees.

The breeds selected to participate in tests were German Shepherds, Belgian Malimois and springer spaniels. In early experiments, the dogs began to detect infection within two to three weeks of inoculation. Gottwald reported that none of the trees became positive until three months post inoculation, and the majority of the trees required multiple months for detection.

The next step was to test the dogs against 1,000 trees in randomized field trials in a gridded array containing varying incidents of citrus greening. The canines identified infected trees by sitting next to them to alert the handler. Those trees could then be removed to help prevent the disease from spreading. The dogs’ accuracy was through the roof at 99.17 percent overall.

Next the dogs were introduced to real-world tests in Florida and South Texas in a variety of environments. These included residential neighborhoods, mobile home parks and even a Buddhist monastery. The dogs successfully detected HLB in all of these scenarios.

Dr. Mary Palm, APHIS’ Director of Pest Management Programs, says the idea is to deploy these trained canines over the next two years to various affected states, particularly in Texas and California.

Growers will be able to contract through the training company to bring citrus Greening detector dogs to their groves.

Dr. Palm stressed the dogs are not being used for regulatory purposes. Currently, there is only one federally approved regulatory method to confirm the presence of HLB — molecular-based PCR testing (polymerase chain reaction). Other promising tools are being tested, however. Among these, the USDA APHIS Center for Plant Health Science and Technology’s Mission Laboratory in South Texas has developed and successfully released large numbers of beneficial insects to help control the Asian citrus psyllid.

In 2010, up to 43 immature psyllids were detected in new leaf growth (flush) in residential citrus, where it is prevalent. Over the five years since biological controls began, the psyllid population has dramatically declined. By 2016, only 3.8 immature psyllids per flush were observed — a 91.2 percent drop.

Exploring Other Options

Other avenues for greening reduction and eventual eradication being explored are breeding, planting and growing strategies. One project tests the use a biocontrol fungus to kill ACP adults. Researchers are also working to develop new types of citrus trees and/or adding new root varieties that would make trees more resistant to HLB.

Example of citrus greening

Citrus greening essentially clogs an infected tree’s vascular system, preventing fruit from maturing and eventually killing the tree.

Additional strategies being examined are raised beds, mulching and various planting densities that may promote healthy trees that can thrive regardless of the presence of citrus greening.

Other promising areas of research being pursued include a potentially controversial research project that focuses on genetic modification to create HLB resistant citrus trees. Another emerging project is thermotherapy — treating trees with heat to suppress citrus greening and encourage diseased trees to become productive again.

If successful, this method could allow infected trees to regain growth after treatment. MAC provided funding to help build the first thermotherapy truck. Since then, several producers and a private company have begun investing in their own thermotherapy machines.

Technology offers yet another promising tool for growers. Researchers in Pennsylvania have developed a low-cost, hand-held smartphone optical sensor (spectrophotometer). It uses cloud-based computing to allow real-time diagnosing of plant health with a high degree of accuracy for early detection of citrus greening that may become useful.

Dr. John DaGraca, of the Texas A&M Citrus Center in South Texas, said there’s still a lot of verification to go through as to what techniques work best. He said one of their scientists recently got funding to explore thermotherapy in Texas, but DaGraca questions the effectiveness and practicality of this process.

“You might be able to suppress HLB aboveground, but at the roots, where we know the bacteria is harbored, you won’t be able to heat the soil.”

DaGraca reported that a commercial grower in Florida was experimenting with growing citrus in pots.

“You can do thermotherapy of the whole pot,” he said. “But it’s an expensive layout; it’s probably not going to be as big a production as (what) they have in the field.”

These possible solutions to the devastating effects of HLB are tantalizingly close, particularly the canine detection program. It is still unknown which if any of these will be the silver bullet citrus growers are seeking. With the future of citrus fruit and billions of dollars at risk, the stakes for the citrus industry couldn’t be higher.

By Debra Atlas. This article appeared in the May 2018 issue of Acres U.S.A. magazine.

A freelance journalist and professional blogger, Debra Atlas is reachable through envirothink.word or

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