One of the most severe crop pandemics in modern times is that to citrus trees, caused by a bacterium that jumped from the animal to plant kingdom within the last 200 years. Once a tree is infected with the exotic pathogen, called huanglongbing (HLB), it will die. There is no cure, and the disease can spread fast. The only method to halt an entire crop collapse? Eliminate the tree.
"Proportionally, this epidemic has affected a larger proportion of the worldwide host population than most emerging zoonotic and botanical epidemics,” write the team in Proceedings of the National Academy of Sciences. "Originating in Asia in the previous century, HLB has spread to the majority of citrus-producing areas of the world, emerging throughout the Western Hemisphere with devastating results."
Florida lost around 80 percent of its citriculture production and around 70 percent of its juice plants. The team warn that if not curtailed, the pathogen could destroy Florida’s citrus industry, previously the second largest in the world. This is nothing to scoff at: Citrus fruits are among the highest-value fruit crop for international trade, with oranges topping the list, followed by grapefruits, lemons, and limes.
"As a consequence, over the last 13 years, citrus industries in the United States alone have dedicated over $100 million to research of this new pathosystem with no imminent solution for HLB control," the team write.
The researchers instead turned to our favorite sniffers out there: detector dogs. They used Belgian malinois, German shepherds, hybrids between the two, and springer spaniels as "an ancient technology", as the team call them, to rapidly survey large plantings without the time-consuming and laborious work of collecting and analyzing samples by hand. The canine species were chosen for their instinct to hunt by odor, ability to traverse long distances, and their endurance.
The "ancient technology" is a throwback to when canines were used by early humans as chemical detectors to hunt some 12,000 years ago. Now, the modern-day dog detectors have switched from animal prey to citrus pathogens, identifying infections with 99 percent accuracy up to weeks to years before a visual survey and molecular methods would have done so. The pet pooches were remarkably specific, discriminating HLB from other pathogens that could have distracted them. When two dogs were used, the detection accuracy was near 100 percent.
Each canine went through 8 to 20 weeks of obedience and sensitization training, consisting of verbal praise and a toy reward for correct identifications. When a tree was infected with HLB, the dog would sit nearby and not move from the target until a reward was received. The number of false-positive errors was low, with 4 to 15 errors (0.0004 to 0.0015) per 950 to 1,000 trees per canine. The study suggests dogs are more effective and economical than current methods used for disease detection.
"Because canine detection is direct and immediate, this method has potential for use in real-time detection of bacterial, viral, and fungal exotic and endemic pathogens in field situations," the team write. "Deployment of canine teams for detection of CLas in commercial and research orchards began in Southern California in 2018 to 2019."