The Nose Knows: How Dogs Are Saving Citrus Trees

Analysis by Dr. Karen Shaw Becker

citrus tree disease

Story at-a-glance -

  • A winged, aphid-like insect and the “citrus greening” disease it causes have been wreaking havoc on citrus production around the world, including Florida and Texas, and in California where the $2 billion industry is also threatened
  • Citrus greening, aka Huanglongbing (HLB), had been described as undiagnosable, incurable and “monumentally destructive” — until researchers found that dogs can detect it as early as two years before growers can
  • For one experiment, dogs were taken to a citrus orchard in Texas to pick which trees were diseased and which were healthy, and they were correct 95% of the time, even among other citrus bacterial, viral, fungal, and spiroplasma pathogens
  • Dogs have been trained to find endangered animals and the scat of bears, track foxes, coyotes, tigers, ringed seals, invasive brown tree snakes in cargo, tree-damaging pests such as termites and weevils and screwworm in animal wounds, single out dairy cattle in heat (aka estrus) and detect bedbugs
  • Scientists say dogs can play detective over large landscapes and do it rapidly without the need for endless samplings and lab processing

For lovers of all things citrus, and certainly growers of grapefruit, limes, oranges and other tropical fruits, it’s become alarming to find that a serious crop disease known as citrus greening, aka Huanglongbing (HLB), has been hitting citrus fruit trees worldwide.

HLB are bacteria that cause rapid deterioration, starting with symptoms like blotchy leaves, then moving to bitter fruit and, eventually, dead tree limbs. But it’s not a new disease; according to The Organic Center,1 the first damage was described in India in 1912, then reported in 1919 in South China. In Asia, it’s known as yellow dragon disease.

Essentially, citrus greening has spread worldwide in the last 100 years and, unfortunately, it’s incurable. It arrived in Florida in 1998, moving county to county. By 2017, every county in the state reported it. From Florida to Texas to California, as well Georgia, Louisiana, areas in Central and South America and throughout Asia, the bacteria’s spread have been noted as costly and “monumentally destructive.”

As recently as April 2018, The National Academies of Sciences Engineering and Medicine deemed HLB “the most serious threat for citrus growers worldwide” and noted that a “single breakthrough discovery for managing” it was “unlikely.”2

Worse, there was no way of detecting a problem. Because “there are no practical methods available for growers to diagnose asymptomatic trees, they often serve as hidden bacterial reservoirs for feeding psyllids,”3 winged aphid-like insects also known as plant lice,4 threatening disaster for the $2 billion citrus industry in California alone.

That is, until scientists discovered a secret weapon. Disease-detecting dogs, proving that the noses that can detect and divert potentially deadly human diseases such as cancer, chemical components in bombs, bad booze and even lost children, are also able to help sniff out disease-ridden pests in citrus orchards. Their report was published in the Proceedings of the National Academy of Sciences (PNAS) in February 2020.5

How Dogs Detect Disease-Ridden Citrus Trees

Scientists reveal that many of our canine companions, being true to their noses and their amazing sense of smell, can sniff out the disease way ahead of time to avert potential disaster for citrus growers and every stop along the proverbial food chain, including disappointed consumers.

In fact, dogs can be trained to smell out HLB weeks and even years before anyone else knows there’s a problem, according to a recent study written in part by Timothy Gottwald, a U.S. Department of Agriculture researcher. Gottwald told PBS News Hour that the “technology” of a dog’s nose is thousands of years old. “We’ve just trained dogs to hunt new prey: the bacteria that causes a very damaging crop disease.”6

With all the doom and gloom over the lack of remedial devices to stop or at least slow the disease, Gottwald was happy to report that in just one experiment during which dogs were taken to a grapefruit orchard in Texas to pick which trees were diseased and which were healthy, they were right on the nose (so to speak) 95% of the time.

He noted that once the disease is sniffed out so growers know which trees are infected, they can cull them, so the chance of stopping an epidemic of the disease is much greater. According to PBS:

“Matteo Garbelotto, who studies plants at the University of California, Berkeley, says the new research elevates the study of dog sleuths in orchards from anecdotal to field-tested, showing that dogs can detect an infection well before current methods. Garbelotto has been involved in similar research but had no role in the new study.

Another plant scientist, Laura Sims, of Louisiana Tech University, said she was impressed by the rigorousness of the research. She applauded the steps taken to determine if the dogs were sniffing out the bacteria itself or a plant’s response to an infection.”7

Not only in orchards, the research also involved a laboratory setting in which dogs were shown a variety of plants infected with the virus. Not all of them were citrus, but the dogs were correct in their smell-sense analyses. Gottwald concluded that soon dogs would be using their expert noses on farms as well as in airports and other crucial settings.

The Role of Dogs in Citrus Greening Detection

Scientists with the USDA’s Agricultural Research Service, along with personnel from Texas A&M University’s Kingville Citrus Center and the Center for Integrated Pest Management, participated in both field and lab testing. According to the study and experts at the USDA, which funded it, trained dogs have proven to be the most efficient way to detect citrus greening. According to the USDA:

“Currently, the only solid hope of curtailing the spread of citrus greening is to eliminate trees with the disease as quickly as possible to prevent further spread. Early detection of the citrus greening pathogen is crucial because trees can be infected and act as a source to spread the disease months or years before showing symptoms that are detectable by the naked eye.”8

Significantly, Gottwald says they found trained dogs could identify trees infected with the citrus greening bacteria, Candidatus Liberibacter asiaticus, within two weeks of a tree’s inoculation, and differentiate between “other citrus bacterial, viral, fungal, and spiroplasma pathogens, including closely related Liberibacter species.”9

Today, the Organic Center warns that “no matter where citrus is grown, the farmer must have a plan assuming that sooner or later HLB will need to be dealt with.”10 There may not be a cure yet, but detection by dogs is a nearly foolproof stopgap until prevention is a reality; at least, that’s the goal.

Experts also say there’s evidence that if the disease can be managed, the fruit can still be successfully marketed. Although the study shows that “Human visual assessment is insufficiently sensitive to detect new plant infections in a responsive timeframe,” dogs can play detective over large landscapes, and do it rapidly without the need for endless samplings and lab processing.

Dogs have a wide array of skills in this area, the study authors noted. They’ve been used to find both domestic and endangered birds and mammals, and the scat of bears. They’re also been trained to track foxes, coyotes, tigers, ringed seals, invasive brown tree snakes in cargo, pests such as termites and weevils that damage trees and screwworm in animal wounds, single out dairy cattle in heat (aka estrus) and detect bedbugs in hotels and homes.

Breeds used in the scientists’ experiments included Mira, a German Shepard/Belgian Malinois mix, and Szaboles, Zsemir and Maci, all Belgian Malinois. To date, 19 dogs chosen for their adept noses as well as their initiative have been obtained from European breeders for disease detection to help save the U.S.’s $3.35 billion citrus industry.11

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