Farmers may have found an unexpected ally to fight the fearsome huanglongbing, also known as «Yellow dragon», an agricultural plague that is wreaking havoc on citrus from much of the world. Scientists from the United States Agricultural Research Service (ARS) have discovered that trained dogs are the most effective tool to detect it, even before the disease begins to manifest itself, according to a study published in “Proceedings of the National Academy of Sciences.”
The yellow dragon has its origin in an Asian bacterium that is housed in lemon, orange and tangerine. Deforms and bitter citrus, and then destroy its seeds. Finally, kill the tree. In just ten years, his arrival in Florida (United States) has caused a decrease of more than 70 percent in the production of oranges. In Spain, its landing worries. Valencian farmers have come to ensure that if they manage to settle in the country, Citrus «would disappear in just a few years». A recent study estimated at 2.5 billion annually what the European Union could lose if it extended through our territory.
Currently, the only option to fight against this plague is eliminate trees with disease as fast as possible and try to stop its spread. It has no treatment. Therefore, early detection of the pathogen is crucial, although everything is against citrus growers: trees can become infected and spread the disease for months or years before showing visible symptoms.
Now, ARS epidemiologist Timothy R. Gottwald has discovered that dogs can be trained to detect the presence of «Candidatus Liberibacter asiaticus», the bacterium that causes citrus greening, with a accuracy greater than 99 percent, he assures.
«We discovered that, once trained, these dogs were able to identify infected trees within the two weeks after inoculation of trees, ”said Gottwald. They were able to distinguish the bacteria responsible for greening and other bacterial, viral and fungal pathogens.
During the tests, the trained dogs had a total of 4 to 15 false negatives and false positives of 950 to 1,000 trees per dog. Occasionally, dogs alerted to trees that were free of the bacteria, but were planted in the same place where an inoculated tree had previously been.
To contextualize these data, the PNAS study explains that the only method currently approved by the United States Department of Agriculture to confirm the presence of the greening pathogen is a DNA-based assay, the polymerase chain reaction test. (PCR) This test detected less than 3 percent of tree infections 2 months after inoculation, 16 of 30 trees inoculated at 16 months and 20 of 30 in 17 months.
So far, the Gottwald program has trained 19 dogs obtained from European breeders for their qualities, and have been deployed for 9 months in California and northern Florida. The training they have received is similar to that of dogs that sniff out explosives: they are taught to recognize a particular smell and sit in front of the fountain, something for which they then get a reward. However, training is somewhat more complex than with explosives, since they are trained to detect a bacterium that is infecting a plant, and they are odors that cannot be separated.
«When we carry out epidemiological models, we discover that canine detection combined with the extraction of infected trees it would allow the citrus industry to remain economically for a period of 10 years, compared to the use of molecular tests or visual inspection combined with the extraction of trees, which could not suppress the spread of the infection, ”explains Gottwald.