Image courtesy of UIHere.
As technological advancement allows the world to become increasingly connected through trade and travel, exotic pathogens spread more easily across the globe. These pathogens are not limited to human disease but include plant pathogens as well. According to Tim Gottwald, the lead researcher of the Pathology Department at the US Department of Agriculture, exotic pathogens are especially dangerous to plant populations. Gottwald explained that most plant species and their pathogens “develop together evolutionarily, whereas citrus developed in the absence of its most devastating exotic pathogen, a bacterium Candidatus Liberibacter asiaticus (CLas).” Rather, CLas was introduced to citrus about one hundred years ago by an insect vector most likely originating in the southeast of Asia near India. Because of this, citrus lacks a natural resistance to CLas.
In recent years, Gottwald’s lab has studied diseases that plague the citrus industry in the United States, specifically the CLas bacteria. Gottwald explained that insects act as “little hyperdermic needles” and spread the CLas bacterium from tree to tree. Infected insects with CLas on their proboscis and in their gut deposit some bacteria into plant cells when they feed on a tree’s phloem. In order to stop a widespread epidemic, it is important to catch a tree in the early stages of infection before the bacteria can spread to the rest of an orchard.
A recent paper published by Gottwald’s team reported that dogs could be trained to smell a CLas infection with near-perfect identification of infected trees when surveying orchards. In the first year of the study, the team trained ten dogs to smell citrus trees and sit next to trees they identified as infected with CLas. To test the dogs’ accuracy, one hundred tree test grids were set up with infected trees placed randomly throughout. The dogs were taken through the ten grids in the same manner they would survey a commercial orchard. Each dog had nearly a perfect hit rate and identified infected trees with over ninety-nine percent accuracy over the one thousand trees tested by each dog.
This research was prompted by a great need for new methods of early detection of plant pathogens. Gottwald’s research group began to study the use of canine olfaction as a more effective and accurate early detection method back in 1998. At that time, citrus canker was an exotic plant disease that was causing an epidemic in fruit trees. Following a suggestion by one of his colleagues, Gottwald’s team explored the use of dogs as a viable method for early detection of the disease, but their research came to an abrupt halt in 2001. After 9/11, canine detection research was diverted away from the agricultural business and instead focused on detection of explosives. It wasn’t until 2005 that funding became available and the Pathology Department could once again study the promising field of canine detection.
Before the use of canine olfaction, farmers previously relied on human visual detection and PCR confirmation to determine if a tree was infected with the CLas bacterium. However, each of these methods presented severe shortcomings. Visual detection consists of a trained surveyor walking through an orchard and looking for host responses to the infection, i.e. symptoms of the disease. There are two main challenges to visual detection: latency and absence of visual clues. “The latency in symptom development can be anywhere from months to years after an infection takes place,” Gottwald said. By the time a surveyor observes an infected leaf, the tree could have already served as a reservoir of bacteria and spread the infection to surrounding trees. Furthermore, even if a leaf does display symptoms, it is often difficult for a person to see them. The orientation and location of a symptomatic leaf on a tree can lead to missed infections. On the other hand, PCR confirmation is a molecular assay run on tissue samples from leaves that indicate whether the tissue is infected (i.e. has CLas DNA) or not. “PCR is almost a perfect assay. If you have infected tissue, [it will test positive],” Gottwald said. However, PCR presents a sampling problem: There are a tremendous number of leaves on a mature tree, and early in the infection, only a few leaves are infected. In more advanced infections, the CLas bacterial infection may still be confined to sectors in a tree, so not every leaf will be infected. Even within a leaf that is infected, not every cell will contain the bacteria.
Canine olfaction solves many of the limitations faced by these other methods. Dogs can detect very early infections in only a few leaves anywhere in a tree. Thus, the latency problem associated with visual signs of infection is eliminated, allowing for earlier diagnosis. Canine detection is much more accurate than a human performing a visual search of tree leaves. There is no reliance on a molecular assay performed only on a few leaves, so canine detection also sidesteps the sampling problem of PCR. Overall, the use of canine olfaction for the detection of plant pathogens is a highly effective and accurate method that does not suffer from the limitations present in conventional detection methods. This new technique is already being implemented in orchards across the country, saving the lives of countless trees and livelihoods of many farmers.
Gottwald et al., “Canine detection of CLas” PNAS, 2020
Interview with Dr. Timothy Gottwald on March 6, 2020