YSM Issue 93.2

Agricultural Science
FEATURE
DOGS ON DUTY
CANINE DETECTION OF PLANT PATHOGENS BY MAKAYLA CONLEY
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
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
IMAGE COURTESY OF
PIXABAY
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, T., Poole, G., McCollum, T., Hall, D., Hartung, J., Bai, J.,
... Schneider, W. (2020). Canine olfactory detection of a vectored
phytobacterial pathogen, Liberibacter asiaticus, and integration
with disease control. PNAS, 117(7). https://doi.org/10.1073/
pnas.1914296117
September 2020 Yale Scientific Magazine 21