Canines and chemistry work to combat powdery mildew

Dogs have many jobs, but one you may not expect is identifying grapevines coated in a destructive and highly contagious fungus. Although dogs can detect serious vine infections by smell, scientists don’t know exactly what odor molecules are triggering the response.

Researchers are now analyzing volatile chemicals emanating from grape leaves infected by a fungus called powdery mildew with the goal of improving training for vineyard canines.

Nayelly Rangel, a graduate student at Texas Tech University, will present the team’s results at the spring meeting of the American Chemical Society (ACS).

“Powdery mildew is one of the most contagious diseases that affects grapevine plants,” says Rangel. “It reduces plant growth, fruit quality and quantity, and it can lead to a decline in wine quality.”

The current method to identify an infection relies on humans looking for tell-tale patches of gray powder along plant leaves. But, by then, the condition is usually serious and requires large amounts of fungicide to eradicate.

Past research showed that dogs can identify powdery mildew by smell. But not much is known about the chemistry of what these animals smell, or whether the plants’ odor profile changes as the infection progresses.

“Our four-legged friends don’t talk, so we’re trying to understand what they are encountering when they’re sniffing,” says Paola Prada-Tiedemann, a professor of forensic science at Texas Tech University who is leading the study.

So, the researchers set out to identify which volatile organic compounds, or airborne scents, grapevine leaves give off at different stages of powdery mildew infection.

First, the team needed a technique that would keep leaf samples intact for dog training. So, they placed a leaf inside a vial and inserted a tiny absorptive fiber into the vial to pick up chemicals from the air above a leaf. From there, the researchers characterized the volatile organic compounds (VOCs) stuck to the fiber by inserting it directly into a gas chromatograph-mass spectrometer.

“Our approach is unique because we’re testing the exact location where a canine sniffs the grape leaf,” says Rangel. “So, we’re analyzing the same airspace in both scenarios, whether we’re in the chemistry lab or the canine lab.”

So far, the team has optimized their process from the VOCs emitted from healthy leaves. Initial results from comparisons of healthy and fungus-impacted grapes revealed that the baseline odors emitted from healthy leaves include more acidic odor compounds than sick ones.

In fact, healthy leaves released fewer vapors over time, says Rangel, in contrast to sick leaves that expelled more VOCs as the infection grew.

Next, the researchers will analyze the chemical composition of what’s wafting off the leaves at different stages of infection. Once they’ve identified a few key molecules, they will present each one individually to the canines, measure the animals’ responses to each, and test the smallest amount needed for detection.

Like how certain scents, such as vinegar, are strong in small amounts, the researchers think that dogs may pick up on certain VOCs more easily than others. Using those compounds for training could enable more sensitive and accurate mildew identification, especially early-stage infections.

“The ultimate goal is to move away from the visual diagnosis of mildew to odor diagnosis as the gold standard,” says Prada-Tiedemann. “Even when we can’t see it ourselves, the dog sitting next to a plant can tell you with their nose, ‘uh oh, that vine’s starting to go.'”

By “bridging the canine to chemistry,” as Prada-Tiedemann says, the team wants to find a more efficient solution for protecting grapevines from a widespread and damaging disease. After all, she adds, “We all want good wine!”