From UF/IFAS
In Summary:
Researchers at the University of Florida Institute of Food and Agricultural Sciences are investigating how bacterial spot disease spreads during fungicide applications in tomato fields. Their study found that bacteria can become airborne during spraying and travel at least 24 feet under warm, humid conditions, potentially increasing disease transmission. Scientists are now exploring new sprayer technologies and application methods to help growers control the disease without inadvertently spreading it.
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Bacterial spot disease isn’t just a nuisance for tomato growers – it’s a relentless, weather-powered villain that can ruin a field of promising fruit, threatening farm profits.
When warm rains roll in and humidity hangs in the air, the pathogen behind bacterial spot comes alive, spreading fast and scarring tomatoes so they can’t be sold.
Related: UF researchers develop AI-powered tools to help strawberry and tomato growers predict yields.
Growers rely on fungicides to fight the disease, but they often don’t work very effectively because pathogens are resistant to them, said University of Florida plant pathologist Gary Vallad.
Interior photos from UF/IFAS
Still, in an unfortunate twist of fate, the very act of applying these materials can unintentionally help the pathogen spread. As Vallad explains, traditional spraying equipment uses high volumes of water at high pressures to deliver the fungicide to the tomato canopy. But that process also creates particles containing the bacteria that can remain suspended in the air, accelerating plant-to-plant spread across fields.
UF scientists are trying to empower tomato growers to spray fungicide without spreading the bacterial spot pathogen. Doctoral student Renzo Ramirez told growers at the May 26 Tomato Field Day that there might be reason for hope.
In previous research from Vallad’s lab, scientists discovered the bacterial spot pathogen can hitch a ride on tiny water droplets created by overhead irrigation in greenhouses.
“This fact raised an important question: whether the fine mist produced by traditional tractor-mounted sprayers could also help move the bacteria in the field,” said Vallad, a plant pathology professor at the UF Institute of Food and Agricultural Sciences (UF/IFAS) Gulf Coast Research and Education Center (GCREC).
To find out, Ramirez, a student in Vallad’s lab, infected tomato plants with the bacterial spot pathogen in a GCREC field. He then used a tractor-driven sprayer to spray the plants with water, mimicking a fungicide application. While spraying, he used air samplers to see how far the bacteria could travel through the air.
Ramirez conducted his experiment in the Spring of 2025, which turned out to be the perfect time for conditions conducive to bacterial spot – hot, humid, and rainy.
Under these conditions, Ramirez detected bacteria as far as 24 feet from infected plants and up to 15 feet above the ground. Those amounts reflect the extent to which scientists sampled the air. It’s likely that the bacteria moved even farther, but scientists didn’t have additional sampling stations to measure the full extent of how far the bacteria moved, Vallad said.
“These results suggest that weather and disease severity influence how easily the bacterial spot pathogen becomes airborne during spraying,” Vallad said. “In other words, with more disease, we get more bacteria, which would increase the probability of long-distance movement – and our ability to detect it. The data also show that, in some situations, spray applications might unintentionally help spread the disease. Understanding when this risk is highest could help farmers make better decisions about when to spray and help limit the movement of bacterial spot in their fields.”
Vallad adds: “We are looking at new sprayer technology, changing nozzle types, use of low-volume air-assisted sprayers, or even changing sprayer configuration as ways to limit the production of bioaerosols.”
Key Points:
- UF researchers found that conventional fungicide spraying may unintentionally spread bacterial spot bacteria through the air.
- In field trials, bacteria were detected as far as 24 feet horizontally and 15 feet above the ground during spray applications.
- Warm, humid, and rainy conditions increased the likelihood that bacteria became airborne.
- Scientists believe disease severity and weather conditions play a significant role in how easily bacterial spot spreads during spraying.
- Researchers are testing new sprayer designs, nozzle types, and lower-volume application methods to help growers reduce disease transmission while protecting tomato crops.
ABOUT UF/IFAS
The mission of the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) is to develop knowledge relevant to agricultural, human, and natural resources and to make that knowledge available to sustain and enhance the quality of human life. With more than a dozen research facilities, 67 county Extension offices, and award-winning students and faculty in the UF College of Agricultural and Life Sciences, UF/IFAS brings science-based solutions to the state’s agricultural and natural resources industries and all Florida residents.
For more information, go to ifas.ufl.edu.
