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.
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.
Growers rely on fungicides to fight the disease, but those chemicals often don’t work very effectively because pathogens are resistant to them, University of Florida (UF) plant pathologist Gary Vallad said.
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).
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 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.”