USA
September 15, 2025
by Chris Branam, Oregon State University
Flowering carrot plants at the Central Oregon Agricultural Research and Extension Center. Credit: Lynn Ketchum
Farmers in Central Oregon produce some of the world’s most valuable hybrid carrot seed, including about 75% to 85% of U.S. hybrid carrot seed supply.
However, Oregon farmers face a persistent and costly threat—a disease called bacterial blight, caused by the bacterium Xanthomonas hortorum pv. carotae (Xhc). This pathogen can cause blighted leaves, damaged flowers and infested seeds — ultimately reducing seed quality and yields.
While previous research showed that Xhc can become airborne during harvest and travel up to a mile, there was a critical knowledge gap: When else and how often is the bacterium airborne throughout the year? Without knowing when the pathogen spreads, farmers and researchers couldn't effectively time disease management strategies.
To find out, Oregon State University (OSU) scientists Jeness Scott and Jeremiah Dung launched a yearlong study in two commercial carrot seed fields in Central Oregon. This study was supported by funding from USDA’s National Institute of Food and Agriculture (NIFA) Specialty Crop Research Initiative, the Western Integrated Pest Management Center and the Agricultural Research Foundation.
Dr. Tom Bewick, NIFA national program leader, called the research effort vital.
“Carrot seed from Oregon is grown across North America,” Bewick said. “Having pathogen-free propagative materials like seeds is possibly the most important aspect of an integrated disease management program.”
The OSU team installed specialized air samplers that ran continuously for over 13 months (excluding winter) to capture airborne particles. Scott and Dung then used DNA testing to estimate the number of Xhc genomes present in the air samples.
What they found changed how we understand this disease. It takes over one year to produce carrot seed. Even before the annual fall harvest of carrot seed begins, the young seedlings intended to produce next year’s crop are already growing in the field. It turns out that activities such as combining during carrot seed harvest are not the only mechanism for Xhc to become airborne.
They found that the pathogen was airborne more than 80% of the time during the growing season in both fields. The pathogen showed up in the air as early as September, shortly after both planting and the harvesting of the previous years seed and reappeared during spring, summer and the autumn harvest months. This indicates that the plants are likely exposed to airborne Xhc not only during harvest but throughout the entire growing season.
The study also confirmed that the bacterium was present on carrot leaves before winter and in early spring, meaning it can establish early and survive cold months. In one field, leaf samples during spring and summer had extremely high bacterial levels, consistent with those known to cause disease symptoms.
These results suggest that disease-causing bacteria in the air, along with the overlapping timing of nearby carrot seed fields, help it survive from one growing season to the next but also that host plants are exposed to airborne bacteria throughout most of the year. This helps explain why bacterial blight has become a recurring and endemic issue in many carrot root and seed production regions.
“The knowledge generated by this NIFA-supported research project at OSU can potentially save growers across the nation millions of dollars,” Bewick said. “These savings could translate into better pricing for consumers.”
Farmers and the seed industry can explore new disease management practices that span the full growing season. By identifying the hidden windows of pathogen activity, this research offers a path forward to protect one of Oregon’s most valuable specialty crops and ensure its long-term viability.
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