In the high desert of Central Oregon, farmers produce some of the world’s most valuable hybrid carrot seed. But their fields face a persistent and costly threat: a plant 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.
For farmers and the seed industry, this means they can start exploring new disease management practices that span the full growing season — not just during harvest.
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 scientists Jeness Scott and Jeremiah Dung at the OSU Central Oregon Agricultural Research and Extension Center (COAREC) in Madras launched a yearlong study in two commercial carrot seed fields in Central Oregon. They installed specialized air samplers that ran continuously for over 13 months (excluding winter) to capture airborne particles. Scott and Dung then used sensitive 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 that routinely occur during the carrot seed harvest are not the only mechanism for Xhc to become airborne.
The found that Xhc 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, published in the journal Plant Disease, 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 the bacteria survive from one growing season to the next — a process known as the “green bridge” — but also that the host plants are exposed to airborne bacteria throughout most of the year. This in part explains why bacterial blight has become a recurring and endemic issue in many carrot root and seed production regions.
Thanks to this study, researchers now have a much clearer picture of how and when Xhc spreads. For farmers and the seed industry, this means they can start exploring new disease management practices that span the full growing season — not just during harvest. These might include the application of treatments to the carrot plants to slow the establishment of Xhc on the leaf surfaces, more precise field sanitation, and timing of fieldwork to reduce bacterial dispersal.
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 sustainability.
This study was supported by funding from the U.S. Department of Agriculture National Institute of Food and Agriculture (USDA-NIFA), Specialty Crop Research Initiative project no. 2020-51181-32154, the Western Integrated Pest Management Center project ID 1616, and the Agricultural Research Foundation.