Detection and Occurrence of Airborne Xanthomonas hortorum pv. carotae in Carrot Seed Production Systems

Bacterial blight of carrot, caused by the plant pathogenic bacterium Xanthomonas hortorum pv. carotae (Xhc), is an important disease of carrot and carrot seed crops. Previous research demonstrated that airborne Xhc could be detected up to 1.6 km downwind of carrot seed fields being harvested, but more research is needed to identify dispersal periods of the pathogen during all stages of carrot seed production. The objective of this research was to quantify and characterize temporal dynamics of airborne Xhc during a biennial seed-to-seed carrot seed production season. During the 2018 to 2019 season, a Burkard volumetric air sampler was used to collect air samples on a total of 263 and 274 days in a carrot seed crop in each of two fields in central Oregon. Tape samples from the Burkard spore traps were subsequently subjected to a quantitative PCR assay to determine the number of Xhc genomes captured on a daily basis. Xhc was detected on 216 (82.1%) and 228 (83.2%) of the sampling days, respectively. On average, 2.67 × 10³ and 4.28 × 10³ Xhc genomes/day were detected in the two fields over the period of sampling. The results of this study suggest that airborne Xhc inoculum likely contributes to the annual and apparently endemic presence of Xhc on carrot seed crops in the region.