Detection and occurrence of airborne Xanthomonas hortorum pv. carotae, causal agent of carrot bacterial blight, in carrot seed production systems

Abstract Bacterial blight of carrot, caused by the plant pathogenic bacterium Xanthomonas hortorum pv. carotae (Xhc), is an important seedborne disease of carrot. Previous research demonstrated that airborne Xhc could be detected up to 1.6 km downwind of carrot seed fields being harvested, but more information is needed to identify dispersal periods of the pathogen during other stages of carrot seed production. The objectives of this research were to: 1) quantify and characterize temporal dynamics of airborne Xhc during carrot seed production; and 2) identify weather factors that contribute to airborne Xhc levels in carrot seed fields. During the 2018–2019 season, air samples were collected on a total of 273 and 289 days in two fields and Xhc was detected on 223 (81.7%) and 245 (84.8%) of sampling days. On average, 2.6 x 103 and 7.0 x 103 Xhc/day were detected. Principle component analysis identified three components associated with airborne Xhc levels. Temperature variables (daily mean, maximum, and minimum air and soil temperatures) accounted for 53.2% of the total variance. The second component was associated with wind (wind speed, direction, and run) and 19.1% of the total variance. Factors related to moisture (daily precipitation, relative humidity, and dew point) were associated with the third component and accounted for 13.0% of the total variance. Together, these data can be used to build predictive models to inform growers of inoculum risk and improve control through better-timed bactericide applications, modified cultural practices, and increased knowledge of bacterial blight epidemiology in carrot seed crops.