Airborne Transport of the Pathogen Xanthomonas Hortorum pv. Carotae

During the summers of 2021 and 2022, continuous and intensive sampling of airborne particle concentration and airborne Xanthomonas Hortorum pv. Carotae (Xhc) was conducted within commercial carrot seed crop fields in Central Oregon, USA. Approximately 65% of all carrot seed produced in the United States is sourced from Central Oregon.

Xhc is a pathogenic bacteria directly linked to decreases in annual carrot seed crop quality and health. It produces a stress response and can cause total partial or total blight on the leaves and umbels. Prior research by du Toit et al. 2005 indicates that airborne inoculable Xhc, either as standalone bacterial aggregates or coupled to suspended plant matter, is capable of traveling up to 1600 meters from a source. The commercial combining of carrot seed crops is now believed to be the primary source of airborne Xhc inoculum. There is compelling evidence to suggest that correlations between airborne particle size, distance traveled in the air, and Xhc concentration exist.

In situ, non-invasive, dispersion and deposition experiments were performed during the commercial combining of carrot seed crops. Linear arrays of Cascade Settling Trap (CST) devices and colocated micro-meteorological stations were deployed in the downwind and perpendicular to path of travel of the combining tractors. The byproduct of combine seed separation is a quasi steady stream of shredded plant material which forms a cloud. Variability in both wind magnitude and direction existed during the sampling events. The transient wind vectors recorded at the micro-meteorological stations within the field indicate that analytical solutions to dust cloud pathlines are impossible and numerical approaches should be utilized.

The effective distance a given dust cloud travels prior to being intercepted by a given CST particle trap can be approximated by assuming the wind field over the carrot seed crop field at 1.5 meters is homogeneous and varies in time according to the array of 2D anemometers equipped on the micro-meteorological stations. By numerically integrating the velocity field in time and invoking the aforementioned assumption, an effective dust cloud arc length pathline can be produced and an uncertainty analysis can be performed. This allows for the Xhc concentration captured by the CST particle traps to be assessed as a function of effective distance. This talk will highlight selected results from the 2021 and 2021 Airborne Xhc bacterial dispersal study with an emphasis on methods to spatially correct bacterial concentration recordings as a function of distance in a transient wind field.