The first objective of this research was to quantify and characterize the temporal dynamics of airborne X. hortorum pv. carotae, the causal agent of bacterial blight of carrot. In this study, Xhc was detected in air samples at a high frequency in the two fields sampled during the carrot seed cropping season (81.7 to and 84.8% of sampling days), suggesting a high potential for aerial dispersal of the pathogen within carrot seed production systems. The amounts of airborne Xhc detected in each field (2.6 to 7.0 x 103 Xhc/day) were similar to the average bacterial immigration estimates of 104 cells/month described by Lindow (1996).
Plant canopies can be strong sources of airborne bacteria (Lindemann et al. 1982; Lindemann and Upper 1985), including epiphytic bacteria like Xanthomonas, and it is likely that the high frequency of Xhc detection was due in part because of the close proximity of the air samplers to the crop canopy (2 to 3 m); however, the relative contribution of carrot debris or soil as sources of airborne Xhc was not determined. Sampling was not performed between December and March, so the frequency and amount of Xhc in the air during the overwintering and vernalization period is still not known.
The protocols used in this study relied on molecular methods to quantify Xhc genomes and did not quantify viable bacteria using a semi-selective medium. Quantitative PCR used in this study for several reasons: 1) the uncertainty of bacterial viability on spore trap tape during the one week sampling interval; 2) the large number and amount of non-target organisms that are also able to grow on various semi-selective media for Xhc; and 3) increased sensitivity and specificity of qPCR compared to plating, especially in the presence of non-target organisms. Regardless, others have isolated viable Xhc cultures in air samples (du Toit et al. 2005); the authors have similarly collected Xhc isolates from air samples that are pathogenic on carrots (data unpublished). More research is needed to determine the relative proportions of viable and non-viable Xhc cells in aerosols and airborne debris.
Carrot seed is a biennial crop, with seed-to-seed production fields are typically planted in August and harvested in September or October of the following year. Harvesting practices often generate plumes of dust and plant material that can contain viable bacteria (Lighthart 1984), including Xhc (du Toit et al. 2005), which can potentially be dispersed and deposited in newly planted fields. We detected large numbers of airborne Xhc in September and October during both years of the study, which were likely due to fields being harvested during these months. Additionally, leaf samples taken in November 2018 and in other studies (du Toit et al. 2005) show that considerable Xhc populations can become established on seed-to-seed carrot fields prior to winter. These results lend further support for a green bridge effect that contributes to the annual and seemingly endemic presence of Xhc in the region.
There are other cultural practices associated with carrot seed production that have the potential to generate airborne Xhc, including plant thinning, mechanical weed management, applying pesticides, and the destruction of pollen donor lines prior to harvest. McInnes et al. (McInnes 1988) observed more significantly more X. campestris pv. vesicatoria in the air above tomato transplants after clipping and harvest. In carrot seed production, some of these activities (e.g. mechanical weed management, pesticide applications) occur multiple times throughout the year, while other practices only occur once per season (e.g. plant thinning, destruction of pollen donor lines), and the exact timing of these activities will vary depending on cultivar, weather conditions, and logistics. In this study, we detected airborne Xhc during the majority of sampling days and on days when field activities were not occurring, including immediately after seedling emergence, suggesting that the pathogen was either originating from neighboring fields or there are other mechanisms for Xhc to become airborne from carrot seed crop canopies.
Another objective of this research was to identify weather factors that contribute to airborne X. hortorum pv. carotae. In this study, principle component analysis identified temperature, wind (speed, direction and run) and factors related to moisture as the primary factors accounting for the variance. Previous studies have also demonstrated that the airborne dispersal of bacteria in aerosols is dependent on temperature, wind, and moisture. Lindemann and Upper (Lindemann and Upper 1985) reported that upward aerial fluxes of bacteria from bean phyllospheres occurred during the middle of dry, warm days and not in the evening or when moisture was present on the leaves in the form of rain or dew. In their study, bacterial numbers were stronger after precipitation events compared to when soils were dry, and wind speed was positively correlated with aerosol strength. In carrot seed crops, strong winds may promote the release and dispersal of Xhc leaf-to-leaf contact within and between individual plant canopies, which could potentially dislodge bacterial cells from epidermal surfaces or biofilms (Morris and Monier 2003). The central Oregon growing season is typified by sunny and dry days, windy afternoons, and cool nights, which may contribute towards the production of aerial fluxes of Xhc in and around carrot seed fields. Diurnal fluctuations of bacterial aerosols are known to occur in climates similar to the high desert of central Oregon, with greater upward fluxes occurring during the afternoon on warm sunny days and after rain or irrigation events (Lindemann and Upper 1985); it would be reasonable to hypothesize that similar temporal patterns might occur with Xhc aerosols from carrot seed crop canopies.