Aerial dispersion of Xanthomonas fragariae during trimming leaves of angular leaf spot diseased strawberry propagation plants

The bacterium Xanthomonas fragariae is the causative agent of angular leaf spot (ALS) of strawberry, a Regulated Non-Quarantine Pest in Europe (EPPO A2) for plant propagation material. Field experiments were conducted to explore if X. fragariae is dispersed through the air during trimming ALS-diseased strawberry plants with dry or wetted leaves. Trimming the leaves led to dissemination of leaf fragments to the nearby surroundings. A sharp decrease in the amount of leaf fragments within the first 5 m distance downwind from the strawberry plants was found. Furthermore, air quality monitors demonstrated that during trimming 0.5 – 10 μm sized particles were ejected into the air, resulting in short periods with increased particle densities 45 cm above ground level, which could be detected at least 50 m downwind. At this height X. fragariae was detected by means of air samplers, as evidenced with a combination of dilution-plating and TaqMan assays, at 25 m distance downwind from ALS-diseased plants. A sharp decrease in the density of X. fragariae colony forming units (cfu) within the first 10 m distance from the source plants was found. The densities of X. fragariae cfu were strongly associated with the particle densities. Results indicate that during trimming leaves of strawberry propagation crops with ALS-diseased plants there is a considerable risk of deposition of airborne X. fragariae inoculum on nearby nursery beds. Whether this airborne inoculum can result in infections is discussed.