P-225 - Determination of population dynamics of Erwinia amylovora on stigma under controlled conditions

Blossom blight of the fire blight disease, caused by Erwinia amylovora, is a critical phase in fire blight epidemics in apple and pear orchards. Epiphytic colonization of stigma and endophytic colonization of nectary is strongly influenced by host and environmental factors. Predicting disease incidence and improving management requires a comprehensive understanding of bacterial population on stigmas. This study examined the individual and interactive effects of temperature, relative humidity (RH), flower age, apple cultivar, and bacterial concentration on stigmatic population dynamics using a detached flower assay. Our results showed that bacteria grew exponentially at 25°C in the first 48 hours under high RH (90-99%), whereas populations reached similar levels on day five at 12°C with the same RH, suggesting constant exposure to high humidity supports bacterial growth even at lower temperatures. In addition, One-day-old apple flowers consistently supported higher bacterial populations across all inoculum concentrations. Among tested apple/crabapple cultivars, bacterial populations on stigma of Snowdrift, Mt. Everest, and Gala were significantly higher than that of Pink Lady. Furthermore, Mt. Everest supported the highest bacterial generations at both 12°C and 25°C, while Pink Lady had the lowest at both temperatures. Our results suggest incorporating cultivar-specific bacterial population dynamics with varying relative humidity and temperature into the Cougar Blight forecasting model could improve fire blight predictions and refine disease management strategies.