Evidence of Greater Competitive Fitness of Erwinia amylovora over E. pyrifoliae in Korean Isolates

Erwinia amylovora and E. pyrifoliae are the causative agents of destructive diseases in both apple and pear trees viz. fire blight and black shoot blight, respectively. Since the introduction of fire blight in Korea in 2015, the occurrence of both pathogens has been independently reported. The co-incidence of these diseases is highly probable given the co-existence of their pathogenic bacteria in the same trees or orchards in a city/district. Hence, this study evaluated whether both diseases occurred in neighboring orchards and whether they occurred together in a single orchard. The competition and virulence of the two pathogens was compared using growth rates in vitro and in planta. Importantly, E amylovora showed significantly higher colony numbers than E. pyrifoliae when they were co-cultured in liquid media and co-inoculated into immature apple fruits and seedlings. In a comparison of the usage of major carbon sources, which are abundant in immature apple fruits and seedlings, E. amylovora also showed better growth rates than E. pyrifoliae. In virulence assays, including motility and a hypersensitive response (HR), E. amylovora demonstrated a larger diameter of travel from the inoculation site than E. pyrifoliae in both swarming and swimming motilities. E. amylovora elicited a HR in tobacco leaves when diluted from 1:1 to 1:16 but E. pyrifoliae does not elicit a HR when diluted at 1:16. Therefore, E. amylovora was concluded to have a greater competitive fitness than E. pyrifoliae.