Erwinia amylovora, the causative agent of fire blight, leads to important economic losses of apple and pear crops worldwide. This study aimed to investigate the potential of the resident bacterial flora of the apple blossom in combatting plant disease-causing organisms, with a focus on strains that show potential for controlling fire blight. We collected samples of apple blossoms from dif-ferent sites around Canton Zurich, some grown with and some without the use of conventional pesticides, and isolated 538 strains which we tested for activity against Pectobacterium carotovorum and E. amylovora. We also tested the isolates against oomycete and fungal pathogens, to evaluate them for additional potential as biocontrol agents. Nine isolates showed activity against P. ca-rotovorum and eight of these against E. amylovora. Furthermore, 117 showed antifungal and 161 anti-oomycete activity. Five strains showed activity against all classes of pathogen and were tested in a detached apple model for anti-E. amylovora activity. Of these five strains, two were able to antagonise E. amylovora; Bacillus velezensis #124 and Pantoea agglomerans #378. We also show that B. velezensis #124 exhibits strong activity against three different fungi and two oomycetes in vitro, suggesting that this strain might have broad potential for biocontrol. Our results showcase the potential of the natural apple blossom flora. We isolated two candidate biocontrol strains from apple blossoms, suggesting that they can persist at the most common entry point for the causative agent of fire blight. Furthermore, they are probably already part of the human diet, suggesting they are safe for consumption and thus are promising candidates for biocontrol applications.

Fire blight, caused by Erwinia amylovora, is a devastating bacterial disease that threatens world-wide production of apples and pears. In Canada and the US, streptomycin and kasugamycin are commonly used for the control of this pathogen. There is an urgent need for development of biocontrol agents (BCAs) that can be easily incorporated into integrated pest management pro-grams with high efficacy against streptomycin sensitive and resistant strains of E. amylovora. In addition, excessive use of antibiotics in agriculture has increased awareness of the possibility of antibiotic resistance gene transfer to human, animal and environmental microbes. To address this issue, a phage-carrier system (PCS) that combines Pantoea agglomerans as an antagonistic bacterial carrier of Erwinia phages as a double-pronged treatment for fire blight have been de-veloped. The low viability of P. agglomerans cells following spray drying challenged the industrial scale production of this PCS. In the present research, a spray drying protocol was developed for P. agglomerans by modifying the growth medium and the bacterial cell formulation using D(+)-trehalose and maltodextrin. The developed protocol is amenable to industrial scale pro-duction of the BCA/PCS. The P. agglomerans viability was greater than 90% after spray drying, had a prolonged shelf life of 4 months at 4 C, and the reconstituted cells showed an approximate 3 log reduction in E. amylovora counts with a green pear disc challenge assay.