Biocontrol of fire blight via microcapsule-mediated delivery of the bacterial antagonist Pantoea agglomerans E325 to apple blossoms

Fire blight, incited by the pathogen Erwinia amylovora, is the most economically important bacterial disease of apple, as well as of some other members of the Rosaceae family. The common method of controlling fire blight is with the use of repeated applications of antibiotics, most commonly streptomycin, beginning at early bloom. However, these applications are often not completely effective or reliable from one growing season to another, particularly under high humidity conditions that are favorable to E. amylovora, as well as in instances of the evolution of new strains that are resistant to streptomycin. While biocontrol of fire blight using antagonists is deemed as a promising alternative to antibiotics, these also often fail in field applications due to the rapid and wide fluctuation of moisture and nutrient levels. In this study, the effectiveness of the bacterial antagonist Pantoea agglomerans strain E325 (E325), encapsulated in alginate microcapsules (AMCs), is demonstrated to suppress the growth of E. amylovora strain Ea153 (Ea153) under different relative humidity (RH) conditions. The effects of RH on the water retention property of AMCs and the suppressive activity of E325 against Ea153 are investigated using detached blossoms of apple cv. Gala. The encapsulated E325 exhibits the following advantages: (1) preservation of an effective E325 population and (2) improved suppressive activity of E325 against Ea153 at different RH conditions, especially at the hypanthium, the main infection site. This study demonstrates that the microcapsule-mediated delivery of biocontrol agents may serve as a viable strategy for managing fire blight for apples.