Field performance of antagonistic bacteria identified in a novel laboratory assay for biological control of fire blight of pear.
The antagonistic effect of different bacterial strains against Erwinia amylovora, which causes fire blight in pear, was investigated in flower-freezing assay in the laboratory and under field conditions. Ten different bacterial strains comprising of antagonistic (Pseudomonas fluorescens strains A506, 22, 73 and 199 along with 2 unidentified strains) and non-antagonistic strains (Erwinia herbicola [Pantoea agglomerans] strain 56, Acinetobacter strain 64 and 2 unidentified strains), were used in the study. In the flower-freezing assay, all antagonistic strains reduced the relative frequency of freezing of flowers inoculated with the Ice+ strain of E. amylovora to an average of 0.18-0.34, indicating antagonistic effect of the bacterial strains. However, none of the strains were able to produce antibiotics inhibitory to E. amylovora on the culture media. In two experiments (greenhouse and orchard) conducted during spring 1996 in Albany, California and in a field experiment conducted during 1999 in Hopland, California, USA, all antagonistic and non-antagonistic strains survived well on pear flowers, reaching population sizes of 105-106 cells/flowers after 3 days post-application. Significant correlations were observed between the antagonists' ability to suppress growth of E. amylovora in the flower-freezing assay and their ability to control fire blight in the field.