Field performance of antagonistic bacteria identified in a novel laboratory assay for biological control of fire blight of pear

A laboratory assay to identify antagonists of Erwinia amylovora on flowers was tested for its accuracy in predicting the performance of antagonists in controlling fire blight disease of pear trees. Bacterial strains isolated from pear flowers were evaluated in the laboratory for their ability to suppress growth of E. amylovora on flowers on cut branches. The growth of E. amylovora was assessed with a flower-freezing assay using a recombinant strain of the pathogen exhibiting an ice-nucleating phenotype. A number of antagonistic and nonantagonistic bacterial strains selected by this method were tested for their control of flower infection by E. amylovora on trees in the greenhouse and the field. There was a significant (P less than or equal to 0.01) relationship between the ability of antagonists to suppress growth of E. amylovora in the flower-freezing assay and their ability to control fire blight on trees. Some of the strains identified conferred similar or better disease control than the biocontrol agent Pseudomonas fluorescens A506. None of the strains produced antibiotics inhibitory to E. amylovora on the culture media tested. All antagonistic and nonantagonistic strains survived well on pear flowers on trees and reached population sizes of approximately 10(5) to 10(6) cells/flower 3 days after their application. The use of this flower-freezing assay has promise in the rapid identification of biocontrol agents of fire blight. (C) 2001 Academic Press.