Establishment of bacterial antagonists of Erwinia amylovora on pear and apple blossoms as influenced by inoculum preparation.
The influence of inoculum preparation on the establishment of bacterial antagonists that suppress fire blight and Erwinia amylovora on blossoms was evaluated in field trials conducted at the Oregon State University Department of Botany and Plant Pathology Field Laboratory, near Corvallis, Oregon, USA, during 1993 and 1994. Aqueous suspensions of Pseudomonas fluorescens A506, E. herbicola [Pantoea agglomerans] C9-1R, or E. amylovora 153N were prepared from cells harvested from the surface of an agar medium or from cells that were lyophilized after culture under similar conditions. Bacterial suspensions (1×108 CFU/ml) were sprayed on pear (cvs. Bosc and Bartlett) and apple (cvs. Rome Beauty and Golden Delicious) trees at 50% bloom near midday. The incidence of recovery (proportion of blossoms containing detectable populations) and the population sizes of the bacteria on individual blossoms with detectable populations were followed over a period of several days. Fluorescent microspheres (1 µm in diameter) were added to sprays at a concentration of 1×l07 microspheres per ml to mark blossoms that were open during application of bacteria. After dilution-plating, the stigmas and styles of each blossom were examined for the presence of microspheres with an epifluorescence microscope. In three of five trials, bacteria applied as suspensions of lyophilized cells were recovered from a greater proportion of blossoms than bacterial cells harvested directly from culture media. Every blossom harvested within 6 days after spraying had microspheres present on the surfaces of the styles and stigmas; thus, lack of establishment of detectable populations, rather than escape of blossoms from spray inoculation, accounted for the differences in proportion of blossoms colonized by the different preparations of bacteria. The use of lyophilized cells in field trials decreased variability in the establishment of bacteria on blossoms.