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 bacteria l antagonists that suppress fire blight and Erwinia amylovora on bloss oms was evaluated. Aqueous suspensions of Pseudomonas fluorescens A506 , E. herbicola C9-1R, or E. amylovora 153N were prepared from cells ha rvested from the surface of an agar medium or from cells that were lyo philized after culture under similar conditions. Bacterial suspensions (1 x 10(8) CFU/ml) were sprayed on pear and apple trees at 50% bloom near midday. The incidence of recovery (proportion of blossoms contain ing detectable populations) and the population sizes of the bacteria o n individual blossoms with detectable populations were followed over a period of several days. Fluorescent microspheres (1 mu m in diameter) were added to sprays at a concentration of 1 x 10(7) microspheres per mi to mark blossoms that were open during application of bacteria. Af ter dilution-plating, the stigmas and styles of each blossom were exam ined for the presence of microspheres with an epifluorescence microsco pe. In three of five trials, bacteria applied as suspensions of lyophi lized cells were recovered from a greater proportion of blossoms than bacterial cells harvested directly from culture media. Every blossom h arvested within 6 days after spraying had microspheres present on the surfaces of the styles and stigmas; thus, lack of establishment of det ectable populations, rather than escape of blossoms from spray inocula tion, accounted for the differences in proportion of blossoms colonize d by the different preparations of bacteria. The use of lyophilized ce lls in field trials decreased variability in the establishment of bact eria on blossoms.