SECONDARY COLONIZATION OF PEAR BLOSSOMS BY 2 BACTERIAL ANTAGONISTS OFTHE FIRE BLIGHT PATHOGEN

Dispersal of the bacteria Pseudomonas fluorescens strain A506 and Erwi nia herbicola strain C9-1S from treated to nontreated pear blossoms, a nd the effect of their spread on fire blight, were investigated in an orchard block of 10 rows containing 4 trees per row. Center rows of tr ees were sprayed with a mixture of P. fluorescens A506 and E. herbicol a C9-1S at 30, 15, and 50% bloom in 1994, 1995, and 1996, respectively . Immediately after spraying, antagonists were detected only on treate d blossoms. In 1994 and 1996, as bloom progressed, both P. fluorescens A506 and E. herbicola C9-1S were detected on nontreated blossoms loca ted up to 4 rows (10 m) from the treated rows. In 1995, establishment of the antagonists on treated blossoms was poor and spread to nontreat ed trees was limited, apparently because of cold temperatures. Each ye ar, honey bees were used to inoculate all trees with E. amylovora at 8 0% bloom. After full bloom in 1994 and 1996, the proportion of blossom s with E. amylovora populations >10(5) CFU per flower were highest in the outermost rows, and decreased linearly (P < 0.05) with proximity t o treated rows. In 1994, diseased blossom clusters decreased significa ntly (P < 0.05) from the outermost rows to the treated rows, but there was no significant effect of distance on disease incidence in 1995 or 1996. Secondary colonization of blossoms by P. fluorescens A506 and E . herbicola C9-1S can play a role in disease suppression, but, among s easons, rates of secondary colonization by P. fluorescens A506 and E. herbicola C9-1S were variable, indicating that multiple applications o f antagonists may be necessary to optimize biological control.