Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/1999
Publication Date: N/A
Citation: Interpretive Summary: Fire blight is an important disease of pear and apple, caused by Erwinina amylovora, a bacterial plant pathogen. For the past forty years, growers have relied on sprays of streptomycin to manage fire blight, but these sprays are no longer effective in many orchards due to the presence of streptomycin-resistant strains of Erwinia amylovora. Biological control poses a promising alternative to streptomycin for management of fire blight, and a bacterial biocontrol agent is now available commercially. Bacterial biological control agents can be sprayed onto blossoms, where they quickly establish populations large enough to suppress the pathogen, which would otherwise infect the tree through blossoms. Bacterial biocontrol agents will establish populations most reliably on blossoms in which other bacteria are not already well established. For this reason, we determined the presence and identity of bacteria, fungi, and yeasts on pear blossoms in pear orchards in three locations in Oregon and Washington. We found that microorganisms are absent from a vast majority of blossoms immediately after those blossoms open. Very quickly after opening, however, blossoms are colonized by microorganisms. Our data indicate that growers should spray orchards with biocontrol agents at early stages of bloom to allow them to colonize blossoms with minimal competition from other microorganisms. These data are important because they provide a rationale for optimizing the success of biological control.
Technical Abstract: Pear blossoms were sampled during various stages of bloom in 1991 and 1992 from orchards in Cashmere, Washington and Corvallis and Medford, Oregon for epiphytic populations of culturable bacteria. On stigmatic surfaces, bacteria were isolated from 2-32% of blossoms prior to petal expansion and the isolation incidence increased to 47 to 94% by petal fall. In general, a lower percentage of hypathia than stigmas supported bacterial populations. Randomly-selected bacteria isolated at populations ? 104 CFU/tissue were identified by FAME analysis. Diverse genera of Gram-negative and Gram-positive bacteria were identified from Medford and Cashmere field sites. Pseudomonas syringae and Pseudomonas viridiflava were isolated from all sites, being the predominant species detected in Corvallis, where they were found on up to 28% of the blossoms sampled on a given date. The presence of filamentous fungi and yeasts on floral tissues was evaluated in Medford in 1991 and in Cashmere in 1991 and 1992. Generally less than 15% of pear blossoms sampled from orchards in Medford and Cashmere supported culturable populations of yeasts and filamentous fungi prior to petal expansion. Within a few days of petal expansion, however, yeasts and filamentous fungi were detected on stigmas or hypanthia of 10 to 85 % of blossoms. Because most pear blossoms do not support detectable populations of culturable bacteria or fungi prior to petal expansion, we speculate that introduced biocontrol agents may become established with minimal competition from indigenous epiphytes at early stages of bloom.