Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 7, 2009
Publication Date: May 1, 2009
Citation: Pusey, P.L., Stockwell, V.O., Mazzola, M. 2009. Epiphytic bacteria and yeasts on apple blossoms and their potential as antagonists of Erwinia amylovora. Phytopathology. 99:571-581. Interpretive Summary: Fire blight, a serious bacterial disease of apple and pear trees, limits the production and international trade of pome fruit. The causal organism, Erwinia amylovora, colonizes the blossom stigmas during warm weather, and rain or heavy dew facilitates its movement to the floral cup (hypanthium) where infection occurs through nectary openings. Knowledge of the microflora on these plant surfaces will lead to a better understanding of interactions affecting disease development and to technologies for utilizing beneficial organisms to reduce disease incidence. New information regarding the incidence and population sizes of bacteria and yeast on apple blossom surfaces over the bloom period was generated. Diverse taxonomic groups of microorganisms were identified, and blossom assays revealed that members of two taxa (Pantoea agglomerans and Pseudomonas spp.) show particular potential for use in biological control of fire blight.
Technical Abstract: Apple blossoms were sampled for indigenous epiphytic populations of culturable microorganisms during different stages of bloom at two orchards in or near Wenatchee, WA, and one in Corvallis, OR. Frequencies and population sizes of bacteria on stigmas of apple were lower at Wenatchee than Corvallis, where higher average relative humidity may have been more favorable for colonization. Bacteria from flowers in Corvallis were mainly Pseudomonads, whereas those from Wenatchee sites were diverse, composed of several other genera. At Wenatchee, yeast as well as bacteria were isolated from both stigmatic and hypanthial surfaces. Sampled blossoms were processed immediately to assess microbial populations, or after a 24-h incubation at 28°C, which broadened the range of detectable taxa evaluated for use in biological control. Identifications were based on FAME and rRNA-sequence analyses. Yeasts (or yeast-like organisms) were detected at frequencies similar to, or greater than, bacteria, particularly in the hypanthium. When isolates were tested for their capacity to suppress E. amylovora on stigmas of detached crab apple flowers, yeasts were not effective, but their adaptation to flowers and known osmotolerance may be advantageous for preempting the pathogen on the nectariferous hypanthium. Bacteria identified as Pantoea agglomerans were most effective in suppressing E. amylovora on stigmas, followed by isolates of Pseudomonas spp. Bacillus spp. and members of other diverse genera were generally poor performers.