Characterization of epiphytic bacteria from Australia and Europe as possible fire blight antagonists

Several epiphytic bacteria were isolated from apple and pear trees at three places in Australia and classified as the novel species Erwinia tasmaniensis by microbiological data and DNA/DNA hybridization assays as well as nucleotide sequence analyses of several house keeping genes. They are non-pathogenic for apple and pear, produce levan from sucrose and can induce a hypersensitive response (HR) on leaves of non-host plants. Taxonomically different strains, classified as Erwinia billingiae, were isolated in England, and were assayed similarly as E. tasmaniensis. Most of the investigated plant-associated bacteria produced the autoinducer AI-2. A Vibrio harveyi mutant was used as a sensor for induction of bioluminescence by culture supernatants of the epiphytic bacteria. E. billingiae additionally produced an acyl-homoserine lactone (AHL, AI-1), which was monitored by complementation of a Chromobacter violaceum mutant for pigment secretion. When slices of immature pears were soaked with E. billingiae or E. tasmaniensis the bacteria interfered with the growth of E. amylovora for formation of ooze and necrotic symptoms. When inoculated in excess to E. amylovora into apple flowers, the pathogen propagation was strongly reduced. First trials in an experimental orchard confirmed their ability to prevent symptom formation of fire blight in blossoms of apple trees to a significant extent.