Competitive Exclusion of Epiphytic Bacteria by IcePseudomonas syringae Mutants

The growth of ice nucleation-active and near-isogenic ice nucleation-deficient (Ice) Pseudomonas syringae strains coexisting on leaf surfaces was examined to determine whether competition was sufficient to account for antagonism of phylloplane bacteria. The ice nucleation frequency spectra of 46 IceP. syringae mutants, obtained after mutagenesis with ethyl methanesulfonate, differed both quantitatively and qualitatively, but the mutants could be grouped into four distinct phenotypic classes. The numbers of ice nucleation-active bacteria and ice nuclei active at -5 degrees C were reduced on plants colonized with IceP. syringae mutant strains before challenge inoculations with an IceP. syringae wild-type strain. Frost injury to plants pretreated with IceP. syringae strains was also reduced significantly compared with that to control plants and was correlated with the population size of the IceP. syringae strain and with the numbers of ice nuclei active at -5 degrees C. An IceP. syringae strain colonized leaves, flowers, and young fruit of pears in field experiments and significantly reduced the colonization of these tissues by IceP. syringae strains and Erwinia amylovora as compared with untreated trees.