Competitive Exclusion of Epiphytic Bacteria by Ice − Pseudomonas syringae Mutants

Abstract

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 Ice − P. 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�C were reduced on plants colonized with Ice − P. syringae mutant strains before challenge inoculations with an Ice + P. syringae wild-type strain. Frost injury to plants pretreated with Ice − P. syringae strains was also reduced significantly compared with that to control plants and was correlated with the population size of the Ice + P. syringae strain and with the numbers of ice nuclei active at −5�C. An Ice − P. syringae strain colonized leaves, flowers, and young fruit of pears in field experiments and significantly reduced the colonization of these tissues by Ice + P. syringae strains and Erwinia amylovora as compared with untreated trees.