EPIPHYTIC GROWTH OF A PATHOGENIC AND AN AVIRULENT STRAIN OF ERWINIA AMYLOVORA ON PEAR AND APPLE FLOWERS IN RELATION TO BIOLOGICAL CONTROL

Prior to infection, Erwinia amylovora grows epiphytically on floral stigmas, which provide a conducive but nonselective habitat for bacterial growth. This nonselectivity allows for biocontrol of fire blight; although, in practice, it is very difficult to exclude E. amylovora completely from this habitat. We investigated the dynamics of growth suppression of E. amylovora by comparing the ability of virulent and avirulent strains of E. amylovora to compete with each other on stigmas of pear and apple and to compete with a co-inoculated mixture of effective bacterial antagonists. When strains were inoculated individually, virulent E. amylovora strain Ea153 attained the highest population size on stigmas with 'epiphytic yields' that were approximately double those of the avirulent derivative or the bacterial antagonists. In competition experiments, growth of the avirulent hrpL mutant of Ea153 was suppressed by the antagonist mixture to a greater extent than the virulent strain. Unexpectedly, the virulent strain enhanced the epiphytic yield of the antagonist mixture. Similarly, a small dose of virulent Ea153 added to inoculum of Ea153 HrpL- significantly increased the epiphytic yield of the avirulent strain. These results are consistent with the hypothesis that virulent E. amylovora modifies the epiphytic habitat presented by the stigma through the expression of pathogenesis-related genes, which increases resources available to itself and coincidentally to nonpathogenic competitors.