Temporal Dynamics of the Biocontrol Agent Pseudomonas fluorescens Strain A506 in Flowers in Inoculated Pear Trees

The colonization of individual flowers in mature pear orchards by Pseudomonas fluorescens strain A506 applied at different times during bloom was measured to determine the receptivity of flowers to colonization and the extent of intra-tree movement over time. Strain A506 populations in flowers open at inoculation were initially about 10 4 cells per flower and increased to approximately 10 6 cells per flower in flowers that were inoculated within about 5 days of opening. However, eventual populations decreased with further increases in flower age at inoculation to as few as about 10 3 cells per flower when inoculated flowers were more than 10 days old. Populations of strain A506 on flowers that opened after inoculation was initially very low at the time of petal expansion (<100 cells per flower) but increased rapidly with time after flower opening. The maximum population of strain A506 that developed on such flowers decreased with increasing time between inoculation and petal expansion; 10 4 to 10 5 cells of strain A506 eventually colonized flowers that opened within 7 days of inoculation, whereas fewer than 100 cells colonized flowers that opened 24 days or more after inoculation. Large total bacterial populations on A506-treated trees were associated with significant reductions in populations of Erwinia amylovora and reduced incidence of fire blight and severity of fruit russet.