CONTROL OF FIRE BLIGHT BY PSEUDOMONAS FLUORESCENS A506 INTRODUCED INTO UNOPENED PEAR FLOWERS

Pseudomonas fluorescens strain A506 is a bacterium that confers biological control of fire blight by preemptive competitive exclusion of Erwinia amylovora and is commercially available as Blightban A506^®. Best control is achieved if the antagonist is applied to newly opened flowers prior to the arrival of the pathogen. However, since flowers of pear cultivars such as Bartlett (Williams) can emerge for several weeks, the bacterium must be applied three or more times even though it can spread throughout the tree after application. A means to ensure that it is the initial colonist of flowers was investigated over a multiple year period by introducing it into flower buds with a penetrating organo-silicon surfactant, Breakthru^®. The proportion of flowers colonized with strain A506 throughout the spring generally increased with increasing concentrations of surfactant in which the bacteria were applied in a single application at the time of first bloom. Importantly, throughout the main bloom and into delayed bloom, populations of the antagonist on most flowers on trees inoculated only one time at “first bloom” with strain A506 in 0.2% or 0.5% Breakthru^® remained as high or higher than those on trees receiving weekly applications of the same amount of A506 alone. Colonization of flowers from early-season applications was poor if flower buds were too tightly closed at the time of application. No fruit russet caused by the high surfactant rate was observed at harvest when treatments were applied before substantial bloom had occurred, but russet occurred at later application times. A single application of strain A506 at very early bloom reduced the costs of control versus multiple sprays.