ISHS Acta Horticulturae 338: VI International Workshop on Fireblight
INTERACTIONS BETWEEN THE BIOLOGICAL CONTROL AGENT PSEUDOMONAS FLUORESCENS A506 AND ERWINIA AMYLOVORA IN PEAR BLOSSOMS |
| Authors: | M. Wilson, S.E. Lindow |
| Abstract:
Pseudomonas fluorescens strain A506 is an effective biological control agent of fire blight of pear in northern California and is currently being tested for control of fire blight in several other states, including Oregon, Washington and Utah. This study of the mechanism of control by strain A506 was initiated in order to provide information which may enable us to enhance the field efficacy of this strain, or aid in the selection of strains superior to or complementary to A506. Unlike other biological control agents of fire blight, such as Erwinia herbicola, strain A506 does not exhibit antibiosis against Erwinia amylovora in vitro and hence it was hypothesized that competition rather than antibiosis was involved in the mechanism of control. Blossoms of pear (cv. Comice) on trees in the greenhouse were spray inoculated with A506 alone, E. amylovora alone, or both strains. Populations of each strain were determined on pistils and nectaries of blossoms at 12 h intervals. Strain A506 colonized pistils very effectively to a level of ≈106 cfu/pistil. Strain A506 had no significant effect upon pathogen population size on pistils when the two strains were co-inoculated, but the pathogen population size was significantly reduced compared to the pathogen-only control when A506 was inoculated 72 h in advance of the pathogen. These data suggest that while A506 is unable to competitively exclude E. amylovora from the pistil, prior colonization of the pistil by A506 resulted in the pre-emptive utilization of nutritional resources required for growth of the pathogen. Strain A506 did not colonize nectaries as effectively as the pistil. Populations of A506 were maintained at inoculation levels for only 60–72 h. Strain A506 had no significant effect upon the pathogen population size on nectaries when the two strains were co-inoculated. When blossoms were inoculated with A506 72 h in advance of E. amylovora and the blossoms were incubated at high RH, there was a delay in pathogen multiplication and the final population size of the pathogen was significantly reduced. When blossoms inoculated in the same way were incubated at low RH, the population size of the pathogen dropped significantly prior to multiplication and the final population size of the pathogen was also significantly reduced. It was observed in blossoms colonized by A506 that nectar secretion would cease prematurely and also that there was an accumulation of a purple/red compound in the nectarial tissues. These data suggest that although pre-emptive utilization of nutritional resources may contribute to the suppression | |
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