Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 3, 2008
Publication Date: August 1, 2008
Citation: Pusey, P.L., Rudell Jr, D.R., Curry, E.A., Mattheis, J.P. 2008. Characterization of Stigma Exudates in Aqueous Extracts from Apple and Pear Flowers. HortScience. 43(5):1471-1478. Interpretive Summary: Fire blight, a serious bacterial disease of apple and pear trees, limits the production and international trade of pome fruit. The causal organism, Erwinia amylovora, initially becomes established on floral parts, particularly the stigmas. Antibiotics used for suppressing this bacterium on blossoms have become ineffective in many production areas due to resistance development. An alternative control strategy is the application of beneficial bacteria that compete with E. amylovora for nutrients on flower stigmas. Since knowledge of stigma chemistry could lead to a better understanding of these microbial interactions, we analyzed stigma exudates for free sugars, free amino acids, and other available nutrients. The results were used to develop an artificial liquid medium that mimics the stigma. Experiments with this medium and with detached crab apple flowers have so far indicated that some beneficial bacteria reduce the pH of the substrate to a level that is inhibitory to E. amylovora. It is hoped that the new knowledge and techniques generated will lead to advances in biological control of fire blight.
Technical Abstract: Fire blight of apple and pear is most commonly initiated by epiphytic populations of Erwinia amylovora established on flower stigmas. Knowledge of stigma chemistry could lead to advancements in biocontrol. Analyses of stigma exudates from apple and pear indicated that predominant free sugars are glucose and fructose; predominant free amino acids present in low quantities are asparagine, glutamine, proline and serine. These nutrient sources were incorporated in a liquid stigma-based medium (SBM), to which selected bacterial antagonists were added in advance of E. amylovora. Differences in the suppression of the pathogen by antagonists in SBM were comparable to results of inoculations performed with detached crab apple flowers. Alterations of SBM provided insight into the importance of substrate components in pathogen-antagonist interactions. For instance, the antagonist Pantoea agglomerans strain E325, originally selected based on its suppression of E. amylovora on stigmas of detached flowers, was more effective against the pathogen in the presence of both glucose and fructose than either sugar alone or sucrose. Additional findings have led to a reevaluation of acid production as a mode of antagonism, something speculated by early workers. Preliminary investigation with apple flowers inoculated with strain E325 indicated a significant decrease in the pH of stigma exudates.