Submitted to: International Symposium on Biological Control of Bacterial Diseases Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2006
Publication Date: 10/19/2006
Citation: Pusey, P.L. 2006. Chemistry of apple and pear stigma exudates related to bacterial antagonism toward erwinia amylovora. International Symposium on Biological Control of Bacterial Diseases Proceedings. Mitt. Biol. Bundesanst. Land- Forstwirtsch. v 408:228-232 . 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, becomes established on floral parts prior to infection. Antibiotics have been used to suppress it; however, resistance has developed in many production areas. An alternative control strategy is the application of beneficial bacteria that compete with E. amylovora for nutrients, particularly on the flower stigma. Since knowledge of the stigma chemistry could help us understand these microbial interactions, we analyzed stigma exudates for free sugars, free amino acids, and other available nutrients. This information was used to develop an artificial liquid medium that simulates stigma exudates. Studies with the medium, supported by preliminary work with flowers, have so far led to the discovery that pH modification may be an important mode of action by some biological agents. It is hoped the new information 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 that first become established on flower stigmas. Since microbial activity on the stigma is largely supported by the presence of fluidal exudate, knowing the chemistry of this substance could lead to advancements in biocontrol. When stigma exudates from apple were analyzed, identifiable components by weight were 48% complex carbohydrates, 46% protein, 6% free sugars, and <0.1% free amino acids. Predominant free sugars in exudates from apple and pear were glucose and fructose in near-equal proportions; predominant free amino acids were asparagine, glutamine, proline and serine. These sugars and amino acids were incorporated in a liquid stigma-based medium (SBM), to which selected bacterial antagonists were added in advance of E. amylovora. Differences in antagonist capacity to suppress growth of E. amylovora in SBM were comparable to results of inoculations performed with detached crab apple flowers. Also, SBM results mimicked those of flowers better than did other synthetic media or altered forms of SBM. For instance, replacement of the glucose-fructose combination in SBM with the weight equivalent of glucose alone had an outcome less similar to that of flowers. Efforts to identify modes of antagonism in SBM point to competitive exclusion and antibiosis; however, pathogen suppression correlated best with the capacity of antagonists to increase medium acidity to a level unfavorable for E. amylovora, a phenomenon observed in vitro by early workers (Goodman, 1965; Riggle and Klos, 1972). Preliminary investigations with inoculated apple flowers also indicate pH modification on the stigma. Further study of stigma chemistry related to microbial interactions should prove advantageous in maximizing the use of biocontrol agents for fire blight.