Submitted to: Acta Horticulture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2005
Publication Date: 2/1/2006
Citation: Pusey, P.L. 2006. Biochemical analyses of pomaceous stigma exudates and relevance to biological control of fire blight. Acta Horticulture Proceedings. v. 704. p. 375-377. 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 the stigmas of flowers during warm weather, and upon wetting by rain or heavy dew, moves to the floral cup where it invades through the nectary openings. One widely demonstrated control strategy is to apply beneficial bacteria that compete with E. amylovora for nutrients on the stigma. Since knowledge of the chemical and nutrient environment on stigmatic surfaces could advance this technology, we began investigating this by analyzing stigma exudates for free sugars and amino acids. It was discovered that predominant sugars are glucose and fructose, and predominant amino acids are asparagine, glutamine, proline, and serine. This information was used to develop an artificial liquid medium that simulates stigma exudates. It is hoped that the medium and modifications of it will lead to greater understanding of interactions between beneficial bacteria and E. amylovora on apple and pear flowers, and to advances in biological control of fire blight.
Technical Abstract: Microbial antagonism toward Erwinia amylovora on stigmas of pear and apple flowers has been widely demonstrated. Since knowledge of the chemical environment on stigmas could advance the selection and use of antagonists, stigma exudates from different pomaceous cultivars and ages were analyzed for sugars and amino acids. Flowers were collected in groups of 50, and one-by-one, their stigmas were submerged and sonicated in the same aliquot of 600 'l water. Pollen and macromolecules were removed by centrifugation and filtration. Sugars were converted to trimethylsilyl derivatives and analyzed by GC. Amino acids were converted to fluorescent derivatives using o-phthaldialdehyde and separated by HPLC. The predominant sugars were glucose and fructose; predominant amino acids were asparagine, glutamine, proline and serine. A liquid medium was developed with these compounds to evaluate antagonist-pathogen interactions. The stigma-based medium was placed in microplates, 250 'l/well, then inoculated with selected bacterial antagonists 24 h prior to inoculation with E. amylovora. The same antagonist strains were applied to stigmas of detached crab apple flowers 24 h in advance of the pathogen. Differences among antagonists according to their effect on the size of pathogen populations in the artificial medium were similar to the differences indicated in tests with flowers. The stigma-based medium will likely be modified as we learn more about the chemistry of apple and pear stigma exudates. It remains to be proven whether the medium is more predictive of antagonist performance on flowers than other artificial media used for this purpose.