Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2007
Publication Date: N/A
Interpretive Summary: Fire blight is a serious disease of apple and pear trees that limits production in many fruit-growing regions. The causal bacterium, Erwinia amylovora, initially becomes established on blossom stigmas and rain or dew facilitates its movement to the floral cup where infection occurs. Antibiotics used to suppress E. amylovora on floral parts have become less effective due to resistance. Biological control with beneficial microorganisms is now a viable alternative. The active ingredient in the product Bloomtime Biological (Superscript)TM, Pantoea agglomerans strain E325, was originally isolated from apple blossoms and selected in screening assays with detached crab apple flowers. Given its high efficacy, E325 was suspected to not only compete for space and nutrients, but to possess additional modes of action. Since E325 caused a reduction of pH that correlated with its suppression of E. amylovora in an artificial medium, pH modification was investigated. Results with Gala apple did not implicate pH modification as a primary mode of biological control, but did indicate a low pH range on stigmas conducive for the activity of an inhibitory compound produced by E325. The inhibitor was shown under low-phosphate and acid conditions to be highly effective and specific in suppressing E. amylovora. Additional characteristics of the E325 compound distinguish it from known antibiotics produced by other strains of P. agglomerans. The new knowledge could lead to enhancement of strain E325 as a biocontrol agent and improved fire blight management.
Technical Abstract: Pantoea agglomerans strain E325, the active ingredient in Bloomtime Biological (superscript)TM, was originally isolated from apple blossoms and selected based on broad screening with detached crab apple flowers. To evaluate pH modification on Gala apple stigmas as a possible mode of antagonism, exudates were extracted from flower stigmas and tested with a pH electrode. Measurement of pH in field samples resulted in only slight differences, but indicated a low pH range (between 5 and 6) conducive for antibiotic activity based on laboratory assays. An extracellular product of E325 was shown under low-phosphate and low-pH conditions to inhibit E. amylovora, but not strains of 17 other microbial species tested so far. A minimum of 20 to 40 ng of the compound, purified using RP-HPLC, caused visible inhibition of the pathogen in plate assays. Inhibition was relatively heat stable and unaffected by amino acids, ferric ions, or enzymes previously used to characterize antibiotics from other strains of P. agglomerans. Conversely, activity was critically affected by pH and phosphate buffering capacity. It was deactivated under basic conditions, and at pH 6 and 7, it diminished or disappeared at a rate that increased with the phosphate buffer concentration. The inhibitory activity was often undetectable at phosphate concentrations commonly used in tests for antibiosis. Work is in progress to develop methodology for direct evaluation of pH on flower stigmas, and to further characterize the inhibitory compound produced by strain E325.