Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/1/2008
Publication Date: 10/1/2008
Citation: Pusey, P.L., Stockwell, V.O., Rudell Jr, D.R. 2008. Antibiosis and acidification by Panoea agglomerans strain E325 may contribute to suppression of Erwinia amylovora. Phytopathology. 98:1136-1143. 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. Although antibiotics have become less effective for suppressing E. amylovora on floral parts due to resistance, biological control with beneficial microorganisms is now a viable alternative. The active ingredient in the product Bloomtime BiologicalTM, Pantoea agglomerans strain E325, was originally isolated from apple blossoms and selected in screening assays with detached crab apple flowers. Given the high efficacy of strain E325, it was suspected that this bacterium not only competes with E. amylovora for space and nutrients, but inhibits the pathogen through other mechanisms. Since E325 caused a reduction of pH that correlated with its suppression of E. amylovora in artificial media, pH 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 towards 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 to improved fire blight management.
Technical Abstract: Pantoea agglomerans strain E325, a commercially-available antagonist for fire blight of apple and pear, was originally selected through broad screening based on suppression of Erwinia amylovora on flower stigmas, but specific mechanisms were unknown. Bacterial modification of pH was evaluated as a possible mechanism by analyzing stigma exudates extracted from ‘Gala’ apple stigmas. The pH values for field samples were only slightly different, but indicated a low range (pH 5-6) conducive for antibiotic activity according to subsequent assays. Under low-phosphate and low-pH conditions, an antibacterial product of E325 with high specificity to E. amylovora was effective at low levels. A minimum of 20 to 40 ng of a ninhydrin-reactive compound purified using RP-HPLC caused visible inhibition in assays. Activity was heat stable and unaffected by amino acids, ferric ions, or enzymes known to affect antibiotics of P. agglomerans. It was deactivated, however, under basic conditions, and degraded with increasing phosphate concentrations at pH 6 and 7. Inhibition was reversed by phosphate at levels commonly used in antibiosis assays. E325 not only competes for nutrients on the stigma, but is now thought to produce an antibiotic specific to E. amylovora. Further work is necessary to substantiate that the compound is produced and active on flower stigmas.