Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/2/2012
Publication Date: 5/19/2012
Citation: Pusey, P.L., Wend, C. 2012. Potential of osmoadaptation for improving Pantoea agglomerans E325 as biocontrol agent for fire blight of apple and pear. Biological Control. 62:29-37. Interpretive Summary: Fire blight, caused by the bacterium Erwinia amylovora, is a destructive disease of apple and pear initiated in blossoms. Though managed in recent decades with antibiotics, resistance has developed in the disease organism. An alternative approach is the use of beneficial microorganisms that preemptively colonize floral surfaces and prevent E. amylovora from becoming established. The commercially available biocontrol agent, Pantoea agglomerans strain E325, first discovered by ARS scientists, is effective against fire blight but may be enhanced through improved production and formulation procedures. Osmoadaptation, which involves growing microorganisms in a high-saline medium, has been used to increase their tolerance to desiccation in the environment. It was investigated as a means of improving the survival of E325 on apple floral surfaces under dry western conditions. Although results of experiments under dry orchard conditions (less than 50% average relative humidity) didn’t indicate this, laboratory studies showed an advantage of the osmoadapted E325 in flower nectaries at moderate relative humidity (e.g., 70%); thus, osmoadaptation may actually be more advantageous in wetter orchard environments. The greatest benefit of osmoadaptation discovered was the increase in stability of preparations during freeze drying and subsequent storage prior to orchard applications. It is anticipated that this will insure better product stability for fire blight management.
Technical Abstract: Pantoea agglomerans biocontrol strain E325 is the active ingredient in a commercial product for fire blight, a destructive disease of apple and pear initiated by Erwinia amylovora in flowers. Osmoadaptation, involving the combination of saline osmotic stress and osmolyte amendment to growth media, was investigated as a means of improving the survival of this bacterial antagonist on apple flowers, particularly in dry climatic regions. E325 was osmoadapted in two complex media, nutrient yeast dextrose broth and the commercial fermentation medium, amended with NaCl and glycine betaine according to previous research. It was cultured and freeze dried with cryoprotectants at the ARS-USDA laboratory or the commercial production facility prior to treating ‘Gala’ apple flowers in the orchard. During 3 years of orchard tests when average relative humidity (RH) was below 50% and precipitation was minimal, osmoadaptation did not affect the growth and survival of E325 on flower stigmas and hypanthia. Results were similar in the laboratory with detached flowers; however, osmoadapted E325 was at a significant advantage in the nectar-rich hypanthium at an intermediate RH of 70%, indicating a potential benefit of osmoadaptation under moderate moisture conditions. Laboratory evaluation of E325 preparations indicated that the greatest benefit of osmoadaptation is likely to be the increase in stability during freeze drying and subsequent storage prior to orchard application. The study provided evidence that osmoadaptation in high-saline growth media could complement the addition of cryoprotectants to bacterial cells to increase the overall stability of freeze dried preparations of E325 and other biocontrol agents.