Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2002
Publication Date: June 20, 2002
Citation: Pusey, P.L. 2002. Biological control agents for fire blight of apple compared under conditions limiting natural dispersal. Plant Disease. 86(6):639-644 Interpretive Summary: Fire blight is a serious disease of apple and pear trees. Antibiotics applied to blossoms have become less effective because of resistance in the causal organisms. Biological control with beneficial microorganisms is a viable alternative, and one commercial product recently became available. Advances will partly depend on improved testing methods. In the laboratory, new screening assays involving crab apple flowers led to the discovery of superior biocontrol organisms. In the present study, one such organism, Pantoea agglomerans strain E325, was evaluated in the field on apple trees. A major advantage of biocontrol agents is that they multiply and spread from flower to flower. This, however, could interfere with the comparision of microbial treatments in the same orchard. Plastic enclosures were used to minimize natural spread via honeybee activity. Cross-contamination between treatments was reduced and differences in efficacy more discernible on enclosed trees than on non-enclosed trees. Strain E325 proved to be more affective than standard biocontrol strains and is presently being developed commerically for fire blight management.
Technical Abstract: The efficacy of Panoea agglomerans strain E325 for control of fire blight of apple was determined in comparative field trials involving other bacterial antagonists. Concurrently, the importance of the natural dispersal of bacteria by honey bees as a complicating factor was assessed. Tests were performed under two sets of conditions, those that allowed for the dispersal of bacteria via honey bee activity and those that minimized it through the use of translucent polyethylene enclosures around single trees. The enclosures also raised daytime temperatures and allowed for controlled wetting, two factors important to the development of blossom blight. Single-antagonist treatments with strain 325, Pseudomonas fluorescens strain A506, and P. agglomerans strain C9-1 were represented on each of 10 enclosed trees and 10 non-enclosed trees. During bloom, suspensions of antagonists (10**8 CFU/ml) were applied twice with a brush and a suspension of Erwinia amylovora (10**7 CFU/ml) was subsequently applied once using the same method. Two days after inoculation with the pathogen, trees were misted to simulate precipitation. Flower to flower spread of antagonistic bacteria was less frequent on trees surrounded by plastic enclosures than on non-enclosed trees. The range and statistical separation of means for population size of E. amylovora and disease incidence among treatments were greater for enclosed trees than for non-enclosed trees. Results indicate that the natural spread of antagonists being compared may mask difference in their efficacy as biocontrol agents. Such distortions and resulting misinterpretations could be lessened by separating treatments widely in large orchard blocks and by monitoring microbial populations.