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Effect of Antagonistic Bacteria on Establishment of Honey Bee-Dispersed Erwinia amylovora in Pear Blossoms and on Fire Blight Control. K. B. Johnson, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902; V. O. Stockwell(2), R. J. McLaughlin(3), D. Sugar(4), J. E. Loper(5), and R. G. Roberts(6). (2)(5)USDA-ARS Horticultural Crops Research Laboratory, Corvallis, OR 97330; (3)(6)USDA-ARS Tree Fruit Research Laboratory, Wenatchee, WA; (4)Department of Botany and Plant Pathology, Oregon State University, Southern Oregon Experiment Station, Medford 97502. Phytopathology 83:995-1002. Accepted for publication 25 June 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/Phyto-83-995.

In field trials conducted in 1991 and 1992 at Medford, OR, and in 1992 at Wenatchee, WA, Pseudomonas fluorescens strain A506 and Erwinia herbicola strain C9-1 established epiphytic populations on pear blossoms and were effective antagonists for the biological control of fire blight. Both bacterial antagonists, water, or streptomycin sulfate were applied to trees at 30% and full bloom. Pear trees were challenged-inoculated with freeze-dried cells of E. amylovora vectored to blossoms by honey bees. One week after full bloom, the antagonists were established in more than 95% of treated blossoms in Oregon in 1991 and Washington in 1992, but in less than 50% of blossoms in Oregon in 1992. At the same bloom stage, 41% (Oregon, 1991), 27% (Oregon, 1992), and 49% (Washington, 1992) of water-treated blossoms had detectable populations of E. amylovora, whereas trees treated with bacterial antagonists always had a significantly lower (P < 0.05) percentage of blossoms with detectable E. amylovora populations: 1820% (Oregon, 1991), 915% (Oregon, 1992), and 817% (Washington, 1992). In Oregon in 1991, only 4% of blossoms treated with bacterial antagonists supported populations of E. amylovora that exceeded 105 cfu per blossom compared with 19% of blossoms treated with water; however, suppression of population size of E. amylovora by bacterial antagonists was not apparent in 1992. In 1991, fire blight symptoms developed in 8, 0.1, and 1% of blossom clusters treated with water, streptomycin, or bacterial antagonists, respectively. In 1992, the percentage of diseased blossom clusters in these same treatments in Oregon averaged 44, 2, and 22%, respectively, and 9, 2.5, and 4%, respectively, in Washington.