EFFECT OF ANTAGONISTIC BACTERIA ON ESTABLISHMENT OF HONEY BEE-DISPERSED ERWINIA-AMYLOVORA IN PEAR BLOSSOMS AND ON FIRE BLIGHT CONTROL

In field trials conducted in 1991 and 1992 at Medford, OR, and in 1992 at Wenatchee, WA, Pseudomonas fluorescens strain A506 and Erwinia her bicola strain C9-1 established epiphytic populations on pear blossoms and were effective antagonists for the biological control of fire blig ht. Both bacterial antagonists, water, or streptomycin sulfate were ap plied to trees at 30% and full bloom. Pear trees were challenged-inocu lated with freeze-dried cells of E. amylovora vectored to blossoms by honey bees. One week after full bloom, the antagonists were establishe d in more than 95% of treated blossoms in Oregon in 1991 and Washingto n 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% (Was hington, 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 detect able E. amylovora populations: 18 20% (Oregon, 1991), 9-15% (Oregon, 1 992), and 8-17% (Washington, 1992). In Oregon in 1991, only 4% of blos soms treated with bacterial antagonists supported populations of E. am ylovora that exceeded 10(5) cfu per blossom compared with 19% of bloss oms 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 t reated with water, streptomycin, or bacterial antagonists, respectivel y. 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.