ISHS Acta Horticulturae 338: VI International Workshop on Fireblight
ESTABLISHMENT ON PEAR BLOSSOMS OF BIOCONTROL AGENTS FOR FIRE BLIGHT |
| Authors: | V.O. Stockwell, J.E. Loper, K.B. Johnson |
| Abstract:
Under the semi-arid conditios of the pear production areas of the western United States, Fire Blight is generally limited to blossom infections. Establishment of bacterial biocontrol agents on blossoms may prevent colonization and infection by Erwinia amylovora. In 1991 and 1992, two potential biocontrol bacteria, Pseudomonas fluorescens strain A506 and Erwinia herbicola strain C9-1, were sprayed at 108 cfu/ml (separately and as a mixture) onto pear (Pyrus communis L., c.v. Bartlett) trees in 10–30% and 70–80% bloom in experimental plots near Corvallis and Medford, Oregon. Other trees were treated with streptomycin (100μg/ml) as a comparative chemical control method or water as a non-protected control at 10–30%, 70–80% bloom, and four days after full bloom. A nalidixic acid-resistant strain of E. amylovora was applied by two methods at full bloom in 1992. In one inoculation method, a 106 cfu/ml aqueous suspension of the pathogen was sprayed onto trees until run-off. The second method involved dusting blossom clusters eith a freeze-dried preparation of the pathogen with a cell concentration of 109 cfu/g. Population sizes of the bacterial biocontrol agents and the pathogen were estimated by dilution plating the stigmas, styles, and nectaries from individual blossoms onto selective media. After spray inoculation of the pathogen, population sizes of E. amylovora between 102 to 103 cfu/blossom were recovered from 5–10% of the blossoms from water-treated trees. Population sizes of E. amylovora averaged 103 to 104 cfu/blossom on 40% (Medford) and 50% (Corvallis) of the blossoms on water-treated trees inoculated by dusting the freeze-dried pathogen preparation. A506 was detected in 50–90% of the sampled blossoms in Medford and 25–50% of the blossoms in Corvallis in 1991. In 1992, incidence of recovery of A506 from flowers was lower (40% in Medford, 30% in Corvallis) than in 1991. The average population sizes of A506 ranged from 104 to 105 cfu/blossoms in both sites in 1991 and 1992. C9–1 was recovered from a higher proportion of blossoms in Medford (50–90% in 1991, 70–100% in 1992) than in Corvallis (20–30% in 1991, and 30–50% in 1992). The average population sizes of C9-1 in both sites and years was between 104 to 105 cfu/blossom. | |
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