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Interactions of Antibiotics with Pseudomonas fluorescens Strain A506 in the Control of Fire Blight and Frost Injury to Pear. Steven E. Lindow, Department of Environmental Science, Policy, and Management, University of California, Berkeley 94720-3110; Glenn McGourty(2), and Rachel Elkins(3). (2)University of California, Cooperative Extension, Mendocino County, 579 Low Gap Road, Ukiah 95482; (3)University of California, Cooperative Extension, Lake County, 883 Lakeport Blvd., Lakeport 95453. Phytopathology 86:841-848. Accepted for publication 17 April 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-841.

Pseudomonas fluorescens strain A506 and the antibiotics streptomycin and oxytetracycline, to which this antagonist is resistant, were evaluated individually and in combination in field trials over a 16-year period to assess the interactions and relative efficacy of bactericides and antagonistic bacteria in the control of fire blight and frost injury to pear. Strain A506 maintained population sizes of greater than about 105 cells per fruiting spur for over 30 days on trees inoculated once with this strain. Over 90% of the flowers on inoculated trees harbored detectable populations of strain A506. The population sizes of strain A506 were as high on inoculated trees, to which streptomycin was applied, as on trees sprayed only with strain A506. The population sizes of ice nucleation active bacteria were about 100-fold lower on trees treated with strain A506 than on untreated control trees or trees sprayed only with antibiotics. The incidence of frost injury to pear fruit in natural frosts and of fire blight was reduced to about 40 and 50%, respectively, of that on untreated trees. The incidence of frost damage and fire blight was lower on trees treated both with strain A506 and antibiotics than with either agent alone. Strain A506 and antibiotics acted additively in the control of frost and disease.