INTERACTIONS OF ANTIBIOTICS WITH PSEUDOMONAS-FLUORESCENS STRAIN A506 IN THE CONTROL OF FIRE BLIGHT AND FROST INJURY TO PEAR

Pseudomonas fluorescens strain A506 and the antibiotics streptomycin a nd oxytetracycline, to which this antagonist is resistant, were evalua ted individually and in combination in field trials over a 16-year per iod to assess the interactions and relative efficacy of bactericides a nd antagonistic bacteria in the control of fire blight and frost injur y to pear. Strain A506 maintained population sizes of greater than abo ut 10(5) cells per fruiting spur for over 30 days on trees inoculated once with this strain. Over 90% of the flowers on inoculated trees har bored detectable populations of strain A506. The population sizes of s train A506 were as high on inoculated trees, to which streptomycin was applied, as on trees sprayed only with strain A506. The population si zes of ice nucleation active bacteria were about 100-fold lower on tre es treated with strain A506 than on untreated control trees or trees s prayed only with antibiotics. The incidence of frost injury to pear fr uit in natural frosts and of fire blight was reduced to about 40 and 5 0%, respectively, of that on untreated trees. The incidence of frost d amage and fire blight was lower on trees treated both with strain A506 and antibiotics than with either agent alone. Strain A506 and antibio tics acted additively in the control of frost and disease.