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Ecology and Epidemiology

Competition Between Ice Nucleation-Active Wild Type and Ice Nucleation-Deficient Deletion Mutant Strains of Pseudomonas syringae and P. fluorescens Biovar I and Biological Control of Frost Injury on Strawberry Blossoms. Julianne Lindemann, Plant Pathology/Biological Control Group, Advanced Genetic Sciences, Inc., 6701 San Pablo Ave., Oakland, CA 94608; Trevor V. Suslow, Plant Pathology/Biological Control Group, Advanced Genetic Sciences, Inc., 6701 San Pablo Ave., Oakland, CA 94608. Phytopathology 77:882-886. Accepted for publication 20 November 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-882.

Ice nucleation-deficient (INA^-) mutants of Pseudomonas strains derived by site-directed mutagenesis were tested for their efficacy as biological control agents of frost injury on blossoms of greenhouse-grown strawberry plants. Inhibition of one bacterial strain by its near-isogenic counterpart was dose dependent rather than strain dependent. The INA^- deletion mutants of P. syringae and P. fluorescens biovar I, inoculated at 10⁷ cfu per blossom, inhibited growth of their ice nucleation-active (INA⁺) parental strains inoculated at 10² cfu per blossom. The INA⁺ parental strains inhibited INA^- derivatives when inoculum doses were reversed. Inhibition was incomplete unless doses differed by about 10⁴-fold. No inhibition occurred when two strains were inoculated simultaneously at equal doses (either 10² or 10⁷ cfu per blossom). The INA^- P. syringae strain protected blossoms against freezing by other P. syringae strains but did not inhibit or protect against INA⁺ P. fluorescens. The INA^- P. fluorescens strain was much more effective as an inhibitor of P. syringae strains than of other P. fluorescens strains.