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Ecology and Epidemiology

Streptomycin Resistance in Erwinia amylovora. M. N. Schroth, Professor, Department of Plant Pathology, University of California, Berkeley, 94720; S. V. Thomson(2), and W. J. Moller(3). (2)Assistant professor, Department of Plant Pathology, University of California, Berkeley, 94720; (3)Extension plant pathologist, University of California, Davis, 95616. Phytopathology 69:565-568. Accepted for publication 16 November 1978. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1979. DOI: 10.1094/Phyto-69-565.

Streptomycin resistant (Str^r) Erwinia amylovora were detected in California pear orchards in areas where little or no streptomycin had been used for fireblight control. Both Str^r and streptomycin susceptible (Str^s) E. amylovora strains frequently were isolated from both healthy and diseased pear parts. Only strains resistant to high levels of streptomycin (100μg/ml) were detected from the field during a 7-yr period. Culture media greatly affected the sizes of inhibition zones. Based on the Luria-Delbrück fluctuation test, the mutation rate for high-level Str^r (500 μg/ml) was 4.1 × 10^–9. Mutants with a low level of resistance (10 μg/ml) occurred at 0.1 × 10^–9 but these were unstable and died after several transfers. Generation times for Str^r and Str^s E. amylovora strains ranged from 78 to 118 min with no significant difference between the two groups. Increasing the concentrations of streptomycin in media up to 1,000 μg/ml increased the generation times of Str^r strains but did not prevent growth. Virulence among Str^r and Str^s strains varied but there was no consistent difference between the two groups. The Str^r strains appeared to be relatively stable; they were detected in orchards 6 yr after termination of streptomycin application.