Streptomycin Resistance inErwinia amylovora

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Abstract
Streptomycin resistant (Strr) E. amylovora were detected in California [USA] pear [Pyrus communis ''Bartlett''] orchards in areas where little or no streptomycin had been used for fireblight control. Both Strr and streptomycin susceptible (Strs) E. amylovora strains frequently were isolated from both healthy and diseased pear parts. Only strains resistant to high levels of streptomycin (100 .mu.g/ml) were detected from the field during a 7-yr period. Culture media greatly affected the sizes of inhibition zones. Based on the Luria-Delbruck fluctuation test, the mutation rate for high-level Strr (500 .mu.g/ml) was 4.1 .times. 10-9. Mutants with a low level of resistance (10 .mu.g/ml) occurred at 0.1 .times. 10-9 but these were unstable and died after several transfers. Generation times for Strr and Strs E. amylovora strains ranged 78-118 min with no significant difference between the 2 groups. Increasing the concentrations of streptomycin in media up to 1000 .mu.g/ml increased the generation times of Strr strains but did not prevent growth. Virulence among Strr and Strs strains varied but there was no consistent difference between the 2 groups. The Strr strains apparently were relatively stable; they were detected in orchards 6 yr after termination of streptomycin application.

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