Streptomycin Resistance ofPseudomonas syringaepv.papulansin Apple Orchards and Its Association with a Conjugative Plasmid

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Abstract
Pseudomonas syringae pv. papulans strains resistant to streptomycin were isolated from blister spot lesions in 10 of 22 western New York apple orchards. The growth of a streptomycin-sensitive strain was inhibited by 2.5 .mu.g of streptomycin per milliliter of culture medium. Resistant strains grew on medium containing 100 .mu.g/ml but not on medium containing 25 0 .mu.g/ml. There was a significant asssociation between the number of streptomycin sprays applied in orchards in 1985 and the detection of streptomycin-resistant strains of P. s., pv. papulans. Streptomycin-resistant strains were isolated from dormant buds and from fruit lesions the year after initial detection of resistance. A 68-megadalton (MDa) plasmid was present in streptomycin-resistant strains of P. s. pv. papulans but was not observed in streptomycin-sensitive field strains. Streptomycin resistant was transferred from streptomycin-resistant donor strains Psp 34 and Psp 36 to streptomycin-sensitive recipient strains after matings on nitrocellulose membranes at frequencies of 4.7 .times. 10-3 and 2.0 .times. 10-2 per recipient cell, respectively. Transconjugates contained the plasmids of recipient strains plus the 68-MDa plasmid of the donor strain. The 68-MDa plasmid of Psp 36 was designated pCPP501. Transfer of streptomycin resistance from donor strain Psp 33 occurred at the much lower frequency of 2.0 .times. 10-6 per recipient cell and resulted in more complex changes in the plasmid content of transconjugants.