Copper and Streptomycin Resistance in Pseudomonas Strains Isolated from Pipfruit and Stone Fruit Orchards in New Zealand

Abstract

Strains of plant pathogenic Pseudomonas syringae and some non patho- genic strains of Pseudomonas isolated from apple orchards in Hawke's Bay, New Zealand, were found to be resistant to streptomycin. Fifty of the 54 P. syringae strains tested carried the genes strA strB on a transposon identified by PCR and DNA sequencing as Tn5393. Other strains, including all strains of Erwinia amy- lovora, were streptomycin resistant due to a mutation in the rpsL gene. Strains of P. syringae able to grow on minimal medium containing 2 mM of copper sulphate were selected from the collection of streptomycin resistant strains. From one of these strains a 1.3 kb fragment of DNA was isolated by PCR using primers designed on two genes reported to be associated with copper resistance (the copA gene P. syringae pv. tomato and the pcoA from Escherichia coli). Sequencing of this fragment revealed a high level of similarity (95-98%) with a gene from P. syringae pv. actinidiae that codes for copper resistance. DNA homologous to this 1.3 kb fragment was detected in eight other strains of P. syringae isolated from Hawke's Bay. These strains were subsequently found to be resistant to copper. When the genes which conferred copper resistance were identified they were always carried by a plasmid. In five strains, the plasmid that carried genes coding for copper resist- ance also carried genes for streptomycin resistance. Therefore, the use of only one of those compounds could lead to selection of strains that carry resistance to both compounds. Strains of Pseudomonas resistant to streptomycin and to copper were also isolated from stone fruit orchards in Hawke's Bay (North Island) and Central Otago (South Island). Treatment of nectarines with copper seemed to affect within one day and for at least up to seven days, the percentage of pathogenic strains resistant to this compound.