|Authors: ||S.V. Thomson, S.C. Gouk, J.L. Vanneste, C.N. Hale, R.G. Clark|
Streptomycin (Str) resistant isolates of Erwinia amylovora were isolated from fire blight infections in apple, pear and hawthorn in the Hawkes Bay fruit growing area of New Zealand.
Seven orchards with resistant isolates were detected in a 10 km diameter area in a major apple and pear growing region.
In most of the seven orchards, 100% of the E. amylovora isolates and many of the saprophytic bacteria were Str resistant.
Only Str sensitive isolates were detected in 28 other orchards in the vicinity.
Attempts to determine the distribution of Str resistant E. amylovora in other geographical areas of New Zealand were hampered by the low disease incidence in 1991. However, a limited number of infections were tested and all found to be Str sensitive in the regions of Waikato, Bay of Plenty, and Gisborne.
All Str resistant isolates were resistant to 100 μg/ml of Str incorporated into KB medium or to 1000 μg/ml in filter disks.
The total DNA of the Str resistant strains of E. amylovora did not hybridize with SMP3, a 500 bp probe that is specific for the Str resistance gene in Pseudomonas syringae pv. papulans strain Psp36, indicating that the genetic determinant of the New Zealand Str resistant E. amylovora strains is not related to that of P.s. pv. papulans isolated in New York and Michigan or to that of Str resistant E. amylovora isolated in Michigan, USA. Resistance to Str in E. amylovora was not transferred in conjugation attempts to Escherichia coli, and no extra plasmids were detected in the New Zealand Str resistant E. amylovora strains.
This implies that Str resistance is probably not plasmid borne and might be comparable to the chromosomal Str resistance found in California isolates of Str resistant E. amylovora.
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