A survey of the occurrence of strains of Erwinia amylovora resistant to streptomycin in certain Egyptian pear orchards was earned out during April and May 1988. Twenty‐two isolates out of 604 isolates collected from 11 orchards showed resistance to streptomycin. All the streptomycin resistant (Strr) strains isolated in the present work were resistant to high levels of streptomycin with minimal inhibitory concentrations ranging from 1000 to 3000 μg/ml. The occurrence of Strr strains in Egypt is still limited and the population of resistant strains was at relatively low level. However, such occurrence of E. amylovora with resistance to streptomycin is a potentially serious situation.
Five isolates of Badllus subtilis were found to be antagonistic to 3 isolates of Erwinia amylovora in vitro on sucrose agar medium. However, B. subtilis or its culture filtrate did not exhibit any activity against E. amylovora in planta tests in the laboratory on immature green pear fruits or in a commercial pear orchard under natural growing conditions.
Five isolates of Bacillus subtilis isolated from the soil, were found to be antagonistic to 6 isolates of Agrobacterium tumefaciens in vitro.
Inoculation of B. subtilis in wounded castor bean plants 30 min before or simultaneous inoculation with A. tumefaciens resulted in excellent control of the crown gall symptoms on the host within 50 days of inoculation.
Application of B. subtilis 30 min after inoculation with A. tumefaciens did not result in appreciable disease reduction.
Treatment of the tested plants by B. subtilis did not induce any phytotoxic injury or growth retarding side effects.
It appears that B. subtilis could potentially be incorporated for crown gall control. However, further tests are needed to test this biological control agent with other plant species especially fruits, nuts and vine nursery stock.
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