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During the survey of sunflower fields in Mazandaran province and Gorgan plain between 1995 and 1996, fifty six downy mildew samples from 31 areas were collected. The disease causal agent in these samples , Plasmopara hatstedii,was mass-produced and maintained separately using the whole seedling immersion(WSI)method on susceptible record cultivar. The maintained isolates were inoculated on differential lines received from Canada, according to the proposed-internationally standardized method. The differential lines showed similar reaction, to all isolates tested. Comparison of the reactions of isolates with differential line reactions, in relation to the designated known physiological races, showed that these isolates were different from the existing known races.