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The host ranges of 33 isolates of the downy mildew fungus Peronospora parasitica from different Brassica hosts and diverse geographic origins were assessed on a standard set of Brassica accessions Isolates from each host species had a distinct host range, and in some cases isolates from the same host species but different geographic origins differed in host range. Most isolates were capable of infecting hosts other than their species of origin. These results are discussed in relation to species specificity and the conservation of certain virulence gene combinations. The epidemiological implications for cross‐infection of Brassica hosts by P. parasitica isolates in the field are also considered.