Pathogenicity tests were carried out on the bark of Alnus glutinosa with 19 isolates of the standard (near-tetraploid) hybrid alder phytophthora, nine isolates representing its known heteroploid variants and 11 isolates of P. cambivora, a probable parent species of the hybrid. Over a 4-year period, 12 experiments were conducted on living alder logs incubated at 20°C. Most isolates of the standard hybrid and those of the ‘Dutch variant’ were highly aggressive to alder bark. Isolates of the ‘Swedish’, ‘UK’ and ‘German variants’, and of P. cambivora, were only weakly pathogenic. Also, isolates of P. fragariae, P. cinnamomi, P. sp. ‘O-group’, P. cryptogea, P. megasperma, P. gonapodyides and P. citricola were either weakly or nonpathogenic. Rates of lesion development were greatest on logs cut during July–October, slower on logs cut between November and March and zero on logs cut during April, indicating a strong seasonal effect. Other evidence indicated that lesion development was subject to critical thresholds of host resistance. The standard hybrid was nonpathogenic to the bark of four other hardwood and two conifer species, indicating that it is relatively host specific. In contrast, P. cambivora was an aggressive pathogen on live bark of Quercus and Castanea. The significance of these results is discussed.