Comparative aggressiveness of standard and variant hybrid alder phytophthoras, Phytophthora cambivora and other Phytophthora species on bark of Alnus, Quercus and other woody hosts

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 degreesC . Most isolates of the standard hybrid and these of the 'Dutch variant' wer e highly aggressive to alder bark. Isolates of the 'Swedish', 'UK' and Germ an variants', and of P. cambivora, were only weakly pathogenic. Use, isolat es of P. fragariae, P. cinnamomi, P. sp. 'O-group', P. cryptogea, P. megasp erma, 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 l esion development was subject to critical thresholds of host resistance. Th e standard hybrid was nonpathogenic to the bark of four other hardwood and two conifer species, indicating that it is relatively host specific. In con trast, P. cambivora was an aggressive pathogen on live bark of Quercus and Castanea. The significance of these results is discussed.