Variation in aggressiveness of Canadian isolates of Phytophthora infestansas indicated by their relative abilities to cause potato tuber rot

Studies were conducted in 1995, 1996, and 1997 to determine the ability of Canadian isolates of Phytophthora infestans to cause tuber disease in a var iety of potato cultivars. Most isolates of recently introduced multilocus g enotypes (US-7, US-8, g11, g26, g29, and g40) were more aggressive on tuber tissue than isolates of the traditional US-1 genotype, based on surface ne crosis (SN), lesion depth (LD), and compound aggressiveness index (CAI = SN x LD) components. Other multilocus genotypes (g30, g41, g42, and UN-3) wer e similar in aggressiveness to US-1. The g11 (US-11) genotype consistently displayed aggressiveness that was intermediate to that of the US-8 and US-1 genotypes. Variation also occurred within a multilocus genotype, and an is olate of the US-8 genotype from New Brunswick was consistently less aggress ive than other US-8 isolates. Cvs. Dorita and Island Sunshine were consiste ntly the most resistant to infection, regardless of isolate, relative to th e more susceptible responses of Green Mountain, Russet Burbank, Kennebec, S uperior, Shepody, Red Pontiac, Sebago, and Bintje. Even so, isolates of the newly introduced US-8 genotype were able to cause significantly more disea se on these resistant cultivars than isolates of the US-1 genotype. The pre dominant genotypes currently found in potato production areas in Canada (US -8 and g11) have higher fitness on tuber tissue than displaced, preexisting strains (US-1).