Changes in metalaxyl resistance among glucose phosphate isomerase genotypes of Phytophthora infestans in Canada during 1997 and 1998

A total of 930 isolates of Phytophthora infestans collected during 1997 and 1998 from commercial potato and tomato fields, home gardens, and potato st orages in Canada was analyzed for glucose phosphate-isomerase (Gpi) genotyp e, mating type, and resistance to metalaxyl. Both mating types and diversit y in resistance to metalaxyl were detected within and among Gpi-genotypes. Pathogen populations were dominated by the 100:111:122 Gpi-genotype (charac teristic of US-8) in all the provinces where late blight was detected, exce pt in 1997 in British Columbia (BC) where the 100:100:111 Gpi-genotype (cha racteristic of US-II) predominated. Gpi-genotypes 100:111, 111:122, and 100 :122 (characteristic of US-7, US-10 and US-14/US-17, respectively) were les s common. Nearly all of the isolates with the 100:100:111 Gpi-genotype were the Al mating type, except for a few collected in BC in 1997. A majority o f isolates with the 100:111:122 Gpi-genotype mere A2 except for a few found in Quebec in 1998. Based on colony growth on media amended with 1, 10, or 100 mug/ml metalaxyl, the 100:100:111 Gpi-genotype isolates had higher leve ls of resistance to the fungicide in 1998 compared to 1997. In 1998, level of metalaxyl resistance Ras higher within the 100:100:111 Gpi-genotype wher eas it was lower within the 100:111:122 Gpi-genotype, when compared to 1997 . BC was the only Canadian province where an increase in metalaxyl resistan ce within the 100:111:122 Gpi-genotype was recorded between 1997 and 1998. Isolates from leaves had higher metalaxyl resistance levels than stem or tu ber isolates, regardless of genotype.