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 storages in Canada was analyzed for glucose phosphate-isomerase (Gpi) genotype, mating type, and resistance to metalaxyl. Both mating types and diversity in resistance to metalaxyl were detected within and among Gpi-genotypes. Pathogen populations were dominated by the 100:111:122 Gpi-genotype (characteristic of US-8) in all the provinces where late blight was detected, except in 1997 in British Columbia (BC) where the 100:100:111 Gpi-genotype (characteristic 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 less 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 of 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 levels 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 whereas 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 resistance within the 100:111:122 Gpi-genotype was recorded between 1997 and 1998. Isolates from leaves had higher metalaxyl resistance levels than stem or tuber isolates, regardless of genotype.