Abstract: It is necessary for late blight disease control to characterize populations of phytophthora infestans. The characteristics of mating type, metalaxyl sensitivity and mtDNA haplotype of P.infestans isolates collected from 32 potato and tomato production regions in Yunnan were analyzed. The result showed that the A2 mating type and self-fertility isolates were rare on potato, average 3.4％and 4.4％respectively, which were detected at only 4 sites. Only were A1 mating type isolates collected from tomato detected and the composition of mating type had been changed dramatically in Yunnan since 2002. In vitro tests of sensitivity to the phenylamide fungicide metalaxyl showed that insensitive and sensitive isolates occurred on potato and tomato and the ratio of insensitive and sensitive ones was 22.6％and 77.4％respectively. The ratio of resistant isolates from tomato was higher than that from potato. Three mtDNA haplotypes Ⅰa,Ⅱa andⅠb were detected from P.infestans populations in Yunnan. Both Ⅰa and Ⅱa haplotypes of P.infestans collected from potato were the ‘new’ populations and the former predominated, dispersing in all potato production regions.Ⅰa and Ⅰb haplotypes from tomato isolates showed that the “old” and “new” populations of P.infestans coexisted. The result of mtDNA haplotypes revealed that replacement of P.infestans had occurred. Migration and rare sexual reproduction may play an important role on variation and replacement of P.infestans populations in Yunnan.

Key words: mating type, self-fertility, Metalaxyl sensitivity, mtDNA haplotype, late blight disease