AbstractIsolates of Phytophthora infestans (Mont) de Bary (the potato and tomato late blight pathogen) resistant to phenylamides appeared in Europe and North America in the late 1970s and early 1990s respectively. Concurrent, but coincidentally, with both these events there were radical structural shifts in the pathogen populations as immigrant genotypes from Mexico displaced the indigenous populations. Both A1 and A2 mating type isolates are now present in blighted crops, permitting alternative inoculum via germinating sexually produced oospores to influence dynamics of late blight populations. Studies of inheritance of ploidy, host‐specific pathogenicity, mating type and resistance to antibiotics and phenylamide fungicides have provided insight into mechanisms of variation in this potent pathogen.© 2002 Society of Chemical Industry