The escape from Mexico of the A2 mating type of P. infestans raises the specter of even higher virulence of the pathogen and disturbing changes in the epidemiology of the fungus. What the world's farmers have been battling for the past 150 years is the A1 mating type of the pathogen. Conventional wisdom holds that both the A1 and A2 types originated in the Toluca Valley of Mexico. But for reasons that we do not fully understand, the A2 remained behind while the A1 spread around the world. The situation changed dramatically in the 1970s when the A2 was unintentionally exported to Europe on contaminated Mexican potatoes.
Most scientists agree that it was then re-exported on infected seed potatoes.
Were you to take a map of the countries where the A2 is present and overlay it with a map of countries that purchase seed potatoes from Europe, you would find you have an almost perfect match. The problem comes to a head when A1 and A2 are present in the same environment and recombine sexually. The result is an explosion of genetic variation of the fungus and the enhanced probability that even more virulent forms of the disease will be produced. The new forms of the disease could be resistant to some of the most powerful fungicides and to existing sources of genetic resistance within the crop. Moreover, due to the formation of oospores, which are the product of sexual reproduction between the A1 and A2 types, late blight could become an earlier blight and begin infecting the plant immediately after it emerges.