Until recently, the only widespread mating type of Phytophthora infestans had been the A1. Without its complementary A2 mating type (that occurred in Mexico), the fungus could only reproduce asexually, which limits its genetic variation. When both mating types exist at the same location, however, they reproduce sexually, forming hard-hulled oospores that can survive in the soil for many years. In the mid-1980s, scientists reported finding the A2 mating type in western Europe. This was the first indication of new and far-reaching developments that would invariably lead to population shifts and epidemiological changes in the fungus. V. Umaerus, a well-known scientist, noted that oospore formation has been observed in many countries. He further suggested that improved monitoring and control strategies are now required because of increased diversity in race formation and adaptation due to sexual reproduction, as well as changes in the epidemiological patterns caused by oospore longevity.

Before the A2 mating type spread, all late blight damage outside of Mexico–the center of origin of the fungus–was assumed to come from mutation or parasexual recombination in strains of A1. The migration of the A2 type was recently shown to be composed of several strains, but also of additional A1 strains. Furthermore, some of the new A1 and A2 strains were fitter and replaced the originally disseminated A1 strain. The sexual recombination of fitter strains of the two mating types is expected to produce more aggressive strains of the fungus. Worst of all, some new strains are resistant to metalaxyl, one of the most effective late blight fungicides.

The A2 type has now been reported on all continents except Australia (and of course Antarctica, where potatoes are not grown). Scientists believe that the spread of the new strains will inevitably result in dramatic increases in the use of fungicide sprays and the development of even more powerful chemicals, both of which have important consequences for environment and health. In addition, farmers face substantial annual increases in production costs. Nowhere, however, are the challenges likely to be greater than in the developing world, where farmers often lack the cash required to buy chemicals and sprayers, and where potatoes are helping to meet future food challenges.