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Our understanding of the population genetics of Phytophthora infestans has increased dramatically during the past decade. In addition to satisfying scientific curiosity, we expect that this understanding will lead to im provements in tomato and potato late blight management. We briefly describe the classical population biology/genetics of P. in Jestans to provide context for this review. Potato late blight caused by P. infestans was probably first observed outside of central Mexico in 1842 in the USA; it was first recorded in Europe in 1845 (61). Subsequently, this organism has appeared wherever potatoes are grown (5). Phytophthora in Jestans is heterothallic, so both Al and A2 mating types are required for sexual recombination. Only A l mating types were known before the early 1950s, when A2 mating types were discovered in the Toluca Valley in the highlands of central Mexico (39), now generally considered to be the …