Phytophthora infestans, the fungus that causes blight in potatoes, probably originated in Mexico1, where two sexually compatible mating groups (A1 and A2) occur with equal frequency and oospores, resulting from normal sexual reproduction, are produced2. Elsewhere, by contrast, only one mating type (A1) of the species has been recognized, so that in this situation the sexual stage of the fungus is not considered to have a role in the development of blight3. P. infestans is variable and highly adaptable to its different host genotypes, and much potato breeding experience shows that, even outside Mexico, the fungus adapts remarkably rapidly to varieties that bear R-genes from the wild species, Solanum demissum4. Indeed, it now seems clear that no R-gene resistance can be expected to last for more than a few years, for up to eleven specificities occur frequently in the population of P. infestans in Britain5.