Molecular genetic evidence for a new sexually reproducing population of Phytophthora infestans in Europe

In the early 1980s the old A1 mating type population of the potato late blight pathogen, Phytophthora infestans, was displaced by new A1 and A2 mating type isolates in Europe. Analyses of virulence characteristics and DNA fingerprint patterns of a large number of isolates revealed that with the introduction of new P.infestans isolates the level of genetic diversity in the population has increased dramatically. Experiments under controlled conditions and under natural conditions in the field demonstrated that oospores are formed in large numbers after inoculation of potato leaves with a mixture of A1 and A2 mating type isolates of P.infestans. Oospores in soil, exposed to natural weather conditions during the winter, remain viable for at least eight months. It is concluded that after the introduction of the new A1 and A2 mating type population in The Netherlands P.infestans reproduces sexually and forms oospores. It is likely that oospores play an important role in the epidemiology of potato late blight.