The return of Phytophthora infestans, a potato pathogen that just won't quit

Since the early 1980s and continuing through the end of the 20(th) century, potato late blight caused by Phytophthora infestans has become noticeably more problematic throughout the world. Exotic strains of the pathogen are l argely responsible for the increased problems. The new strains were initial ly detected in Europe, but subsequently they have been detected in Asia, th e Middle East, South America and North America. Populations in the Middle E ast and South America may represent secondary migrations from Europe. Sever al independent migrations have occurred such that migrant populations in No rth America are very different from those in Europe. Also, population struc tures of P. infestans are different in different worldwide locations. In so me locations the migrant strains are resistant to metalaxyl, but in other l ocations either migrant or indigenous strains may be resistant. In most loc ations, the migrant strains are more aggressive than the previous indigenou s strains.