Plant Pathology Department, Cornell University, 334 Plant Science Bldg, lthaca, NY 14853, USA

Potato late blight became noticeably more problematic throughout the world during the final quarter of the 20th century. The first locations to experience the more severe problems were countries in north-west Europe. Initially the more severe disease was thought to be caused by the appearance and then predominance of metalaxyl resistance in north-west European populations of Phytophthora infestans. Subsequently from analysis of neutral markers (alleles at the glucose-6-phosphate isomerase and peptidase loci, mating type, and DNA fingerprint, determined via RG57) it was established that new genotypes had been introduced to Europe, and these were replacing the previous dominant strain (US-1). These reports stimulated investigations throughout the world which revealed migrations into South-East Asia, South America and then North America. There is clearly geographic substructuring of P. infestans populations in Europe, and sexual recombination appears to be important to the population structure in certain locations. At least some populations in South America appear to be derived from those in north-west Europe. However, populations in South America still appear to be primarily clonal and quite simple. In Japan and Korea, populations are very simple, and are dominated now by one or a few recently introduced genotypes. There are not yet any reports of sexual recombination in South-East Asia.

Migrations into the USA and Canada were initiated during the 1980s, but were confined to the Pacific Coast. With introductions to the eastern USA and Canada in the early 1990s, the situation changed dramatically, and one strain (US-8) quickly predominated in potato production areas throughout the USA and Canada.

While metalaxyl resistance provided a clear fitness advantage to strains recently introduced to the USA, there is now evidence that the recently introduced strains are also more aggressive. Whereas sexual reproduction with the appearance of true hybrids can contribute to a fitness advantage of newly introduced strains in some locations, such a mechanism is not possible in all locations. Explanations for the greater fitness of introduced strains are being sought. Some possibilities include the gradual accumulation of debilitating mutations in the previously dominant asexual lineages (Muller's ratchet), and the possibility that new strains may have additional survival stages.

While central Mexico is clearly the centre of diversity and the probable centre of origin of P. infestans, populations that evolve outside central Mexico appear to be influenced by different/additional selection forces from those operational in central Mexico. Whether central Mexico will continue to be the major source of new strains of P. infestans remains to be determined.