Phytophthora infestans: the itinerant invader; “late blight”: the persistent disease

In most locations worldwide, populations of Phytophthora infestans are asexual and are often dominated by one or a few clonal lineages. The dominance in a population by any of these clonal lineages can be ephemeral. Studies in the USA and in the United Kingdom illustrate that dominance by any one clonal lineage may range from a year to two to fifteen years. In the eastern USA, lineage US22 dominated in 2009 and 2010. However, from 1994 to 2009, US8 was consistently dominant on potatoes throughout most of the continental USA. Migrations of exotic clonal lineages contribute substantially to changes in the population composition in most locations worldwide. Because these exotic strains can have “novel” phenotypic characteristics, they may not respond to traditional disease management techniques and thus may cause considerable damage which can lead to significant human suffering. Thus, while the disease persists in a location, the genotypes of P. infestans may change dramatically.