Genetic Structure and Subclonal Variation of Extant and Recent U.S. Lineages of .

The oomycete is an important plant pathogen on potato and tomato crops. We examined the genetic structure of extant 20th and 21st century U.S. lineages of and compared them with populations from South America and Mexico to examine genetic relationships and potential sources of lineages. US-23, currently the most prevalent lineage detected in the United States, shared genetic similarity primarily with the BR-1 lineage identified in the 1990s from Bolivia and Brazil. Lineages US-8, US-14, and US-24, predominantly virulent on potato, formed a cluster distinct from other U.S. lineages. Many of the other U.S. lineages shared significant genetic similarity with Mexican populations. The US-1 lineage, dominant in the mid-20th century, clustered with US-1 lineages from Peru. A survey of the presence of RXLR effector revealed that some lineages carried , its resistance-breaking variant , or both. Minimum spanning networks developed from simple sequence repeat genotype datasets from USABlight outbreaks clearly showed the expansion of US-23 over a 6-year time period and geographic substructuring of some lineages in the western United States. Many clonal lineages of in the United States have come from introductions from Mexico, but the US-23 and US-1 lineages were most likely introduced from other sources.