Population Genetic Analysis Infers Migration Pathways of Phytophthora ramorum in US Nurseries

Recently introduced, exotic plant pathogens may exhibit low genetic diversity and be limited to clonal reproduction. However, rapidly mutating molecular markers such as microsatellites can reveal genetic variation within these populations and be used to model putative migration patterns. Phytophthora ramorum is the exotic pathogen, discovered in the late 1990s, that is responsible for sudden oak death in California forests and ramorum blight of common ornamentals. The nursery trade has moved this pathogen from source populations on the West Coast to locations across the United States, thus risking introduction to other native forests. We examined the genetic diversity of P. ramorum in United States nurseries by microsatellite genotyping 279 isolates collected from 19 states between 2004 and 2007. Of the three known P. ramorum clonal lineages, the most common and genetically diverse lineage in the sample was NA1. Two eastward migration pathways were revealed in the clustering of NA1 isolates into two groups, one containing isolates from Connecticut, Oregon, and Washington and the other isolates from California and the remaining states. This finding is consistent with trace forward analyses conducted by the US Department of Agriculture's Animal and Plant Health Inspection Service. At the same time, genetic diversities in several states equaled those observed in California, Oregon, and Washington and two-thirds of multilocus genotypes exhibited limited geographic distributions, indicating that mutation was common during or subsequent to migration. Together, these data suggest that migration, rapid mutation, and genetic drift all play a role in structuring the genetic diversity of P. ramorum in US nurseries. This work demonstrates that fast-evolving genetic markers can be used to examine the evolutionary processes acting on recently introduced pathogens and to infer their putative migration patterns, thus showing promise for the application of forensics to plant pathogens

First report of alder Phytophthora closely related to P. uniformis on Alnus glutinosa seedling in Finland

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Phytophthora ramorum in Canada: evidence for migration within North America and from Europe

Phytophthora ramorum, the cause of sudden oak death on oaks and ramorum blight on woody ornamentals, has been reported in ornamental nurseries on the West Coast of North America from British Columbia to California. Long distance migration of P. ramorum has occurred via the nursery trade, and shipments of host plants are known to have crossed the United States-Canadian border. We investigated the genotypic diversity of P. ramorum in Canadian nurseries and compared the Canadian population to US and European nursery isolates for evidence of migration among populations. All three of the P. ramorum clonal lineages were found in Canada, but unexpectedly the most common was the NA2 lineage. The NA1 clonal lineage, which has been the most common lineage in US nurseries, was found relatively infrequently in Canada and these isolates may have been the result of migration from the US to Canada. The EU1 lineage was observed almost every year and shared multilocus genotypes with isolates from Europe and the US. Estimation of migration rates between Europe and North America indicated that migration was higher from Europe to North America than vice versa, and that unidirectional migration from Europe to North America was more likely than bidirectional migration