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.

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