Submitted to: Digger
Publication Type: Trade Journal
Publication Acceptance Date: July 15, 2011
Publication Date: September 1, 2011
Citation: Grunwald, N.J. 2011. Phytophthora ramorum: How it got here and how it spread. Digger. 9:41-45. Technical Abstract: The sudden oak death pathogen might have arrived on the U.S. West Coast circa 5-15 years before its first detection in the mid 1990s. Sudden oak death has caused disease of epidemic proportions on tanoak, Japanese larch and live oak. This article provides a brief chronology of the sudden oak death epidemic since its first discovery in California, showing emergence of different clones and major migration events. The sudden oak death pathogen is evidently of exotic origin both in North America and Europe, the only ranges in which it has been detected to date. There are now 3 known variants of the pathogen that propagate clonally and do not cross-breed. These clones are referred to as NA1, NA2 and EU1 for the continent on which they were first found. The first documented clone, NA1, is the one responsible for causing sudden oak death in California and Southern Oregon. Detailed population studies have shown that the NA1 pathogen was most likely introduced into California, potentially via imports of exotic ornamentals. Further studies have demonstrated that the NA1 clone has spread up and down the West Coast and has migrated west to east. Most recently, NA1 has also been moved with shipments of ornamentals across state boundaries on the East Coast. Recent studies showed that the NA2 lineage was introduced into the Pacific Northwest through either British Columbia or Washington. The NA2 lineage is rarely found in nurseries and has not been found in forests. The EU1 clonal lineage is more widespread than the NA2 clonal lineage and also exists predominantly in the nursery environment. It too was introduced into the Pacific Northwest and was moved trans-continentally from Europe to North America. Scientists are still looking for the origin of the NA1, NA2, and EU1 clones. All we know at this point is that they are exotic to Europe and North America. This exotic pathogen has repeatedly moved among continents, and continued caution in importing and moving nursery material is warranted.